one thing ive read up on... if my memory serves me right... the average human body needs 8 hours (adult) of sleep to recharge... if you get say 6 hours of sleep on day 1, 6 on day 2... then by ur third day when u wake up you're effectively in the minus by 4hours... it accumulates
you all know the feeling, you can get your 6 hours of sleep on a sunday night and feel fine on monday... but if you keep it up.. by friday you're gona be sleepign at work and hating every moment of it... if you did that all week you'd be 10hours sleep deprived... the interesting fact is by that friday you wouldnt have to sleep a crap load to make up for it... youd just need that 8 hours the next night to bring you back to a refreshed point