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Are We Wired to Need 8 Hours of Sleep? Options
Daemon
Posted: Sunday, October 18, 2015 5:00:00 AM
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Are We Wired to Need 8 Hours of Sleep?

Modern life's sleep troubles — the chronic bleary-eyed state that many of us live in — have long been blamed on our industrial society. The city lights, long work hours, commutes, caffeine, the Internet. When talking about the miserable state of our ... More...
Elsayyed Hassan
Posted: Sunday, October 18, 2015 3:50:00 AM

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Sleep guidelines from the National Heart, Lung, and Blood Institute recommend that adults get between seven to eight hours of sleep daily but findings of a new study involving present-day primitive societies suggest that most people may only need six to seven hours of sleep at night.

A growing number of studies blame artificial light, particularly those from smartphones and other modern-day devices, for people's inability to get sufficient amount of sleep at night.

It appears though that prehistoric people, who are known to recharge their body naturally by sleeping and waking according to the rhythm of the sun, are also "sleep-deprived."

In a study published in the journal Current Biology on Oct. 15, Jerome Siegel from the University of California in Los Angeles and colleagues looked at the sleep patterns of three of the world's last remaining groups of hunter-gatherers who live in Africa and South America.
JUSTIN Excellence
Posted: Sunday, October 18, 2015 6:09:20 PM

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The author of that somehow misleading post has asked:
Quote:

Should we blame the lack of sleep as causing our obesity, mood disorders and the like?

NO!

Sleep has persisted in evolution even though it is apparently maladaptive with respect to other functions. While we sleep, we do not procreate, protect or nurture the young, gather food, earn money, write papers, etc. It is against the logic of natural selection to sacrifice such important activities unless sleep serves equally or more important functions. (Rechtschaffen, 1998, p. 360)

Some animals sleep when it would be much simpler not to do so. For example, the blind Indus dolphin lives in very murky water, so it does not need to see, it uses sonar to navigate, and it eventually became blind. Yet, in spite of the difficulties of sleeping in an aquatic environment - difficulties equal to those of trying to see in murky water - the Indus dolphin has retained sleep. Among mammals and birds, at least, there is some compelling reason to sleep. Yet, there is no complete understanding of, much less complete agreement about, the functions of sleep. It seems as if we are just on the threshold of understanding why we sleep. It is like being in a room with several doors that are just a bit open allowing us a peak of what is inside. With a bit of effort, we may be able, in the future, to open one or more of those doors and get a clearer view. Meanwhile, we shall peek into many doors in order to review many of the current theories of sleep, especially those that remain active and viable or that are new and exciting.

First, let us be clear on what is meant by function. Function means purpose. Function equals instrumental value, that is, it makes a difference. Function means the effect or effects accomplished. But there are different levels of purposes, instrumental values, and effects, ranging from essential to enhancing to convenient to trivial to detrimental. Consider the functions of your nose. It is a vital component of the respiratory system, a convenient place upon which to rest glasses, an enhancer or detractor of facial beauty, and a most annoying place of irritation when you have a cold.

A function may be but a component of a larger system or sequence. Again, your nose is a part of the respiratory system and a component of your face. Some of its functions are in relation to the greater whole.

Some functions of things are absolutely necessary; the system would fail without them. Other functions may not be necessary, but the system works better or faster with them. You can breathe through your mouth, but it is generally better to do so through your nose ... Still other functions may be convenient but could be easily replaced or done without. There are ways that glasses could be kept on your face rather than to rest them on your nose. Finally, other functions may be superfluous or even detrimental. A stuffy nose is a good example.

Often the search for function has concentrated on that which is essential or primary (Kreuger & Obal, 2002; Nicolau et al., 2000). The organism has no other way to initiate this function and would suffer from its loss. It is why the function developed during evolution. Yet, the next two levels of functions, enhancing and convenient are also important and are worthy of search and research efforts and should not be quickly discounted. We may find that the functions of sleep are of these latter types more than of the former type and are just as important as practical answers to the version of the question, “What is the function of sleep?” that means "What is sleep for?" We also must realize that it is more difficult to discover a non-essential function than an essential function. Yet, it is possible to determine such functions by careful research and reasoning. Then, too, the essential function of sleep may not be readily apparent from the characteristics of sleep or its mechanisms of generation.

