David Cohen, M.D.·Published 22 August 2007·8 min read
Key points
This includes a variety of mechanisms that influence sleep but which are independent of the time spent awake and of circadian and ultradian rhythms.
These include motivation, attention and other psychological responses to the environment.
Mental stimulation before falling asleep makes doing so difficult, particularly if there are worries or anxieties which cannot be resolved.
The level or lack of sensory stimulation influences the sleep–wake state, and can be broken down into the following elements.
This includes a variety of mechanisms that influence sleep but which are independent of the time spent awake and of circadian and ultradian rhythms. They modify the sleep–wake cycle according to changes in the environment which are significant for the individual. They are important but complex and ill-understood components of the sleep–wake control system. They have three main elements.
Behavioural factors
These include motivation, attention and other psychological responses to the environment. The voluntary choice of, for instance, deciding whether to take exercise or to move to a more stimulating situation influences the probability of remaining awake or falling asleep. The degree of conscious or subconscious awareness of the significance of different sensory inputs influences whether arousal from sleep occurs. Once aroused, the conscious brain is able to recognize the source of the stimulus and to respond accordingly.
The conscious decision either to try to fall asleep or to stay awake and knowledge of the clock time then influence whether or not the individual stays awake or falls asleep.
Social activity usually induces alertness but, through its association with physical activity and exposure to light, may have complex effects on sleep. Sleep is more likely after social interactions, and the resulting synchronization of sleep and wake patterns within a group or community may have a biological survival advantage.
Psychological factors
Mental stimulation before falling asleep makes doing so difficult, particularly if there are worries or anxieties which cannot be resolved. The ability to relax both mentally and physically affects whether or not sleep can be entered. Watching a television or video can be mentally stimulating because of the changing pictures, sounds and often exciting action, but equally they can induce sleep through boredom, immobility and low ambient light levels.
The degree to which the bedroom is mentally associated with sleep and how much it is, for instance, used as an office or a kitchen, affects the mental associations between going to bed and initiating sleep. Most people also find it more difficult to sleep in an unfamiliar environment, such as a hotel or sleep laboratory.
Reflex factors
The level or lack of sensory stimulation influences the sleep–wake state, and can be broken down into the following elements.
{pagebreak} Light exposure Light exposure has complex and important effects on sleep and wakefulness [14, 15]. The seasonal changes in the duration of daily light exposure lead to cyclical alterations in the duration of rest and activity. The duration of melatonin secretion and of sleep is longer in winter than in summer in non-tropical latitudes.
Written and revised by
David Cohen, M.D.
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At high latitudes with prolonged nights the long nocturnal sleep phase in winter may break into two episodes with an intervening stage of quiet wakefulness. Seasonal changes in light duration also influence mood and behaviour, as exemplified by the seasonal affective disorder.
During sleep, 5–10% of ambient light reaches the retina even through closed eyelids. This has an alerting effect during NREM sleep, increasing the electroencephalogram frequency and causing arousals from sleep, with the result that there may be a later rebound increase in NREM sleep. The effects on REM sleep are less clear, but this can be promoted by brief episodes of darkness.
During wakefulness, light exposure has several important physiological effects that are independent of vision and visual pathways to the brain. These include an increase in the level of alertness, which is particularly marked with bright light and at night.
Light exposure also significantly improves motor performance, for instance in shift workers, and elevates mood. Acute light exposure also increases the heart rate and core body temperature.
A separate non-visual effect is the influence of light on the timing of the circadian rhythms. Exposure to light resets these each day so that they are coordinated with environmental time. These effects are mediated by the influence of light on the suprachiasmatic nuclei and its suppression of melatonin secretion (Fig. 2.4).
{pagebreak} The effects of light on the circadian rhythms depend on the following:
Intensity. There is probably an exponential or logarithmic relationship between the intensity of the light exposure and its effect in modifying circadian rhythms. Exposure to 25lux for around 20min will entrain the circadian rhythm in around 75% of subjects, and while the maximum effect can be seen with around 1000lux, around 50% of this is obtained with only 100lux, which is less than the light level in most domestic situations during the day (Table 2.3).
Duration. Brief light pulses can influence the circadian rhythms, but continuous exposure for 10–20min has a significantly greater impact.
Wavelength. The individual wavelengths of light influence the circadian rhythms to different extents.
The most effective in resetting them are between 445 and 475nm, particularly around 460nm, which is close to the wavelength to which melanopsin in the retinal ganglion cells is most sensitive.
