Hello Koh Elaine.

I had to look this up to be certain.

"Lesser" is not wrong but it is very rare and seems unnatural to me.

The normal, common usage is "little", "less", "least".
She ate little food.
She had less than six hours sleep. (comparative)
She had the least sleep of all of the prisoners. (superlative)

However, in some very formal usages, and in mathematics (statistics theory) "lesser" and "lesser than" are used as the comparative form.

Also some fairly common phrases use "lesser" -
"The lesser of two evils" (a situation in which both possibilities are bad, but one is worse than the other, you choose the lesser of two evils - the one which is "less bad")
When it has the meaning of "less valuable, honourable, lower quality, less important" - "He's a lesser man than his father was" - "The Father came neither first nor last and is neither greater, nor lesser, than the Son." [St Anselm - On the Trinity modern re-translation]

Common pairs:
More or less
Greater or lesser.

According to the figures I found, "lesser" is becoming slightly more popular.
In 1960 (sixty years ago or so) for each time "lesser than" was used in a published item, "less than" was used about 10,000 times.
In 2008 (about ten years ago) for each time "lesser than" was used in a published item, "less than" was used about 6,000 times.