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Sleep and age
The pattern of sleep varies considerably with age [9], partly because of intrinsic changes in the sleep-wake cycles and sleep structure (Figs 1.4 and 1.5), but also because of the presence of age-related medical disorders. The most important stages in the development of sleep patterns are as follows.
Prenatal phase
A rhythmic cycling of motor activity is detectable at around 20 weeks gestation and at 28-32 weeks a regular sleep-wake cycle is detectable with alternating periods of body movements, rapid eye movements and irregular respiratory movements ('active sleep', probably equivalent to REM sleep), interrupted by brief episodes of inactivity ('quiet sleep', probably equivalent to NREM sleep) [10]. By 24 weeks gestation the EEG recordings show intermittent high amplitude signals alternating with periods in which the EEG is 'flat', and which gradually become more prolonged. From 32 weeks onwards, active and quiet sleep become more easily distinguishable and resemble REM and NREM sleep respectively. Any circadian influence on rest and activity prenatally is due to maternal rather than fetal rhythms.
Early postnatal phase
At birth the fetus moves from an environment of continuous darkness into one with intermittent light exposure, and the nature of other external stimuli such as feeding and social contact also changes vastly.
Normal full-term infants show almost randomly timed phases of wakefulness, active sleep, and quiet sleep on behavioural criteria, although about 3% of their time is spent in an indeterminate state [11].
Around 16h per day is spent asleep, which is entered through REM sleep. Premature infants at 30 weeks appear to be in REM sleep for 80% of their sleeping time, at 36 weeks for 60%, and at 40 weeks for 50%.
3-12 months
At this age, as in the early postnatal phase, the interpretation of the electrophysiological tracings is difficult. The total sleep time gradually falls. Quiet sleep predominates by around 3 months. Conversely, active sleep, which may at this age be equivalent to REM sleep or possibly an immature form of wakefulness, occupies around 50% of the total sleep time in normal infants at birth and is responsible for only around 40% at 3 months and 30% at 6-12 months.
The latency before entering REM sleep gradually increases during the first year.
By 3 months K-complexes and sleep spindles are detectable on the EEG and the latter increase in number by 6 months, although until the age of 2 years there is often interhemispheric asynchrony of spindles.
High-voltage, low-frequency theta (4-8Hz) or delta (0.5-4Hz) waves are detectable, particularly over the occipital area from 1 month of age. By 6-12 months the four stages of NREM sleep are distinguishable electrophysiologically. By 6 months 30% of sleeping time is REM sleep and the total sleep time is 14-15h per night. REM density remains high.
Rapid eye movement (REM) sleep
Rapid eye movement sleep, or REM, is one of the five stages of sleep that most people experience nightly....
The sleep pattern until around 3 months is thought to be related to feeding which probably increases the tendency to sleep afterwards. Both sleeping and feeding occur at 3- to 4-h intervals, but by 3-6 months a circadian rhythm in the child's sleep patterns, body temperature, and melatonin secretion becomes detectable. This is initially free-running but gradually comes under the control of external stimuli, including light exposure. By 6 months it is common to sleep throughout the night and signs of a predominantly biphasic pattern with an afternoon sleep appear.
Cycles of REM sleep develop by around 3 months.
These are initially approximately of 45min duration, but gradually lengthen to around 60min by 1 year of age. Intermittent retention of muscle tone during REM sleep is responsible for smiling, grimacing facial movements, brief vocalizations, irregular respiratory movements and twitching of the limbs and whole body.
1 year to puberty
By the age of 1 year children spend around 11h asleep at night and up to 2 1/2 h asleep during one or two naps in the daytime. Thirty per cent of their sleep is REM sleep. The morning nap is usually discontinued at the age of 2-3 years, but the afternoon nap is often retained until around 4-5 years.
The total sleep time at 2 years is around 13 h, at 4 years 12 h, at 5 years around 11h and at the age of 10 years around 10 h. The REM sleep cycles lengthen, but the first REM sleep episode is shorter than those later in the night. Body movements become much less common in REM sleep at this age, and by the age of 5 years REM sleep occupies only around 25% of the sleeping time.