Often the question is put too simply: "What is the function of sleep?" Turn the question around and ask, "What is the function of wakefulness?" There is no one, simple answer to either question. Thus, we must ask, "What are the functions of sleep?" and seek answers on many levels, from molecular to behavioral. Furthermore, we need to remember that "sleep and wake are mutually interacting and cyclic phenomena", and thus a theory of sleep necessarily involves implications for wakefulness, also. Then too, a theory or explanation of the functions of sleep must be consistent with the other myriads of details that are known about it. Most importantly to Kreuger and Obal (2002), the loss of consciousness which occurs during sleep must be explained in an adequate theory of sleep.

There are essentially four scientific strategies used in the search for the functions of sleep: description, correlation, stimulation, and deprivation (Rechtschaffen, 1998). Each has its strengths and weaknesses.

• Description: points to possible functions, but there may easily be other explanations for the observations. For example, it is easily observed that sleeping animals typically close their eyes, but that does not mean that the function of sleep is to protect the eyes. It is more likely that closing the eyes facilitates sleep.

• Correlation: sleep correlated with something suggests possible function, but other explanations are possible. For example, if sleep is correlated with some aspect of personality, we do not know if sleep causes that aspect, or that aspect causes sleep, or a third factor causes both.

• Stimulation (experimentation): suggests internal changes to which sleep responds, but stimulation can increase or decrease sleep independent of need. For example, a sleeping pill is shown to stimulate sleep. Research can show the brain mechanisms by which the pill stimulates sleep, but it may have nothing to do with natural mechanisms, hence function.

• Deprivation: reveals what happens without sleep, but the results could be responses to sleep-preventing stimuli themselves rather than affecting sleep directly. For example, rats kept awake by forced running show consequences, but it is not clear if the consequences are from the lack of sleep or the continuous running.

The best clues to the functions of sleep come when there is research using several different methodologies from more than one type of strategy that consistently leads to the same conclusion.

You may ask, “Why is it important to seek out and understand the functions of sleep and dreaming?” The answer is, we can never really know what sleep is until we understand its functions. This in turn influences our (Meddis, 1979):

• attitudes toward our own sleep

• research endeavors (a good example is the dramatic change in sleep research that occurred when it was realized that sleep is active and not passive)

• treatments for sleep-wake disorders

• and contributions to our basic understanding of human beings and the world in which we live.

In an interesting new paper, Nicolau and colleagues (2000) posit that it is not so much that sleep has evolved, but that it is waking that has evolved. With the development of the forebrain during evolution, homeotherms developed a new wake that is different from their reptilian forbearers. The old wake of reptiles became SWS.

We will begin our review by examining the overall functions of sleep and NREMS...then look at the functions of REMS and dreaming.



Functions of Sleep and NREM

As a tour exploration of the possible functions of sleep, let us briefly review the many unique characteristics of sleep (after Rechtschaffen, 1998). Sleep:
✔ is found in all mammals, birds, and apparently reptiles; it may also occur in some or all amphibians, fish, and invertebrates cannot be replaced by waking rest
✔is homeostatic-deprivation leads to rebound
✔is rhythmic-it tends to occur at regular times each nychthemeron
✔results in physiological and psychological changes that do not easily occur otherwise
✔is a time of unconsciousness with reduced interaction with the external environment
✔is actively produced by the brain has two distinct components that alternate in mammals has similar development in all mammals

Generally sleep is somewhat responsive to what happens in the daytime. Some waking activities have been found to affect sleep, such as increasing body temperature by exercise or sitting in a hot tub of water, weight gain or weight loss. Equally important are things during waking that do not affect sleep or do so to a very limited extent. Among them are exercise, increased metabolic rate, prolonged bed rest, intense sensory stimulation, sensory deprivation, and mentally stimulating activities (Rechtschaffen, 1998). Certainly what waking influence there is cannot be said to be the determining factor of why we sleep. Instead, sleep appears to be a need in its own right. In fact, the most consistent thing that sleep deprivation does is increase the need for sleep and the need for the things that happen during sleep (Rechtschaffen, 1998).

