Is it possible to operate on only 90 minutes of sleep a day? Polyphasic Sleeping involves taking multiple naps per day, of about 30 minutes in length, instead of one long 8 hour sleep period.
The theory is your body adapts to the shorter sleep cycles and ensures REM sleep kicks in earlier. It's the REM period of sleep that is the important part to avoid sleep deprivation. So your 30 minute nap is almost entirely REM sleep. With normal sleep patterns you still only get the same amount of REM sleep and the rest of the sleep time is effectively wasted.
Steve Pavlina tried the polyphasic sleeping idea and blogged about it. It's a very interesting read and he's apparently decided to continue with it indefinitely.
Reading through his blog entries he seemed to find the first week the hardest, dealing with 'foggy thinking', and fatigue. But once over the worst of it his energy levels improved and he added many extra hours to his day. His whole concept of time changed.
Link from Keith Devens pointing to Steve's 'early riser' posts.