Sleep Architecture defines the amount of each phase of sleep occurs during a night's sleep. In the first months of life, approximately one third of each day is spent awake, and the remainder is approximately equally divided between REM sleep and Non-REM sleep. At the age of 10, REM sleep accounts for 25% of sleep, and in adults it falls to ~20%. In old age the amount of time spent sleeping decreases further.
Hypnotic drugs generally cause a decrease in the latency to onset of sleep (time to fall asleep), an increase in the duration of Stage 2 (at the expense of Stage 4, deep sleep), and the duration of REM sleep is decreased. This deprivation of REM sleep seems to have no behavioural effects, and there is a rebound - an excessive increase in REM sleep after the hypnotic is withdrawn. Benzodiazepines, alcohol, barbiturates and MonoAmine Oxidase inhibitors all reduce the amount of Stage 4 sleep and REM sleep.
Deprivation of Slow Wave (Stage 4 ) Sleep causes a marked change in behaviour, and when sleep is again possible, the amount of Stage 4 sleep increases and the subject sleeps for long periods.
In sleep apnoea, there is a delay in taking a breath, and the failure to start a new breath causes a number of reflex physiological changes; the EEG also shows a sudden arousal pattern, and becaus of the nature of breathing these events can occur ~500 times a night. Sleep Apneoa is common in the elderly, but can occur in all age groups, and deep slow wave sleep (Stages 3 and 4)may never be achieved, because the development of these stages is interrupted by repeated arousals.
Narcolepsy is a chronic condition in which irresistible sleep attacks lasting 5-30 minutes occur during the day. These naps are accompanied by a sudden overall loss of muscle tone, quickly followed by a period of REM sleep that lasts about 10-20 minutes. Night-time sleep may also be disrupted. These episodes of REM sleep are accompanied by vivid dreams.
Sleepwalking usually occurs during Stage 3 or Stage 4 of sleep: the patient sits up, gets out of bed, and walks around with eyes open and blank expression, but without any EEG signs of arousal, and often does not remember the episode. Examples include getting out of bed to go to the toilet in the middle of the night.