FUNCTIONS FOR ALL SLEEP

Rest and Restorations

One of the oldest and most intuitive notions is that sleep is for rest and restoration of the body and mind. It seems to be a time of quiescence when the body appears to be able to generally reverse the wear and tear accumulated when awake. We feel bad when we do not get sleep or enough of it. In fact, sleep may have evolved out of rest that alternates with activity in most animals. It is possible that this pattern occurred to enable even greater rest and even restoration of tissues and organs worn down by waking activity.

Sleep researchers are confident that there is some kind of recuperation that occurs during sleep because of the presence of a drive to sleep in all higher animals, the deficits that occur with lost sleep, and the rebound after lost sleep (Bennington, 2001). The greater the sleep lost, the greater the rate of recuperation with subsequent sleep. Yet, there needs to be a minimum amount of continuity during sleep for it to be recuperative, and both REMS and SWS appear necessary for complete recuperation. Additionally, sleep researchers are fairly sure that recuperation needed and obtained is proportional to basic waking activity, and that the recuperation occurs at a cellular or brain circuit level. What is less certain is the precise nature of the recuperation, the relationship between NREMS and REMS, and why there is cycling between them.

In addition to being an intuitive notion, the notion that sleep is for rest and restoration is supported by the fact that some hormones are primarily released during early SWS. Growth hormone is released in its highest levels in young, growing humans during sleep and only during the first SWS period of the night in adults. The levels of other anabolic hormones (prolactin, leutenizing hormone, testosterone) are also highest during sleep. In contrast, catabolic hormones such as the corticosteroids are low during normally phased sleep periods.

More recently, research out of Van Cauter's laboratory at the University of Chicago has shown that sleep is necessary for certain body functions to stay within normal limits. Additionally, NREMS is hypothesized to replenish cerebral glycogen stores depleted during waking (Bennington and Heller, 1994, 1995).

However, there is evidence contrary to the hypothesis of a rest and restoration function for sleep (Rechtschaffen, 1998). There is a mean decrease in protein synthesis during sleep due mainly to fasting and the fact that level of muscle activity, hence wear and tear, during waking has a very low correlation with sleep length. While there is an increase in growth hormone during sleep, there is not an increase of overall protein synthesis. Also, there is not convincing evidence that energy is replenished during sleep. Glycogen restoration is increased only during early sleep with little change during the rest of sleep. Furthermore, sleep deprivation does not result in obvious dramatic and permanent breakdowns of the body or the mind. The physiological effects of acute sleep deprivation are generally small.

Also, logically, it is not clear why unconsciousness, a key component of sleep, is necessary for rest and restoration, when simple inactivity while awake would seem to accomplish the same ends. There is, too, a great variation in sleep in various species of mammals and other animals for it to be necessary for rest and restoration. And, the fact that some mammals sleep half a brain at a time because they need to keep their bodies moving suggests that the benefit of sleep is not primarily for the body. We will look at these concepts more fully later.



Conservation of Energy

Sleep is a time of reduced levels of activity, body temperature, and energy consumption. This notion leads to the hypothesis that sleep serves the function of conservation of energy, especially when little is to be gained from being awake and active. The conservation of energy that occurs during sleep comes from more than a lack of activity, since a period of simple rest would accomplish that goal. Rather, it is the decrease of body temperature during sleep that conserves considerable energy. The cost of maintaining temperature in warm-blooded animals is high-metabolic rate is 7 to 10 times higher in endotherms than in ectotherms at the same body temperature-and a reduction of only a few degrees is cost-effective. A decrease of 1’-2' in endotherms causes a 10% decrease in metabolic rate at neutral ambient temperatures and even greater savings at ambient temperatures that are cooler than neutral.

There is the apparent concurrent evolution of NREMS during evolution as homeothermy evolved with a similar concurrent development during ontogeny. Parallel development of sleep in mammals and birds occurred with the advent of endothermy. NREMS, with its turning down the thermostat of the bodies, matures concurrently with endothermy during maturation. Additionally, there is a greater requirement for sleep in animals that have less energy reserve. Finally, it is noted that there may be a continuation from sleep into hibernation with its even greater reduction in body temperature and energy usage. In contrast, others have pointed out that sleep may simply permit hibernation and, since there is a rebound of SWS following hibernation, sleep and hibernation may not be continuous.

