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Timing of sleep
Regularity of sleep A regular time of going to bed, going to sleep, waking up and getting up in the morning is an important factor in stabilizing and synchronizing the circadian rhythms. The most important of these is the regularity of the waking time. The exact times that are adopted vary according to, for instance, age, social constraints such as work patterns, and the individual tendency to be a long or short sleeper, or a 'lark' or 'morning type' or an 'owl' or 'evening type'.
Nature of Sleep and its Disorders
Regular sleep patterns ensure that the homeostatic drive to sleep is strong because they allow a sufficient interval after the previous main sleep episode. In the elderly especially, restricting the nocturnal sleep phase to 6-7h is usually preferable to sleeping for longer and often improves insomnia.
Monophasic and polyphasic sleep patterns
Sleep in adults is usually taken as a single episode at night (monophasic pattern), but in 85% of mammals, particularly those with a small body mass, sleep is polyphasic. A single prolonged sleep episode may expose these animals to more danger. A monophasic pattern enables the sleep debt accumulated during the waking period to be fully discharged whereas a polyphasic routine pays back this sleep debt in smaller units.
There is evidence that hunter-gatherer communities adopt a polyphasic sleep pattern with at any one time around 25% of the population awake at night and 10% sleeping during the day. A problem with this sleep pattern is that there are repetitive episodes of sleep inertia if naps last for more than 20 min, and particularly if they are longer than 60 min. The polyphasic pattern does, however, reduce the exposure to danger, and can enable a broader division of labour in the society.
Although a monophasic pattern is usual in adults, children obtain polyphasic sleep, usually with around 3-5h during the day at the age of 6 months and 2h during the day at 2 years. Initially these sleep episodes are almost random, but once the circadian rhythms mature at 3-6 months, more sleep is obtained at night than during the day. The polyphasic pattern is often re-entered in the elderly who tend to nap during the daytime, but occasionally it is retained throughout life. Leonardo da Vinci, for instance, is said to have slept for 15min every 4 h.
A polyphasic sleep pattern lessens the performance loss during sleep deprivation. The frequent naps repay the sleep debt exponentially. The deeper NREM sleep at the start of each nap is more time-effective than a more prolonged sleep episode with relatively more lighter NREM and REM sleep. Repayment of a sleep debt is facilitated if the naps coincide with an increased circadian or adaptive drive to fall asleep.
A biphasic sleep pattern is often adopted in Mediterranean countries, with a siesta taken in the mid-afternoon. It is common for nocturnal sleep to be postponed until after midnight. This pattern reflects the circadian tendency to promote sleep during the afternoon and at night, but in addition to this biological element there is probably also a cultural factor.
The siesta also avoids taking physical activity during the hottest part of the day.
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 influence of the REM sleep ultradian 90-min cycle on the timing of sleep is uncertain. A 90-min cycling in reaction time performance and in the tendency to daydream has been found, but it is uncertain whether or not this predisposes to enter sleep at these times.
Most people have a tendency to be more alert early in the morning soon after waking (morning types, larks) or late at night prior to falling asleep (evening types, owls). The tendency to be in one or other category can be assessed using the Horne Ostberg questionnaire .
These patterns merge into the advanced and delayed sleep phase syndromes respectively. There is a tendency during adolescence to become an evening type and from early adult life onwards to progressively becoming a morning type by old age. There is some evidence that morning and evening types tend to marry each other more frequently than by chance.
The biological basis of this may be that it increases the duration during the day when one or other parent is awake and able to protect the children.
Morning types tend to have a shorter latency before entering stages 3 and 4 NREM sleep, are unable to sleep late in the morning and often complain of poor sleep quality with more frequent awakenings, especially late in the night, and early morning waking.
They have an earlier peak melatonin secretion and core body temperature nadir. They may have a shorter circadian periodicity. There is a longer delay between the temperature nadir and waking, so that in effect waking occurs later in the circadian phase, facilitating alertness soon afterwards. They have a high sleep efficiency and are intolerant of working night shifts and can only sleep longer by going to bed earlier.
Evening types tend to have a larger amplitude of circadian rhythms such as temperature. Their sleep is more closely entrained by light. They have a later melatonin peak and temperature nadir and may have a long circadian periodicity . They wake closer to the temperature minimum than morning types. They nap less easily during the day, but for longer than the morning types, and feel more refreshed afterwards.
They obtain more sleep by waking later in the morning rather than going to bed earlier, and if this is impossible because of, for instance, work commitments a sleep debt is built up which is usually discharged at weekends by sleeping longer. Alertness in the morning can be promoted by taking nicotine or caffeine. Evening types have more irregular sleeping times, and adapt better to time zone changes and to nocturnal shift work than morning types. In adolescence, however, they are often more sleepy on school days, with resulting attention problems, under-achievement at school, and more injuries, and they take more caffeine.
Daytime naps are normal in young children and common in the elderly and are taken especially at between 2.00 and 4.00 pmwhen the circadian rhythms favour sleep. They are often taken to compensate for sleep deprivation at night, but may be a manifestation of excessive daytime sleepiness from other causes.
The structure of sleep during a nap depends on the duration since the last sleep episode and its position in the circadian rhythm. Stages 3 and 4 NREM are more likely if the nap occurs after a prolonged period of wakefulness, which is usually later in the day. These stages of sleep are also more likely if a nap lasts for more than around 60 min, but are unlikely if it is less than 20 min. REM sleep may be entered if a nap coincides with the ultradian REM sleep cycle which probably persists in a mild form throughout wakefulness as well as sleep. This is otherwise unusual in normal subjects. Frequent dreaming during daytime naps suggests the diagnosis of narcolepsy.
'Sleep attacks' have been thought to be characteristic of narcolepsy and Parkinson's disease, perhaps triggered by dopaminergic agents, but they can occur with any cause of severe daytime sleepiness. They may be slightly more likely in REM rather than NREM sleep disorders or deprivation, and can be confused with transient loss of consciousness due to cardiac dysrhythmias or epilepsy.
Long naps may be followed by sleep inertia, and brief 'power' naps are often more refreshing. This may be not only because they may contain stages 3 and 4 NREM sleep, but also because the act of entering sleep from wakefulness itself leads to feeling refreshed. This implies that features of stages 3 and 4 NREM sleep may not be the best indicator of whether or not sleep is refreshing.
In narcolepsy naps of 5-30min or even less are refreshing. Longer naps are required in obstructive sleep apnoeas and even prolonged naps of up to 2h may be unrefreshing in idiopathic hypersomnia. Shift workers often have unavoidably modified nocturnal sleep routines and require naps at other times during the day in order to compensate for this.
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