Insomnia comes from the Latin words for “no sleep.” Insomnia is characterized by:
Insomnia may be primary or secondary:
Insomnia is often categorized by how long it lasts:
Insomnia may also be defined in terms of inability to sleep at conventional times. The following examples are referred to as circadian rhythm disorders:
In sleep studies, subjects spend about one-third of their time asleep, suggesting that most people need about 8 hours of sleep each day. Individual adults differ in the amount of sleep they need to feel well rested, however. (Infants may sleep as many as 16 hours a day.)
The daily cycle of life, which includes sleeping and waking, is called a circadian (meaning "about a day") rhythm, commonly referred to as the biologic clock. Hundreds of bodily functions follow biologic clocks, but sleeping and waking comprise the most prominent circadian rhythm. The sleeping and waking cycle is about 24 hours. (If confined to windowless apartments, with no clocks or other time cues, sleeping and waking as their bodies dictate, humans typically live on slightly longer than 24-hour cycles.) It usually takes the following daily patterns:
In addition, daily rhythms intermesh with other factors that may interfere or change individual patterns:
The response to light signals in the brain is an important key factor in sleep:
Sleep consists of two distinct states that alternate in cycles and reflect differing levels of brain nerve cell activity:
Non-Rapid Eye Movement Sleep (NonREM). NonREM sleep is also termed quiet sleep. NonREM is further subdivided into three stages of progression:
With each descending stage, awakening becomes more difficult. It is not known what governs NonREM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones, may be important for deep sleep.
Rapid Eye-Movement Sleep (REM). REM sleep is termed active sleep. Most vivid dreams occur in REM sleep. REM-sleep brain activity is comparable to that in waking, but the muscles are virtually immobilized, possibly preventing people from acting out their dreams. In fact, except for vital organs like lungs and heart, the only muscles not immobilized during REM are the eye muscles. REM sleep may be critical for learning and for day-to-day mood regulation. When people are sleep-deprived, their brains must work harder than when they are well rested.
The REM/NREM Cycle. The cycle between quiet (nonREM) and active (REM) sleep generally follows this pattern:
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