A second version of this purported function for sleep focuses on energy expenditure. Sleep is a time of enforced rest that sets limits on activity and energy expenditure in order to help balance an animal's species-specific energy budget to keep it at a level that is affordable. There is a correlation of 0.63 between sleep length per nychthemeron and metabolic rate in mammals. Metabolic cost of activity varies inversely with body size, and food requirements are proportionally higher in smaller animals. Longer sleeping species tend to be small with higher metabolic rates. There is greater sleep in immature animals when more energy must be directed toward growth. The effects of terminal sleep deprivation studies in rats are also consistent with this version of the hypothesis.

Rechtschaffen (1998) comments on these hypotheses by saying that some support can also be found in the negative correlation (-0.51) between total sleep time per nychthemeron and body weight. Animals with greater body weight have more stored energy and lower cost of locomotion. They also have lower heat loss due to their greater bulk and generally better insulation. Lacking these qualities, there is greater savings in sleeping in smaller animals. Yet, the reductions in energy expenditure during sleep are only modest.



Babevioural Adaptation

One of the behavioral theories of sleep can be summed up by the phrase, "It’s safer to be asleep." There are times in the twenty-four-hour day when an animal may be less safe. The danger might be from other animals attacking it when it is more vulnerable. The immobility of sleep attracts less predator attention and reduced responsiveness to the environment. Webb (1983) has termed this "adaptive non-responding" during times of potential danger. Adaptive non-responding is under circadian control and resembles an evolutionarily developed instinct. It may be more important in more advanced animals.

Another threat might be from an accident when the animal is less able to perceive danger in its environment. For many animals, including human beings, night-time is more dangerous. Our main sensory receptor - our eyes -- is built to respond best during daylight. We can see at night, but not well. This fact makes us, and other animals like us, more susceptible to stumbling over a cliff or other natural dangers. For other animals, daytime is the dangerous time. During such dangerous times, it is safer to be asleep (Meddis, 1983).

Paralleling the safety function of sleep may be a function related to food availability. It is not effective for an animal to be active during those parts of the day when its food is not as available - maybe because its food in the form of other animals is sleeping! From a cost-benefit basis, it is better to be asleep if it takes too much energy and poses too great a risk to be awake and active with little likelihood of securing much food (Meddis, 1975).

This adaptive non-responding hypothesis, intuitive as it is, is almost impossible to test scientifically, because it is difficult to determine degree of predatory susceptibility. Also, it has been argued that sleeping animals may be less aware of potential predators, thus at greater, not less, risk of danger by sleeping. Support for this idea comes from data that show less NREMS and REMS in some animals in the presence of predators, and some species that are more likely to be preyed upon have less REMS. Some researchers counter that the nature of the typical sleeping habitat also needs to be considered. Animals who have a safe sleeping habitat tend to sleep more than those animals that do not have a safe place to sleep. Being immobile in a safe place is clearly an advantage. Also, for all versions of this hypothesis, it is not clear why sleep with unconsciousness is necessary when waking behavioral inactivity would do the same thing.



Benefits for the Brain

Horne (1983a; 1988; 1989) contends that human sleep does much more good for the brain than the rest of the body. At the time he published this conclusion, most of the positive findings on sleep deprivation studies were limited to effects on the brain and to psychological effects. These dysfunctions have been shown to be due to more than simply “lapses” caused by “microsleeps;” rather, they are also the direct result of cognitive deficits caused by the lack of sleep. It is the brain, more than the body, that becomes dysfunctional without sleep.

However, such dysfunctions may be due as much to the disruption of the circadian rhythms that accompany sleep deprivation. Whenever a person is awake, the brain is at maximal activity levels or very near to it. Even during quiet restfulness, when alpha waves are dominating the EEG, the brain cells are still considerably active, rather like a computer whenever it is turned on-it consumes about the same amount of power whether it is running a program or simply waiting for instructions. In order for maintenance to be done, brain activity needs to be reduced. Only during SWS is this condition met. This reduction in activity applies to the forebrain but not to the brainstem. The functions of the brainstem are rigidly determined, with little possibility for adaptability or change. In contrast, the forebrain is very plastic, allowing greater flexibility in behavior and learning. Such a system requires more upkeep and maintenance.

