Sleep Architecture defines the amount of each phase that 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seems to have no behavioural effects, and there is a rebound - an excessive increase in REM sleep after the hypnotic is withdrawn.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, the failure to start a new breath causes the EEG to show a sudden arousal, which can happen ~500 times a night. It is common in the elderly, but can occur in all age groups, and slow wave sleep may never be achieved, because the development of Stage 4 sleep is interrupted by repeated arousals.
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.
Narcolepsy is a chronic condition in which irresistible sleep attacks lasting 5-30 minutes occur during the day. Sleep onset is accompanied by a sudden overall loss of muscle tone, quickly followed by a period of REM sleep that lasts about 10-20 minutes.
Image source: intranet.tdmu.edu.ua
The diagram shows the time spent per day in REM and non-REM sleep, plotted against age. REM sleep dominates in premature infants and gradually decreases with age.
Sleep Architecture and the effects of Age and Maturity
Sleep occurs in 4 stages that can be defined by the EEG. Stge 4 is the deepest form of sleep and is associated with slow (delta) waves in the EEG.
One other variety of sleep is Rapid Eye Movement (REM) sleep which occurs several times during a night's sleep.
The proportion of the different types of slee change with age: REM sleep in 10 week premature infants can account for 80% of sleep and reduced to ~65% by 2-4 weeks of prematurity.
REM sleep falls to 30-35% of the sleeping time by the age of 2
At the age of 10, REM sleep accounts for ~25% of sleep, and in adults the % falls to ~20%.
The amount of time spent sleeping decreases with age in adults
Image source: Häggström, Mikael. Wikiversity Journal of Medicine 1
The effects of sleep deprivation include brain function, stress, endocrine and body weight changes.
Enforced deprivation of sleep has severe consequences, including fatigue, daytime sleepiness, and impaired cognitive function. In addition there may be changes in body weight. it is often caused by bad sleeping habits and sometimes by sleep apnoea.
Deep sleep is associated with Stage 4 slow wave activity in the EEG, and Stage 4 architecture is markedly affected by sleep deprivation.
After a period of sleep deprivation (of slow wave sleep) then there is a rebound effect: the ssubject sleeps for long periods, and attempts to make up the backlog, and the time spent in stage 4 sleep increases.
During sleep deprivation, the percentage of REM sleep is much reduced. However deprivation of REM sleep alone is not accompanied by the same serious consequences.
Deprivation of REM sleep occurs in patients on some drug regimes without effects on stage 4 sleep and is not associated with the behavioral changes characteristic of Stage 4 Sleep Deprivation.
Image source: Kandel and Schwartz: Neuroscience
Deprivation of REM Sleep
Deprivation of REM sleep without any changes in Stage 4 sleep can occur in people on benzodiazapines, and is not associated with the behavioral changes characteristic of Stage 4 Sleep Deprivation.
While it is clear that a major deprivation of REM sleep can occur without gross behavioural changes, there may be some osychological changes, such as an increase in anxiety and irritability.
Deprivation of REM sleep is followed by a rebound effect: there is an excessive increase in REM sleep in the days following the period of deprivation.
Image source: Kandel and Schwartz : Neuroscience
Effects of Drugs on Sleep Architecture
Hypnotic drugs cause drowsiness and are generally associated with changes in sleep architecture.
These changes include:
a decrease in the latency to onset of sleep (time to fall asleep)
an increase in the duration of stage 2 sleep
a reduction in the duration of stage 4 sleep
a reduction in the duration of REM sleep.
This raises the question as to whether hypnotics ever actually produce a ‘good’ deep sleep.
Despite the reduction in slow wave sleep, hypnotics do not produce changes in the secretion of adrenal or pituitary hormones associated with sleep deprivation.
Image source: www.physionet.org
The recording shows changes in the EEG, heart rate and blood pressure during the intermittent inspiration associated with sleep apnoea. In this trace the patient only achieves air intake to the lungs twice in a minute.
Apnoea causes sudden arousal shown by the EEG recordings, and can occur at ~ 500 times a night. Note the slow breathing and the EEG that follows each breath
Sleep apnoea is a failure to take a breath at the normal time (apnoea) because of either a failure of the central neural mechanisms controlling the respiratory muscles, or an obstruction to the respiratory tract causing a failure of air entry into the lungs.
The failure to take air into the lungs causes changes in blood gases and the circulation to the brain, and are associated with changes in the EEG.
During the long period between breaths, there are signs of arousal in the EEG; this can occur ~ 500 times a night.
Sleep Apnoea is common in the elderly, but can occur in all age groups. Slow wave sleep may be absent, largely because apnoea gives rise to arousal in the EEG.
Image source: thebrain.mcgill.ca
The top diagram shows the sleep architecture of a normal person. In a patient with narcolepsy, periods of REM sleep occur suddenly during the day, and there is some disturbance of the nocturnal sleep pattern.
Narcolepsy is a rare chronic neurological condition that affects approximately 0.05% of the population. People with narcolepsy experience frequent bouts of an excessive urge to sleep during the day; this level of sleepiness is comparable to that felt by subjects deprived of sleep for 24 to 48 hours. In addition the amount of nocturnal sleep may be reduced and nocturnal sleep may be disturbed, and sometimes confused with insomnia.
Narcolepsy consists of irresistible sleep attacks during the day; the onset of sleep is accompanied by a loss of muscle tone generally, followed by REM sleep. During the REM sleep, the subject may experience vivid dreams.
At the start of a narcoleptic 'nap', REM sleep happens immediately, and lasts 5-30 minutes during the day.
EEG findings show that REM sleep usually occurs within minutes of falling asleep, unlike normal subjects who do not show REM sleep until at least an hour of deep sleep has occurred.
The discovery of the orexins (sometimes called hypocretins) in 1998 and studies on this system in animals and humans has advanced our knowledge of narcolepsy or similar coniditions characterised by episodes of excessive sleepiness, excessive REM sleep and reduced muscle tone.
Humans who do not produce orexins also have narcolepsy; also narcolepsy in humans is associated with mutations of the human leukocyte antigen (HLA) complex.
Some breeds of dogs have a canine equivalent of narcolepsy which is characterised by a mutation in orexin receptors. Mice lacking the orexin gene also show signs of nacrolepsy.
It has been suggested that in patients with narcolepsy, orexin producing neurones are damaged, possibly by the autoimmune system, leading to a lack of orexin.