Physiology Of Sleep
Sleep is a state of reversible unconsciousness in which the brain is low responsive to external stimuli. We are functionally blind during sleep with no response to visual stimuli and a decreased threshold of response to auditory stimuli. Babies have been exposed to sound of up to 100 dB, which is above the legal limit for ear protection for employees, without waking up. In adults, the action is selective demonstrating continuing cortical function. For example, a sleeping mother is woken by her crying baby but not by other louder noises.
Definition of sleep and sleeping
Phases with specific EEG patterns and physiological changes. Natural sleep is separated into two distinctive states: non rapid eye movement (N.R.E.M) and rapid eye movement (REM) sleep.
NREM sleep is then further seperated into 4 stages where stage 1 is the lightest and stage 4 the deepest level of sleep. REM sleep is divided into phasic and tonic phases. The two distinctive states follow a regular pattern called a sleep cycle which, in an adult, lasts about 1 and half hours and comprises a period of N.R.E.M sleep followed by REM sleep. The cycles may be separated by a period of wakefulness and are repeated 3–6 times each night and are typically displayed as an hypnogram (Fig. 1). The majority of deep (stage 4) NREM sleep occurs in the first and second cycles. As the night progresses, the proportion of REM sleep in a cycle increases and the NREM element is of lighter stage 2 sleep.
Age has a major effect on the duration of sleep and the ratio of NREM/REM sleep. Neonates sleep 16–18 h. It is widely distributed throughout the day with REM sleep accounting for 50% of total sleep time (TST). This may be even greater in premature babies. By the age of 24 months, children should sleep 10 h per day, mainly at night with one or two naps during the daytime and REM sleep has declined to 20–25% of TST. Adults normally sleep 6–8 h per day with 15–20% REM sleep. With increasing age, TST changes little although sleep is more fragmented with more frequent and longer awakenings (decreased sleep efficiency) with less REM sleep and more light NREM sleep. Night-time sleep may be decreased if naps are taken during the day.
Functions of sleep
The functions of sleep are still poorly understood. However, the observation that sleep (or, at least, an activity–inactivity cycle) is present in all species and has been preserved throughout evolution and that sleep deprivation leads to a drastic deterioration in cognitive function and eventually to mental and physical morbidity proves its importance.
It has been suggested that sleep might conserve energy by reducing core temperature slightly and lowering metabolic rate by 10% compared with quiet wakefulness. Sleep would prevent perpetual activity as a response to environmental stimuli leading to excessive energy consumption. However, sleep is a state of starvation and there is no evidence that sleep is important for tissue repair. Sleep has been implicated as an important factor in storage of long-term memory. Facts learned during the day are usually better remembered the next morning whereas facts learned shortly before going to sleep are often poorly recalled.
Electrophysiological features of sleep
The stages of sleep are characterised by typical patterns of electroencephalogram (EEG), electro-myogram (EMG) and electro oculogram (EOG) activity Wakefulness with open eyes is characterised by an EEG with dominant low amplitude, high frequency beta activity of 16–25 Hz. Muscle tone is normally high with high to moderate EMG activity.
Sleep is usually initiated by a transition from wakefulness to a state of drowsiness with closed eyes and a shift from EEG beta activity to alpha activity of 8–12 Hz passing to Stage 1 NREM sleep with a mixed frequency EEG-pattern with low amplitude theta waves of 3–7 Hz accompanied by slow rolling eye movements. Involuntary muscle clonus occurs frequently, resulting in jerky movement of the whole body (hypnic jerks) and EMG activity is moderate-to-low. This stage lasts typically only 5–10 min, during which time minor auditory stimuli will cause arousal.
Stage 2 is characterised by short bursts of high frequency activity (12–15 Hz – sleep spindles) and K-complexes (large amplitude biphasic waves). Bodily movements continue and the EMG activity is low-to-moderate. This stage is generally short (10–20 min) in the first 1–2 cycles but predominates in later cycles. It is the most abundant sleep stage in adults accounting for up to 50% of TST.
Deep NREM sleep stages 3 and 4, sometimes combined as slow wave sleep (SWS) are characterized by high amplitude low frequency delta waves (> 75µV and 0.5–2 Hz) with stage 3 having between 20–50% and stage 4 more than 50% delta activity. EMG activity is low and eye movements are rare. Arousal through auditory stimuli from this stage of sleep is difficult and, if awakened, the individual is often disorientated and slow to react. Return to sleep is easy and short arousals (< 30 sec) are rarely remembered.
