The Process
Sleep cycles through varying degrees of brain activity, consciousness and body movement. A sleep cycle consists of rapid eye movement (REM) sleep and three stages of non-REM sleep. Non-REM sleep is gradually diminishing brain activity and less responsiveness to external stimuli resulting in decreased body movement. Stage three of non-REM sleep is deep sleep where the body and mind are at rest. During REM sleep, brain activity increases, responsiveness to external stimuli is suppressed, and body movement is actively inhibited. REM sleep is also referred to as paradoxical sleep because brain activity in this stage resembles that of an aware state, yet it is difficult to wake. It is a restorative sleep; the brain activity and lucid dreams are necessary for normal function and behavior.
Mechanism
The passive inhibition of movement during non-REM sleep is called disfacilitation. The active inhibition of movement during REM sleep is called REM atony. The physiological mechanisms facilitating disfacilitation and REM atony are not completely understood. What is known is that disfacilitation differs from REM atony. While disfacilitation is related to decreased brain activity, REM atony is a complex and mysterious response to chemical and electrical connections in brain cells. REM atony is a highly selective mechanism suppressing most motor movement while allowing variable heartbeat, breathing, sexual arousal and eye movement (hence REM's namesake). REM atony is so specialized that while some visual areas are highly active, most of the visual cortex is inhibited. This gives rise to the confused nature of dreams.
Significance
The brain needs whatever process occur during sleep much like a personal computer needs to be rebooted regularly to maintain optimum function and processing capacity. While rebooting, a computer cannot print a document or download music. Likewise, REM atony--and to a lesser degree disfacilitation--prevent response to dreams, such as moving the arms and legs in a falling dream. Sleep is not essential to the normal function of other organs and systems. Sleep in correct proportions of deep rest and restorative REM is essential to the normal function of the brain and central nervous system (CNS). Furthermore, REM without atony would be physically demanding if not dangerous. This is exemplified by a night of "tossing and turning," when REM sleep is minimal and non-REM is not sufficiently disfacilitated. The result is fatigue, irritability, slowed reaction times and other behavior changes associated with a bad night's rest. Movement stops while sleeping so the CNS can safely maintain normal function and behavior.
References
- Psychology; Lester A Lefton Phd; 2000
- Fundamentals of Anatomy and physiology; Frederic H Martini Phd; 2004
- Nature; Sleep is of the brain; J. Allan Hobson; Oct 2005
