Sleep is a physiological process characterized by unconsciousness, uncontrolled muscle movement, lack of muscle rigidity and rapid eye movement behind closed eyelids. It is a normal part of human behavior and is also observed in all vertebrates and arthropods although its exact nature may change from species to species.

Most humans have a normal sleep period of eight hours of nighttime sleep for every twenty-four hour period. Too little sleep results in fatigue, irritability and inattention to routine along with physical symptoms such as dark circles below the eyes.

Many diseases can affect sleep patterns. Most infections result in a greater need for sleep, but some rare infections may cause hypersomnia - very lengthy sleep periods. Other disorders may cause insomnia or disturbed sleep. Moreover, individuals who do not sleep regularly, such as those who work at night, shift workers or airline crews, often suffer secondary symptoms from their lack of regular sleep patterns no matter how much sleep they actually get.

Sleep can be disturbed by noise, intense light, pain and excessive heat or cold. This differs it from other forms of unconsciousness, such as those that result from sedatives, where an individual remains unresponsive to these stimuli. Some disorders result in disruption of sleep even when the patient does not realize they are constantly being woken, such as sleep apnea.

Normal sleep constitutes moving back and forth through four phases from light to deep, as well as experiencing REM sleep where dreaming occurs. At birth, babies tend to sleep for very long periods each day, but not regularly, and easily move from light to deep sleep. The need for sleep decreases until puberty, when humans tend to want to sleep later and stay up later - this accommodates rapid growth which generally takes place during sleep. However, after puberty, humans tend to sleep more lightly and do not enter deep sleep as easily. This trend gets worse as people age.

Once sleep patterns are regularly established, they are very hard to change. For example, night shift workers who have worked that way for years may find after leaving those jobs they cannot sleep through the night.

On House Edit

In Paternity, the patient suffers from night terrors and also exhibits a myoclonic jerk which is typical of people entering light sleep even though he is wide awake.

In Sleeping Dogs Lie, the patient suffers from intractable insomnia and has periods of microsleep.

In Fidelity, the patient suffers from hypersomnia.

Sleep on Wikipedia

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