Here's a hypothesis on why we sleep.

Sleep has two parts to it, a deep NREM, no rapid eye movement, sleep mostly
during the first part of the night's sleep, then REM sleep in and out during
most of the rest of the nights sleep.

I suggest that deep sleep or NREM, starts a period of anabolic build up.
For example:

"A full 90%% of your daily supply of growth hormone is released... not when
you work out... not when you do cardio training.., in fact, not even while
you're awake. The fact is, 90%% of your growth hormone supply is released
while you sleep -- not only that, but during the first four hours of sleep!"

And that the later lighter sleep periods or REM part of sleep is mostly a
catabolic period that uses the time to break down and prepare wastes for
excretion.

There is little proof for my idea at this time, but thoughts are welcome.

Tom Hendricks
http://www.daviddarling.info/encyclopedia/U/UV_origin_of_life.html