Rechtschaffen (1998) counters that while sleep deprivation worsens performance and higher order cognitive and creative mental processes in humans, these results would seem to have little parallel in animals, leaving no reason for animal sleep.



Local Cell Benefits

Another hypothesis holds that the primary reason for sleep is to benefit local groups of cells rather than benefit the entire organism or even individual organs. Moruzzi (1966) suggested that sleep is for the slow recovery and stabilization of synapses involved in plastic activities of learning, memory, and consciousness. A related notion IS that sleep strengthens and preserves synapses underused during waking so that they do not weaken so much as to be unavailable when needed In the future (Kreuger et al., 1995). Either way, sleep begins when groups of cells that have been active release chemicals, such as cytokines and nerve growth factors, which initiate several parallel, cascading biochemical processes that help strengthen the active synapses. The result is maintenance of individual synapses, but also the integration of new synaptic patterns initiated by new experiences. These integrated patterns result in greater flexibility in behavior but within a contextual framework.

Kavanau (c.f. 1997) has speculated that use-dependent synapses are stabilized during sleep, free of sensory input interference. The substances released from active synapses also cause the circuits to alter their firing patterns. When enough local circuits are in this altered state, sleepiness occurs. As more local circuits enter this state, sleep ensues. Thus, sleep is neuron use dependent, not wake dependent.

According to Kreuger, whole brain sleep onset and duration is coordinated and organized by specific brain areas, such as the basal forebrain area, thalamus, and pons. These special areas are influenced by local circuits that have entered the functional mode of sleep as well as by the suprachiasmatic nucleus for circadian rhythm control, thermoregulatory centers of the brain, and sensory input from the body. Numerous experiments have shown that destruction of these areas results in the absence of sleep that is only temporary, although the subsequent sleep may not be as well organized. These studies show that it is not these areas that actually initiate sleep but only play a role in organizing it. The difference between NREMS and RELMS is the level of the coordinating brain areas. For REMS, these areas are in the brainstem, while for NREMS, these areas are in the forebrain.

Evidence that sleep occurs at a local level includes:

* Sleep patterns of electrical activity can be shown to occur in local groups of cells;
* Local cellular events combining to produce a coordinated output have been shown to occur elsewhere in the brain. For example, in the SCN, individual cells have their individual circadian firing patterns that coordinate to result in the circadian rhythms of the entire animal;
* Adenosine may build up in local areas because of activity in that area. The adenosine, in turn, causes a local slowing of EEG potentials;
* There is no area of the brain that when destroyed permanently eliminates NREMS Unihemispheric sleep in some aquatic mammals and in many birds shows sleep is not a whole brain phenomenon;
* There are anterior-posterior and right-left differences in EEG power during sleep;
* As sleep progresses through the night, there are different changes in the EEG in different regions of the cortex;
* Extensive use of an area of the brain during waking due to sensory or cognitive activity results in greater intensity of activity in that area during sleep;
* There are regional differences in brain electrical activity during sleep following sleep deprivation. Such differences are not seen during waking;
* There is evidence that the transition from wake to sleep may not occur in all parts of the brain simultaneously;
* Research by Pigarev (c.f. Pigarev et al., 1996) shows that parts of the cortex may be asleep, while other parts are awake and vice-versa for 20 to 30 minutes;
* The symptoms of several sleep disorders, such as narcolepsy and RBD, are consistent with the idea that parts of the brain can be asleep, while other parts are awake. Also, lucid dreaming and the effects of sleep deprivation suggest the same thing.

Thus sleep, for Kreuger, is for synaptic efficacy and organization, which is dynamic and use dependent. The result is important changes in input-output dynamics that benefit the entire organism. This putative function requires loss of consciousness that many other posited functions of sleep cannot explain. The loss of consciousness is both a necessary condition for and a consequence of these local processes.

On the other hand, Rechtschaffen (1998) asks what processes in the brain identify such synapses, how such efforts are directed toward them, and why this has to be done during sleep rather than waking. Further, he points out that there is no direct evidence that sleep or lack of it has an effect on synapses, especially the weak ones.

Emotional Benefits

Sleep deprivation increases negative mood and decreases positive mood, strongly suggesting that a function of sleep is mood regulation. Sleep generally elevates mood. (We will explore this concept further in REM and Dreaming) A number of studies show that morning mood following nighttime sleep in non-depressed people is affected by the amount and quality of the prior sleep. Sleep of sufficient length and quality improves morning mood. When deprived of sleep for one night, morning mood scores are significantly lower.