The sleep latency is shorter than in later life at 5-10min with few arousals by the age of 8. The sleep efficiency is around 95% and the percentage of stages 3 and 4 NREM sleep is greater at around 5 years than at any other age. There is a strong monophasic sleep cycle centred on the night time, possibly related to the high melatonin levels at this age.
Environmental factors often have a significant impact on sleep in childhood. Some physical disorders such as middle ear infections are more painful at night, but may not be recognized, and nocturnal asthma or gastro-oesophageal reflux may also severely disrupt sleep. Bedtime fears and rituals may delay the initiation of sleep, and family situations can cause hyper-arousal of the child shortly before the intended sleep time.
Irregular sleeping times and peer group pressures may also lead to sleep problems.
Puberty
At this stage most children sleep for around 9 h, of which around 40% is stages 3 and 4 NREM sleep and 25% REM sleep. A biphasic sleep pattern begins to emerge with occasional complaints of daytime sleepiness, usually in the afternoon.
Adolescence
Most adolescents begin to go to sleep later and wake up later in the morning. The total sleep time increases slightly, perhaps related to the metabolic changes during the growth spurt. A delayed sleep phase syndrome is common, but whether this is due to an endogenous change in the circadian sleep rhythm or to social factors such as opportunities for late evening and night entertainment or work is uncertain. The sleep latency is usually longer than in childhood and REM sleep comprises 25% of total sleep time. The sleep cycle lengthens to the adult duration of 90 min.
Sleep restriction due to school and social pressures is common, and irregular sleep-wake patterns often develop [12]. Excessive caffeine or alcohol intake and recreational drugs may contribute to these patterns.
Unrecognized depression and anxiety are also common.
In many cultures, napping continues to be a normal part of daily life for both young and old. This practice, which in Spanish-speaking countries is called a siesta, tends to be more common in the tropics than in more temperate latitudes. Naps in these cultures typically take place in mid-afternoon and coincide with the hottest time of day, as well as a lull in the brain's alerting signal that works to counteract the drive to sleep.
As a consequence of these regular daytime naps, nighttime sleep is often shifted to a later hour than it is for societies or individuals that don't nap during the day.
Young and middle-aged adults
At the age of 20 years the sleep efficiency is still usually around 95%, but it then falls progressively.
By 35 years the duration of stage 4 NREM sleep is only around 6% of the total sleep time which is only half of what it is at 20. Wakefulness at night is twice as prolonged and the duration of stage 1 NREM sleep is increased slightly at around 5% of total sleep time.
The percentage of REM sleep remains constant at around 22-25% throughout early and middle adult life, but REM density gradually falls. Changes in the circadian rhythms prevent subjects over the age of around 45 from adapting as fast and as completely to changes in sleep patterns, e.g. shift work. Other environmental factors such as a reduction in exposure to light due to indoor employment, restriction of sleep time and medical and psychological disorders influence sleep patterns.
Old age
The total sleep time during the night is reduced, but if daytime naps are frequent or prolonged, the total amount of sleep during each 24h may be similar to that of younger subjects. The sleep efficiency falls to 70-80% with an increase in the number of awakenings and a reduction in stages 3 and 4 NREM sleep [13]. This is around 18% at age 20 years, but may be only around 10% at the age of 60 and by 75 there may be no stage 4 NREM sleep, especially in males. The amplitude of the delta waves falls by about 75% relative to childhood, probably as a result of loss of cortical synchronization due to degeneration of the sleep regulating processes and to cerebral atrophy which causes fewer cortical neurones to be sampled by the surface electroencephalogram (EEG). The duration of stages 1 and 2 NREM sleep is increased (up to 15% stage 1), sleep spindles become fewer, poorly formed, of small amplitude and their frequency may fall from 16 to 12-14Hz. The elderly often take naps during the day and this may be associated with a poor prognosis.
