Definitions

Sleep occupies approximately one third of the adult life. Although the function of sleep is not understood, the inherent necessity for sleep is widely recognized and is present in almost all mammals. The notion that sleep is a time of rest and brain inactivity persisted until the 20th century when electrophysiological techniques were applied to the study of sleep. Thereafter sleep was found to be a dynamic process, with the cyclical recurrence of different stages. The discovery of rapid eye movement (REM) sleep by Aserinsky and Kleitman in 1953[1i was a major advance in sleep research, stimulating physiological and clinical studies of sleep. Sleep is divided into two types: REM and non-rapid eye movement sleep (NREM). y

REM sleep cycles on and off throughout the night. In a young adult, the first REM period occurs approximately 90 to 100 minutes after sleep onset, following an initial period of NREM sleep. The first REM period is the shortest of the night. Successive REM periods are of progressively longer duration, with the longest REM period occurring in the early morning hours. In infancy, 50 to 80 percent of total sleep time consists of REM sleep. By age 2 years and through adulthood, the percentage of REM sleep falls to approximately 20 to 25 percent of total sleep time.

REM sleep is divided into tonic and phasic events, and has several characteristic features. The tonic REM events include (1) cortical desynchronization, with mixed frequency, fast activity on electroencephalography (EEG), similar to that observed in the waking state; (2) hippocampal synchronous theta activity that also occurs in waking; and (3) muscle atonia that is present in all but respiratory and ocular muscles, and marked by a reduction in chin and limb electromyographic activity. The phasic components of REM sleep include the following features: (1) rapid eye movements, horizontal and vertical, occurring in bursts during REM; (2) muscle twitches punctuating the muscle atonia; (3) ponto-geniculo-occipital spikes (PGO) not observable on routine polysomnography (PSG); and (4) autonomic nervous system lability with fluctuations in respiratory rate, heart rate, and blood pressure. [3]

NREM sleep is divided into four stages: Stages 1 and 2 are considered light sleep. Stage 1 occupies 2 to 5 percent of sleep time and is marked by slow rolling eye movements, low voltage, mixed frequency EEG, and a low arousal threshold. Stage 2 makes up approximately 45 to 55 percent of total sleep time and is marked by the presence of K complexes and sleep spindles on EEG recordings. K complexes are composed of an initial sharp component followed by a slow component. Sleep spindles are episodically occurring rhythmical complexes occurring with frequency of 7 to 14 cycles per second grouped in sequences lasting 1 to 2 seconds. Spindles can occur alone or superimposed on K complexes. Although spindles are a feature of Stage 2 sleep, they may also be present in Stage 3 and Stage 4 NREM sleep, and can even be found in REM sleep.

Stages 3 and 4 sleep are considered deep or slow-wave sleep (SWS). SWS is prominent in youth and diminishes in the elderly. SWS is present for approximately 10 to 20 percent of sleep time and is predominate in the first part of the night. The arousal threshold is high in SWS. The EEG feature of SWS is the delta wave. Delta waves are high voltage (75 microvolts or more) waves with a frequency range of 0.5 to 4 Hz. Stage 3 sleep is defined as sleep consisting of 20 to 50 percent delta waves, and Stage 4 is defined as greater than 50 percent delta waves. [3] , [4

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Sleeping Solace

Sleeping Solace

How To Better Your Sleep For A Better Life. Understanding the importance of good sleeping habits is very beneficial to the overall health of an individual in both mental and physical levels. Learn all the tricks here.

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