For decades doctors have stood by the advice that 8 hours of sleep are needed to properly refresh the body. Now a growing body of evidence is suggesting that natural rhythms may have once caused humans to sleep for four hours, awake for one or two hours, and then sleep again for about four hours. Historical records suggest that the introduction of nighttime lighting reduced the amount of sleep that people got in the evening and pushed the average bedtime further into the night.
Read more details about The Myth of the Eight-Hour Sleep on the Dr. Mercola blog. You can also read more about the stages of sleep shown above at HowStuffWorks.