NREM sleep is followed by REM sleep, the proportion increasing with each cycle. REM sleep is characterised by a fast mixed frequency low voltage EEG with saw-tooth waves and rapid eye movements on the EOG. During the tonic phases of REM sleep, there is marked reduction of muscle tone and EMGactivity in skeletal muscles. The tonic phases of REM sleep are interrupted by short episodes of phasic REM sleep with increased EMG activity and limb twitches. The atonia of REM sleep affects all skeletal muscles, except the diaphragm and the upper airway muscles, and is associated with hyperpolarisation of the α-motor neurones. The purpose of this may be to prevent the acting out of dreams. About 10% of the population have experienced sleep paralysis (i.e. wakening from sleep and finding that the atonia has persisted into wakefulness). It can be frightening but is entirely harmless. Natural wakening usually occurs from REM sleep. Subjects woken from REM sleep are much more likely to recall dream content than those awakened from NREM sleep. NREM dreams are generally vague and formless in contrast to REM dreams.
Physiological changes during sleep
During NREM sleep, there is a decrease in respiratory drive and a reduction in the muscle tone of the upper airway leading to a 25% decrease in minute volume and alveolar ventilation and a doubling of airway resistance accompanied by a small (0.5 kPa) increase in PaCO2 and decrease in PaO2. Hypercarbic and hypoxic ventilator drives are reduced compared with wakefulness. The breathing pattern is regular except at the transition from wakefulness into sleep when brief central apnoeas are common.
During REM sleep there is a further decrease in hypercarbic and, particularly, hypoxic ventilatory drives. The breathing pattern is irregular especially during phasic REM sleep. The loss of skeletal muscle tone in REM sleep affects the intercostal and other muscles which stabilise the chest wall during inspiration. In infants, this may be seen as paradoxical movement of the rib cage and abdomen. In adults, there may be maldistribution of ventilation and impaired ventilation–perfusion matching with consequent arterial hypoxaemia. In normal subjects, this is unimportant but it may be very important in patients with chronic lung disease or abnormalities of the thoracic (e.g. kyphoscoliosis). The great majority of patients with impaired respiratory function will be at their worst during REM sleep.
Blood pressure decreases during NREM and tonic REM sleep but may increase above waking values during phasic REM sleep. Cardiac output is generally decreased during all sleep phases. Systemic vascular resistance (SVR) and the heart rate are both reduced during NREM and tonic REM sleep and increased during phasic REM sleep.
Central nervous system
Cerebral blood flow (CBF) increases by 50–100% above the level of resting wakefulness during tonic REM sleep and is even greater during phasic REM sleep. Cerebral metabolic rate, oxygen consumption and neuronal discharge rate are reduced during NREM sleep but increased above resting values during REM sleep. The autonomic nervous system shows a general decrease in sympathetic tone and an increase in parasympathetic tone, except in phasic REM sleep.
The glomerular filtration speed and filtration fraction are reduced and ADH secretion is increased resulting in a less volume concentrated urine.
The secretion of several hormones is directly linked to the sleep/wake cycle. Melatonin is released from the pineal gland under the control of the supra-chiasmatic nuclei (SCN) in a 4–5h pulse, usually beginning at the onset of darkness (~9 pm). The pulse is inhibited or delayed by exposure to bright light in the evening. It is best regarded as being permissive of sleep (‘opening the gate to sleep’) rather than as an hypnotic, as it is possible to maintain wakefulness during this period. Growth hormone is mostly secreted during the first episode of SWS, particularly during puberty. Prolactin concentrations also increase shortly after sleep onset and decrease with wakefulness. Sleep phase delay delays secretion of both of these hormones. The secretion of cortisol decreases with the onset of sleep and reaches a trough in the early hours of the morning and a peak just after waking.
In contrast to anaesthesia, thermoregulation is maintained during sleep. However, the shivering threshold is decreased and body core temperature decreases by about 0.5°C in humans and 2°C in hibernating mammals. Body temperature is linked to the circadian rhythm and reaches its nadir at about 3 am. Thermoregulation is quite good in human infants compared with other species.
