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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.

  • Stage 1: occurs as the subject falls asleep, low amplitude high frequency activity on EEG
  • Stage 2: sleep spindles (alpha-like bursts of 10-14Hz) on EEG
  • Stage 3: frequency slows and amplitude increased on EEG
  • Stage 4: represents deep sleep, with rhythmic slow waves on the EEG
  • Stages 3 and 4 represent a deeper sleep and are also known as delta or slow-wave sleep

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:

  • Pre-existing disease (eg. COPD with frequent arousal from hypoxia or hypercapnia)
  • Drugs eg. Benzodiazepines abolish stage 3 and 4 NREM sleep
  • Opioid analgesic drugs increase arousal frequency
  • Barbiturates and amphetamines inhibit REM sleep
  • Catecholamines increase wakefulness
  • Tricyclic antidepressants and serotonin reuptake inhibitors prolong slow wave sleep and block REM sleep
  • Anaesthesia and surgery (ie. the stress response to surgery, fever, pain, starvation and age decrease stages 3 and 4 NREM sleep)
  • Environmental factors (ie. noise, lighting, alarms, other patients, round the clock care)
  • Mechanical ventilation
  • Metabolic derangements
  • Sepsis
  • Fear and anxiety related to critical illness

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.