By Dr Jesus Milan Perez [email protected]
Dr Jesus Milan Perez is a specialist in Neurophysiology working exclusively in the Hospital Particular do Algarve
Sleep is characterised by reduced or absent consciousness and the inactivity of nearly all voluntary muscles. During sleep, growth and rejuvenation of the immune, nervous, skeletal and muscular systems takes place.
Sleep is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features.
The various stages of sleep are assessed by polysomnography in a specialised sleep laboratory.
Measurements taken include EEG of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average.
There is relatively little dreaming in NREM, which is divided into three stages, N1, N2 and N3. The first stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep.
Some people may also experience hypnologic hallucinations during this stage. The subject loses some muscle tone and conscious awareness of the external environment.
Stage N2 is characterised by sleep spindles.
During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45-55% of total sleep in adults.
Stage N3 (deep or slow-wave sleep) is when parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur.
Optimal amount of sleep in humans
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual’s circadian rhythms.
A person’s major sleep episode is relatively inefficient and inadequate when it occurs at the “wrong” time of day. One should be asleep at least six hours before the lowest body temperature.
Hours by age
Children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages.
A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM.
Age and condition
Average amount of sleep per day:
Newborn: up to 18 hours
1 – 12 months: 14-18 hours
1 -3 years: 12-15 hours
3-5 years: 11-13 hours
5-12 years: 9-11 hours
Adolescents: 9-10 hours
Adults, including elderly: 7-8 hours
Pregnant women: 8 (+) hours
Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional and physical fatigue.
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiologic and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.
Scientists do not agree on how much sleep debt it is possible to accumulate. It is likely that children are sleeping less than previously in Western societies.
Rapid eye movement sleep, or REM sleep, accounts for 20-25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage.
Sleep timing is controlled by the circadian clock. The circadian clock, an inner timekeeping, temperature-fluctuating, enzyme-controlling device, works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness.
Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. The timing is affected by one’s chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.
A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.