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Sleep is a naturally occurring state of unconsciousness where the response to external stimuli is decreased, but the subject can usually be readily aroused. There are two patterns of sleep.

Non rapid eye movement sleep
This is divided into 4 stages according to EEG activity.

Rapid eye movement sleep
Rapid irregular low amplitude waves occur, this is when dreaming takes place. The eyes make rapid movements associated with tachycardia, tachypnoea and skeletal muscle relaxation.
Typically an adult passes rapidly through Stages 1 and 2, spending approximately 60-90 minutes in stages 3 and 4. This is followed by a 60-90 minute period of REM sleep. This cycle repeats until waking. On average, ICU patients sleep only 2 hours per day, and less than 6% of their sleep is REM sleep.

Sleep disruption on the ICU
Critically ill patients often do not sleep well. Their sleep may be highly fragmented and distributed throughout the day and night, and there is a reduction in slow wave and REM sleep. Reasons for poor sleep on the ICU include:

Studieshave reported that sleep disruption/deprivation may result in impaired cognition, irritability, decreased situational awareness, delirium, loss of concentration, immune function alterations (decrease NK cell and lymphocyte function), negative nitrogen balance, decreased thermoregulation and failure to wean from mechanical ventilation due to its effect on pulmonary mechanics and respiratory muscles.

Prevention of sleep disruption/deprivation
Sleep on the ICU can be facilitated by minimising noise/stimuli, reducing lighting, decreasing unnecessary interventions at night, and provisions of stimulating activities for the patients during the daytime. Use of ear-plugs has also been demonstrated to be helpful. Melatonin at night may help promote sleep without residual hangover psychomotor effects.

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