Maybe you've met a few people who seem to be able to function completely normally on just 4 hours of sleep a night. Those people, sometimes called the 'sleepless elite
' who make up about 1-3% of the population, may have a genetic inclination towards not requiring as much sleep as the rest of us. Short sleepers can get away without 8 hours of sleep a night, but most people can't. Here are just a few links on the importance of sleep.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post