Timing. The sleep phase can be shifted by up to 1–2 hours per day by light exposure according to its timing. The phase response curve of sleep to light is complex (Fig. 2.5). The circadian rhythms are insens- itive to light exposure during the day. There is little sleep phase delaying effect before around 9.00 pm or phase advance after around 10.00 am. Light exposure at night, when there is normally little light and when melatonin is secreted, can have a major effect on the circadian rhythms. A rapid shift in the response to light occurs around the nadir of temperature and at the time of peak melatonin secretion, usually 3.00–5.00 am.
Light exposure before this phase-shift transition point delays the next sleep phase, but afterwards leads to a sleep phase advance. The magnitude of both of these effects is greatest close to the temperature nadir.
{pagebreak} Noise A noisy sleep environment can impair sleep but the threshold for arousal varies according to the age of the subject and the stage of sleep.
Arousal is also more likely if the noise is of significance to the sleeper, such as the crying of a child to the mother. Children sleep through more noise than adults, and older subjects are readily aroused from sleep by noise unless they are deaf. The threshold for arousal increases from stage 1 through to stage 4 NREM sleep.
Arousal is therefore more likely later in the night after the initial episodes of stages 3 and 4 NREM sleep have been completed.
International standards for nocturnal environmental noise have been drawn up. In general they recommend a mean nocturnal noise level within the sleeping area of less than 40dB and the avoidance of peak levels greater than 60dB. Noise greater than 40dB frequently causes difficulty in initiating sleep and greater than 50dB often causes awakenings during the night. These thresholds, however, vary considerably between individuals, between nights and within each night, and with other environmental factors, such as room temperature. In young adults, for example, the threshold for arousal from stage 4 NREM sleep is around 100dB, but this falls to 70dB at age 70 years.
Adaptation to environmental noise occurs rapidly, and people living, for instance, close to railways, motorways and even airports may sleep soundly.
Measures to isolate sleepers from noise can be successful and include earplugs and double glazing.
{pagebreak} Pain and discomfort Both internal and environmental stimuli can cause arousals from sleep despite the pain threshold being increased during sleep. Chronic disorders such as rheumatoid arthritis and multiple sclerosis commonly cause arousals of this type, and while a hard bed may help to reduce backache, it also leads to more frequent changes of position during sleep which are associated with brief awakenings.
Temperature A constant environmental temperature of around 18°C (65°F) is ideal for inducing and maintaining sleep and for the optimal balance of NREM and REM sleep. Both high and low environmental temperatures lead to fragmentation of sleep. Rapid eye movement sleep is particularly sensitive to temperature changes and is significantly shortened in cold environments.
REM sleep is facilitated by a rise, and NREM sleep by a fall, in core body temperature.
Physical exercise Exercise promotes wakefulness not only at the time of the activity, but for around 3h afterwards. This alerting effect is present both in normal subjects and also in, for instance, those with narcolepsy.
Exercise early in the day may cause a phase advance, reducing the sleep latency on the next night. It increases the total sleep time and the duration of stages 3 and 4 NREM sleep, and may reduce the duration of REM sleep. Exercise within around 3h of the desired time of sleep onset may delay this and reduce melatonin secretion.
{pagebreak} Food intake Hunger is associated with wakefulness. This has a survival advantage in that wakefulness is retained until sufficient food has been eaten. After eating, humans tend to relax and readily fall asleep. This is due to complex reflex and humoral factors.
Carbohydrates and milky drinks which contain tryptophan, a precursor of 5HT, and melatonin, have a greater sleep-promoting effect than other foods.
Tryptophan is also present in bananas, peanuts and figs. Lettuce contains lactucarium, an opioid which may promote sleep. Dietary unsaturated fatty acids such as oleamide bind to GABA receptors and can promote sleep and reduce motor activity. Conversely a low serum cholesterol level has been associated with violence.
High protein foods which are rich in tyrosine may promote noradrenaline synthesis and lead to wakefulness.
Large meals taken before sleep can cause gastrooesophageal reflux leading to heartburn and awakenings. Large volumes of liquid should be avoided before sleep since they may lead to nocturia, especially if there is renal impairment or prostatic hypertrophy.
Very severe malnutrition can induce excessive daytime sleepiness and inactivity, but, in general, weight loss, whether it is due to anorexia nervosa, mania or thyrotoxicosis, leads to insomnia and hyperactivity.
Weight loss in depression is associated with insomnia, but if weight is gained, as in seasonal affective disorder, sleepiness is common. There may be a link in otherwise normal subjects between weight loss, hyperactivity and insomnia, and between weight gain, inactivity and excessive daytime sleepiness. The chemical basis of these associations is uncertain, but weight gain may also lead to excessive daytime sleepiness through causing obstructive sleep apnoeas.
Sexual activity Sexual intercourse usually promotes the onset of sleep, in contrast to other types of physical activity.
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