Research has been done with people's circadian rhythms. During one experiment, the subjects lived in an artificial environment which excluded all clues as to what time of day it was, including daylight and clocks. The subjects developed a rhythm of 25 hour days rather than familiar 24 hour days. However, the difference of one hour is not regarded large enough to cause great concern. There are biological rhythms not only within a day, but also within the waking and sleeping periods as well.
Sleep is an activity which consumes approximately one-third of the human life.
The brain's electrical activity varies during sleep. This fact was first observed by the physiologist Eugene Aserinsky in 1953, using an EEG machine which is a machine that can measure electrical activity in the brain. After he had fixed it he wanted to make sure it was working as it should. Not only did he find that his EEG machine was working fine, but that the registered activity varied in the course of the night. This proved to be an important and fundamental insight.