It has also been shown that circadian phase interacts with amount of prior wake to influence mood, but the interaction is not simple. Depending on circadian phase, mood elevates, deteriorates, or remains the same with increasing duration of waking. This interaction is complex and non-additive such that a slight change in timing of the sleep-wake cycle can have notable effects on mood.



THE FUNCTIONS OF INDIVIDUAL SLEEP STAGES

Some of the functions of sleep are best explored by considering each stage of sleep rather than sleep as a whole. Some of the putative functions are derived from the nature of each of the stages, and other functions are suggested by selective sleep deprivation studies. Although rarely occurring in the real world, stages of sleep have been selectively deprived in the sleep laboratory and the resulting changes in physiology or behavior used to infer functions of the deprived stage.

NREMS

We will start examining the functions of NREMS. The functions of REMS are in future we will discuss. First, let us review some of the characteristics of NREMS that have implications for its functions:
● General slowing of the body and brain activity and functions;
● Decreased body temperature;
● Changes in the levels of release of some hormones;
● Burst-pause firing pattern in several major brain areas;
● Certain areas of the brain actively produce NREMS
● Decrease in the turnover of acetylcholine; norepinephrine, and serotonin;
● An increase of many types of pathogens in the body cause NREMS to increase;
● Selective deprivation causes more attempts to enter it NREMS and NREMS rebound occurs when uninterrupted sleep is resumed;
● Awakening from NREMS results in sleep inertia.

Overall, there have been relatively few studies of selective NREMS deprivation, so most of the hypotheses about its functions are derived from its characteristics. They are divided into hypotheses about restoration, conservation, and preparation for REMS.

Restoration

As indicated in the discussion about sleep in general, a very old and prevailing notion is that sleep provides some kind of rest and restoration. This process is most often thought to occur during NREMS. NREMS benefits the body.

NREMS functions to restore or recover some aspects of bodily functions worn down during wakefulness. This homeostatic notion is old and very prevalent (see above). Many lines of evidence are marshaled in support of this hypothesis, but not always without qualifiers or alternate explanations.

✂ The longer you are awake, the more intense your subsequent SWS. Conversely, SWS intensity decreases exponentially with the length of sleep. An explanation is that the longer you are awake, the greater the wearing down of the body and/or using up of vital resources which can only be built back up during SWS. However, this explanation is weak, because the relationship between the length of being awake and subsequent SWS is not found in all animals.

✂ During NREMS, there is a decrease of catabolic hormones and an increase of anabolic hormones in the body. The anabolic hormones tend to build up and restore the body while the catabolic hormones tend to wear the body out.

✂ Growth hormone is present only during the first SWS period of the night in adult humans. It is even more prevalent during SWS in children. However, it has been pointed out that growth hormone may not always do what its name implies (Horne, 1988), and the relationship between SWS and growth hormone has not been found in most other mammals.

✂ Sleep deprivation, or deprivation of SWS, results in a rebound during subsequent undisturbed sleep, showing that the body has a need for SWS and will make an effort to obtain it even if the opportunity is delayed, which implies SWS has an homeostatic function.

✂ Likewise, if deprived of SWS early in the night, there will be more of it later in the night. There is a high amount of SWS in children, with slow declines during adult-hood and much less or no SWS at all in retirees. This decline parallels that of metabolic rate in humans.

✂ In other species with no decline in SWS with advancing age, there is no decline in metabolic rate. However, the increase of SWS following exercise sometimes reported in the past research is no longer viewed as supportive evidence, because these effects are mediated by the resulting increase in body temperature.

✂ There are increases in the capability of the immune system that occur during NREMS. Further, many illnesses cause enhanced sleepiness, and the resulting extra sleep has been shown to be beneficial to the recovery from illness.

SWS primarily benefits the brain. Other hypotheses focus on the brain rather than the body as the beneficiary of SWS. Slowing down of brain activity allows repairs. Horne (1988) points out that in larger, more advanced animals, the body does not need a special period of rest, because it gets enough from quiet wakefulness. Smaller animals spend most of their waking active and may need sleep to get rest. However, this does not apply to the cerebrum, which is constantly active during wakefulness; it may need sleep to rest and rejuvenate. Horne also points out that extra brain stimulation during waking increases SWS but has no effect on REMS. Even in animal experiments that show increased RELMS following learning something new during the day, there is also an increase in NREMS and total sleep time.

