Some alterations in conscious functions are induced by daily changes in biological rhythms. Bodily events that occur in roughly a twenty-four-hour cycle are referred to as circadian rhythms, from the Latin circa ("about") and dies ("day"). It is thought that these cycles are created by natural events, such as the light-dark cycle, and by other cues in the daily routine, such as mealtimes. The sleeping-waking cycle is the major circadian rhythm, but there are others, such as fluctuations in body temperature. This daily temperature cycle appears to be directly related to levels of mental activity. When all external cues are removed, circadian rhythms extend to about twenty-five hours. As a result of prolonged isolation, the cycle can become completely distorted, with periods of up to forty hours of waking followed by periods of up to twenty-two hours of sleep. When the change is gradual in this way, the individual has a distorted sense of time and believes that he or she is experiencing normal periods of sleep and waking. Abrupt changes in circadian rhythms, as when one crosses several time zones, are what lead to that sleepy, uncomfortable feeling known as jet lag.
In addition to biological rhythms, there are other regular daily variations in consciousness. On the way to sleep each night, people enter a kind of "twilight" period known as the hypnagogic state. The state of consciousness that is entered immediately before waking is called the hypnopompic state. In both these states, one is partially asleep and partially continuing to process environmental stimuli. Both are characterized by vivid imagery, and many people have reported creative insight during these periods.
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