The sleep pattern deteriorates in men at a younger age than in women, and men have more arousals from sleep than women between the ages of 60 and 80. At these ages there is no detectable gender difference in the duration of REM sleep and its overall percentage remains almost constant at around 20% even into old age, although it may then fall slightly. REM sleep latency shortens to 70-80 min, but the first episode of REM sleep is often prolonged, possibly related to changes in the circadian rhythm. This results in REM sleep episodes of similar duration throughout the night rather than the pattern of lengthening REM sleep cycles seen in younger subjects.
A number of sleep problems are particular to adult women. Half of them report sleep disturbances during their menstrual periods; three-quarters of expectant mothers report that sleep is more disturbed during pregnancy; and many experience disrupted sleep during menopause, in part due to nighttime "hot flashes."
Insomnia and disrupted sleep in elderly people are a common side effect caused by many chronic medical conditions such as arthritis, congestive heart failure, depression, and gastroesophogeal reflux disorder. Respiratory disorders, such as sleep apnea, which cause multiple arousals during the night, also become more common as people age. Other problems, such as restless legs syndrome, which results in an uncontrollable need to move one's legs while drifting off to sleep, or periodic limb movements, which cause jerking of the feet or legs during the night, can make it difficult to fall asleep or lead to highly fragmented sleep. Unfortunately, sleep problems in older adults often go undiagnosed and untreated simply because many people believe sleep problems are a normal part of aging or that nothing can be done to help them sleep better. Thankfully, treating any underlying medical disorders can dramatically improve sleep.
These changes in the sleep patterns of the elderly are influenced by the following factors.
Changes in circadian rhythms
There is no change in the endogenous length of the circadian cycle, which remains around 24.2 h, but the circadian rhythm reduces in amplitude and often alters its timing. The peak blood melatonin level, which is controlled by exposure to light, falls in the elderly and the amplitude of the temperature rhythm is attenuated. The sleep onset and time of wakening and temperature nadir become almost 1h earlier per decade after the age of 60 years. This is a form of the advanced sleep phase syndrome and is a continuation of the phase change from the delayed sleep phase syndrome of adolescence through the normal sleep phase of middle adult life. There may also be internal desynchronization of circadian rhythms, particularly of sleep, temperature and hormone secretion.
Reduction in homeostatic sleep drive
The sleep-wake controlling mechanisms disintegrate with less delta wave activity, and there are more frequent arousals which may be either spontaneous or due to a lower threshold to, for instance, light and noise. The duration of arousals remains similar to that of young adults at a mean of around 15s [14]. The elderly wake closer to the peak of melatonin secretion and the temperature nadir than younger adults. The increased vulnerability of the elderly to these stimuli reduces the continuity of sleep and together with changes in the circadian rhythm leads to a polyphasic sleep pattern with frequent daytime naps. By the age of 70, 25% of men nap during the day and by 80 the figure rises to 45%.
As we age, there is an increased incidence of medical problems, which are often chronic. In general, people with poor health or chronic medical conditions have more sleep problems. For example, hypertension is associated with both snoring and OSA and heart failure - which affects approximately 5 million Americans - is linked with OSA. In addition, menopause and its accompanying hot flashes, changes in breathing, and decreasing hormone levels can lead to many restless nights.
Gastroesophageal reflux disease (GERD) is another common cause of sleep problems. The pain also makes it difficult to sleep. Medical conditions such as diabetes mellitus, renal failure, respiratory diseases such as asthma, and immune disorders are all associated with sleep problems and disorders. Diseases such as Parkinson's disease and multiple sclerosis also commonly cause problems sleeping.
Changes in environment
It is common for entraining factors of the circadian rhythms to be attenuated in old age. The exposure to light falls because the elderly remain indoors for longer, and often have cataracts and macular degeneration which reduce the amount of light stimulating the retina. Institutionalization in nursing homes, reduction in activity, either due to lack of opportunity or physical restrictions, social isolation and boredom all adversely affect the control of sleep. Conversely, some habits such as physical and social activity [15], regular meal times, bedtime and wake-up time tend to consolidate sleep-wake patterns. Exposure to bright light in the morning may exacerbate the advanced sleep phase syndrome. Light exposure at night, even if it is brief, may reduce melatonin secretion and worsen insomnia.
