Many people sleep 8 hours per day — that means you have 16 hours left in a day. Assuming about an hour/day is non-productive (getting ready to going to bed, waking up, etc), you have only 15 hours of productivity in your day.
If a person gets only 6 hours of sleep, she has an extra two hours of productivity per day … or 13.33% more time. Over a year, that translates to an additional 49 days! Imagine having an extra 49 days to do whatever you want … or start whatever projects you want.
Over a decade it is 490 days — or an additional 1.34 years! That’s an incredible amount of time one spends sleeping an additional amount. For some, those extra two hours of sleep are essential. For others, it is only a crutch and can be slowly worked into one’s rest patterns.
I remember reading a US Navy paper that explained how the sleep cycle works and why they sometimes do shifts where sailors only sleep for no more than 90-120 minutes at a time, with three of four sessions spread throughout the day. Apparently any more or less than that and there is the risk of waking up during an REM cycle, which causes intense drowsiness. I generally try to take 90 minutes naps in the afternoons and then only run on 4 hours of sleep during the night time (just about 2 complete cycles).
Then again, sometimes I fall asleep on the train and really screw myself when I wake up two stops past my office. So much for theories!