Control of sleep
Sleep follows a circadian (~1 day) cycle, the periodicity of which is regulated by an independent genetically determined ‘intrinsic clock’ which is entrained to a 24 h cycle by external cues (Zeitgebers) such as light, darkness, clock time, working patterns and meal times. When a human being is deprived of all external time clues and is exposed to constant levels of illumination (‘free running’), the wake/sleep cycle typically lengthens to about 24.5 h. Subjects who are born blind without any appreciation of light generally free run while those blinded in later lifeor who retain some perception of light remain entrained.
All living organisms, including plants and fungi, have been found to have clock genes and to show an inactivity/activity cycle. In mammals, control of the intrinsic clock is located in the SCN on either side of the third ventricle, just above the optical chiasm. In animal experiments, its destruction leads to a change from the normal sleep cycle into several shorter sleep/activity periods during the day. As noted above, melatonin secretion is prompted by the SCN just before the usual time of sleep onset. A mismatch of this pattern with sleeping time, as occurs in shift workers and after trans-meridian flights, leads to sleep disturbance (‘jet lag’) as the subject is trying to sleep during their circadian day. Light therapy can be helpful in re-setting the circadian clock and the interested reader is referred to the bibliography.
The propensity to fall asleep varies throughout the day and depends upon both circadian factors (process C) and time since the last sleep period (process S). The longer the time since the last sleep period, the greater will be process S. However, its propensity will be modulated by process C. The circadian pressure to sleep is greatest at ~2 am with a secondary peak at ~2 pm. It is least at ~6 am and ~6 pm. If a subject elects to stay awake throughout the night, they will feel most sleepy in the small hours of the morning but will get a ‘second wind’ as morning approaches and the circadian pressure to sleep declines. If wakefulness is maintained, a second period of sleepiness and relative alertness will follow in early afternoon and early evening, respectively. Some of the 8-h sleep debt will be recovered that night but process C will ensure that awakening will occur at or shortly after the normal waking time.
Sleep is normally an actively initiated and not a passive process. Unless a subject is sleep deprived, successful initiation of sleep depends both upon the phase of the circadian clock and external factors (recumbent position, darkness, reducing sensory input). Over the years, considerable effort has been focused on a search for: (i) a ‘sleep centre’, a nucleus or region in the brain where stimulation or ablation would lead to sleep; and (ii) a hormone or transmitter which would reliably induce sleep. Neither have been found because the mechanisms resulting in sleep are complex and diffuse.
During wakefulness, the CNS is dominated by activity of the ascending reticular activating system (RAS) in the brain stem. This formation receives sensory input from all peripheral sensors and projects to the thalamus and the cortex. Its main neurotransmitters are acetylcholine, noradrenaline, dopamine and histamine which explains the sedative effect of antagonists to these substances. A decrease in its activity permits sleep to be initiated by suppressing incoming external stimuli.
The induction of SWS is associated with the secretion of γ-aminobutyric acid (GABA) from basal forebrain neurones. Therefore, it is not surprising that benzodiazepines and barbiturates, which act through stimulation of GABA receptors in the CNS, induce sleep or anaesthesia. Cholinergic mechanisms initiate REM sleep through stimulation of pontine neurones in the lateral portion of the pontine tegmentum and the nucleus reticularis pontis oralis. In animal experiments, injection of carbachol (acetylcholine agonist) induces instantaneous REM sleep.
Recently, orexins (hypocretin) have been isolated in the hypothalamus and appear to be important in the control of REM sleep and appetite. CSF concentrations of orexins have been found to be very low in patients with narcolepsy.
Influence of surgery and anaesthesia on sleep
Anaesthesia and surgery can have a profound effect upon sleep. On the first night after surgery, sleep architecture is severely disrupted with little or no SWS and REM sleep. The light Stage 2 sleep is fragmented with frequent awakenings. The degree of disruption appears to be related to the severity of the surgical insult. The mechanism is unclear but it is probably due to a combination of the surgical stress and the effects of opioid analgesics.
Recovery of lost SWS and REM sleep occurs on postoperative nights 2–5, being later after major surgery. This coincides with the nadir of postoperative pulmonary function and several studies have demonstrated marked hypoxaemia associated with the rebound of REM sleep. It was a logical step to attribute postoperative myocardial ischaemia, myocardial infarction, pulmonary embolism and cerebral disorder (delirium and cognitive impairment) to nocturnal hypoxaemia. However, a number of studies have failed to confirm these presumed associations, although this does not exclude the possibility that the hypoxaemia may be important in some individuals.