cumnulative means u can replace back ur sleeping time tht u lost...eventhough u sleep 8 hours anytime u like,but the time the body heals is only from 11pm-3am..other times is just to get back some energy..

I've always wondered on the authenticity of this claim.
How does the body know what time it is anyway? What if I'm in a different time zone?

You body doesn't know the time...

BUT,
it is still able approximate the time. I can't be sure of the mechanism, but it's something related to the exposure to light and how it affects our biological clock.

It's not the exact time of 11pm-3am that matters. It's the time of non-exposure to intense light, and how long you are into the sleep that can affect your sleep and its cycle.

(This worked well, before the creation of electricity & light bulb, during the stone age =).

As for this idea: the first half of the sleep is for repairing, and the next half is for regaining energy.<-- I have no idea if it's correct or not.

references:
http://healthysleep.med.harvard.edu/health...xternal-factors

http://www.websciences.org/cftemplate/NAPS...cfm?ID=20044171