Cerebral metabolic rate is high during infancy and childhood when SWS is also high and there is more brain organization and information processing occurring. Horne (1992) seems to argue that SWS in humans is most important for the functioning of the prefrontal part of the cortex, since sleep deprivation results in reversible deficits in functions typically associated with this area of the brain. Also, people with psychological disorders known to involve the frontal cortex have less SWS. In short, SWS is a kind of off-line maintenance for the brain.

There is some evidence that NREMS is important for memory. McNaughton's research (c.f. Wilson & McNaughton, 1994) showed that hippocampus cells that are active during a new experience when awake are also active during subsequent NREMS. However, this activity could merely be a carryover of activity from waking. To this point, there is only a little evidence that such reactivation has any subsequent consequences.

Born's group (e.g. Plihal & Born, 1999) has reported that sleep early in the night, that is dominated by NREMS, is important for declarative memory - such as paired associate learning or mental spatial rotation - but not for procedural memory. Other research results suggest that just the SWS component of NREMS is important for declarative memory. Human SWS may also be involved with human non-declarative tasks, but it is not clear if it is exclusive to SWS. Details of brain mechanisms that underlie memory consolidation in SWS are reviewed by Sejnowski & Destexhe (2000).

Other studies have shown that NREMS, like REMS, may increase after positive reinforcement conditioning. For example, rats that failed a two-way avoidance learning task showed improvement on day 2 in proportion to the duration of their NREMS episodes. In humans, it has been observed that intense maze learning and learning a virtual environment both increased sleep spindles and time in stage 2 during subsequent sleep.



Conservation

Earlier the hypothesis that all sleep has a conservation function was discussed. Some researchers maintain that only NREMS or only SWS serves that function, not all of sleep. Specifically they posit that NREMS functions to conserve energy by reducing metabolic rate, energy expenditure, and temperature.

In support of metabolic conservation, they cite as supporting evidence the fact that several studies in humans and other mammals show a high correlation between metabolic rate and amount of NREMS suggesting that as metabolism increases, so does the need to conserve. However, critics say that these data are confounded by equally high correlations of these factors with body size and feeding habits. In altricial species, i.e., those born relatively immature, the development of metabolic rate and SWS parallel each other. Also, some animals increase SWS during times of fasting caused by the reduced availability of food.

A temperature regulation function of NREMS is supported by the fact that:

• NREMS is a time of regulated, controlled, active cooling of the body by a decrease in heat production coupled by changes in mechanisms that allow increased loss of body heat. This process is in contrast to REM, which is a time of uncontrolled body temperature regulation.
• One theory sees the development of NREMS occurring during evolution about the time that warm-blooded animals evolved. This development is not viewed as a coincidence, but rather a necessity to prevent negative effects resulting from being too warm for too long.
• It has been argued that SWS is the first stage on a continuum toward hibernation. While hibernation conserves maximal amounts of energy by maintaining minimal metabolic levels, SWS does so, too, only to a lesser extent.
• The recent work of Rechtschaffen and colleagues at the University of Chicago shows the major effects of prolonged sleep deprivation in rats, which is eventually lethal, to be a disruption of energy metabolism and temperature regulation. However, these conclusions are for all of sleep, not just for NREM.
• Heating the body just a degree or two increases the amount of subsequent SWS. It is as if the body is using the cooling that accompanies SWS to balance off the increased heating of the body when awake, in an effort to maintain a constant daily average body temperature.
• Heating the basal forebrain area, which includes portions of the nearby anterior hypothalamus, increases SWS. This mechanism may regulate average daily body temperature just mentioned above.
• During free-running conditions, SWS occurs during the peak of the circadian body temperature rhythm. Extended sleep of 12 hours or more often includes the return of some SWS, when circadian body temperature is again on the rise.
• To be sure, some of the decrease in body temperature during NREMS is due to its typical co-occurrence with the low of the circadian body temperature rhythm and to the typical recumbent, reduced activity position typically assumed during sleep. However, a significant part of the decrease is due to NREMS sleep itself and occurs whenever NREMS sleep happens.

