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Disruption of the body’s internal clock can result from short-term disturbances such as traveling across time zones (jet lag) or from a longer-term perturbation such as working odd hours or a constantly changing schedule. In some cases, the disturbance can be due to a genetic disorder which impairs an individual’s ability to synchronize with the outside world. Usually a disruption of the circadian system results in deficits in alertness and difficulty sleeping and can contribute to overall fatigue. The study how circadian rhythms function is known as chronobiology, and it is a fairly young field young field. It is only within the last twenty years that scientists have come to understand how the human body "keeps time." The potential for health improvements garnered from greater elucidation of the circadian system is to be tremendous. Pharmaceutical drugs capable of controlling medication release in the body based on the time of day, or even drugs that can change the body's clock itself, may soon be developed. Sleep The function of sleep is not fully understood but it is generally accepted that it allows the body to recover from previous wakefulness and prepare for functioning during the subsequent waking period. Unlike circadian rhythms, the exact location of the brain’s sleep centers is still not fully known. Sleep is believed to be controlled through a diffuse system of neural networks located throughout the brain. During sleep, the brain cycles between a deep, synchronized state of sleep and lighter, more active stage known as Rapid Eye Movement(REM) sleep. Brainwave Changes During Sleep As a person falls asleep, brainwave patterns as measured via an electroencephalograph (EEG) undergo marked changes. The first noticeable change is a slowing of the frequency of the waves as a person moves from wakefulness (left) to drowsiness (right):
During stage 1 sleep brainwaves continue to decrease their frequency and the person becomes more difficult to arouse:
In stage 2 sleep, higher amplitude waveforms appear along with characteristric structures known as spindles and K-complexes:
Stage 3 sleep is characterized by lower frequency and higher amplitude waveforms:
Stage 4 sleep consists mostly of very low frequency, high amplitude waveforms. Sensory information from the outside world is decreased and it is very difficult to arouse a person in stage 4 sleep:
Rapid Eye Movement (REM) sleep, also known as "paradoxical sleep," is a lighter stage of sleep characterized by rapid bursts of eye movement. In this stage the brainwaves more closely resemble waking. Heart rate and respiration become irregular (also similar to waking). Most muscles are paralyzed during this stage and a person’s response to outside sensory stimuli is generally more robust. Much of dreaming is thought to take place in REM sleep:
Over the course of a night a typical person will cycle through these stages about three to five times:
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