Other

See also the discussion of how NREMS is thought to prepare for REMS presented later. Slow waves indicate that the brain is not processing informational input that is complex (Meddis, 1977). Also, it is most difficult to awaken humans or animals from SWS and elicit behavioral responses. SWS is a time of enforced behavioral inactivity.

Stage 2

Little attention has been paid to attributing function to stage 2 sleep (called "light quiet sleep" in cats and some primates, probably because it is harder to manipulate... However, Meddis (1977) has speculated, more than concluded from data, about the functions of this stage of sleep.

Light quiet sleep, or stage 2, is seen only in cats and primates, thus it is most likely of more recent evolutionary development. In cats, it is similar to SWS except for less amplitude of the slow waves. This fact accounts for the lower behavioral thresholds of light quiet sleep. In primates, K-complexes are a kind of isolated slow wave with a characteristic shape. Spindles and K-complexes have been observed to occur in response to stimuli and might serve to jam out stimuli not important enough to need to attend to or to cause arousal.

If these things are true, then it follows that stage 2 sleep is a more advanced way of maintaining the behavioral quiescence of SWS while simultaneously sustaining a higher level of selective vigilance. Indeed, those primates who are more vulnerable while asleep have more light quiet sleep, which would keep them more vigilant.



Conclusion

As a way of summing up this post, I would like to present some of my views, especially those on the author’s question on sleep. My position views sleep as a process which evolved to aid us to adapt our behaviour to an environment of eons ago. The sleep of Babylon is the sleep of today. For those times and places it functioned effectively as a biological system. But, modern times have brought the Edison Age of electric lights and is abolishing the natural rhythm of night and day, the jet aircraft tosses sleep across multiple time zones, and drugs have given promises of bending sleep to our momentary demands. Pervasively, we raise our strident cries and push our self-centered demands that sleep be subservient to our whimsy, bend to our needs, pressures and terrors. We ominously move toward viewing our failures of sleep to be “illnesses” to be "cured."

My view point is to the contrary. In a reasonably natural and stable environment sleep will serve its function as a silent and well-trained servant. It IS rather our “misbehaviors” in relation to sleep, goaded by a changed environment and a thoroughly anthropomorphic arrogance about "nature", which “fails” sleep as it is pushed beyond its natural limits. From my perspective, we must rather than learn the proximal causes of sleep, learn the laws of sleep. In turn we must reach ourselves to act in accord with these laws. I agree with Francis Bacon of 500 years ago: "Nature cannot be commanded except by being obeyed."

Thank you.

References:

✈ Benington, J. H., & Heller, H. C. (1995). Restoration of brain energy metabolism as a function of sleep. Prog. Neurobiology. 45. 347.

✈ Born, J., Hansen, K., Marshall, L., Molle, M., & Fehm, H. L. (1999). Timing the end of nocturnal sleep. Nature. 39, 29-30.

✈ Harrison. Y., & Horne, J. A. (1995). Should we be taking more sleep? Sleep. 18, 901-7.

✈ Kavanau, J. L. (2001). Cornmentary: Brain-processing limitations and selective pressures for sleep, fish schooling and avian flocking. Animal Behavior: 62, 1219-1224.

✈ Kreuger, J. M. & Obal, F. J. (2002). Functions of sleep. In T. L. Lee-Chiong, M. J. Satela, & M. A. Carskadon (Eds.), Sleep Medicine (pp. 23-30). Philadelphia: Hanley & Belfus, Inc.

✈ Lee, K. A, McEnany, G., & Zaffke, M. E. (2000). REM sleep and mood state in childbearing women: Sleepy or Weepy? Sleep. 73, 877-895.

✈ Meddis, R. (1979). The evolution and function of sleep. In D. A. Oakley & H. C. Plotkin (Eds.), Brain, Behavior and Evolution (pp. 99-125). London: Methuen.

✈ Rechtschaffen, A. (1978). The Single-Mindedness and Isolation of Dreams. Sleep. 1, 97-109.

✈ Webb, W. B. (1975). Sleep: The Gentler Tyrant. Englewood Cliffs, NJ: Prentice-Hall.
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