Normal sleep

Sleep is usually described as a series of phases characterized by changes in physiological variables, most notably the electroencephalogram (EEG), as illustrated in Figure 14.1. There is usually a typical body posture (associated with good thermoregulation); physical inactivity; more stimulation required to arouse than during wakefulness; a specific site or nest for this behaviour; and regular daily occurrence. The progression between the phases can be shown by means of a ‘hypnogram’ (Figure 14.2). There are a number of theories regarding the function of sleep (listed in Table 14.1). Evidence is accumulating for each, suggesting that sleep serves a number of functions. Sleep has been found to facilitate mathematical insight, as shown in Figure 14.3.

Figure 14.1 • Physiological concomitants of the stages of normal sleep.

Figure 14.2 • A hypnogram of sleep stage changes of the night in normal young adults.

(Reproduced with permission from Horne J 1985 Why we sleep. Oxford University Press, Oxford.)

Table 14.1 Theories of sleep

Figure 14.3 • Facilitation of mathematical insight by sleep.

• (a) Number Reduction Task (NRT), illustrated by an example trial. On each trial, a different string of eight digits was presented. Each string was composed of the digits ‘1’, ‘4’, and ‘9’. For each string, subjects had to determine a digit defined as the ‘final solution’ of the task trial (Fin). This could be achieved by sequentially processing the digits pairwise from left to right according to two simple rules. One, the ‘same rule’, states that the result of two identical digits is just this digit (for example, ‘1’ and ‘1’ results in ‘1’, as in response 1 here). The other rule, the ‘different rule’, states that the result of two non-identical digits is the remaining third digit of this three-digit system (for example, ‘1’ and ‘4’ results in ‘9’ as in response 2 here). After the first response, comparisons are made between the preceding result and the next digit. The seventh response indicates the final solution, to be confirmed by pressing a separate key. Not mentioned to the subjects, the strings were generated in such a way that the last three responses always mirrored the previous three responses. This implies that in each trial the second response coincided with the final solution (arrow). Subjects who gain insight into this hidden rule abruptly cut short sequential responding by pressing the solution key immediately after the second response. (b) Experimental design (main experiment). An 8-h period of nocturnal sleep, nocturnal wakefulness, or daytime wakefulness separated an initial training phase (three blocks) from later retesting (ten blocks). (c) Columns indicate percentage of subjects gaining insight into the hidden rule in the three experimental conditions of the main experiment (grey), in which subjects either slept (at night) or remained awake (at night or during daytime) between initial training and retesting, and in two supplementary conditions (hatched), where subjects were tested after nocturnal sleep or daytime wakefulness in the absence of initial training before these periods.

(Reproduced with permission from Wagner U et al 2004 Nature 427:352–355)

With less than three hours sleep in 24 hours, humans show increased irritability and a decreased attention span. Long periods of sleep deprivation result in poor concentration, deterioration in general performance, increased suggestibility and, later, hallucinations, paranoia and even seizures. The prominence of psychological effects suggests to some that sleep specifically restores brain functioning.

Normal sleep-dependent consolidation of memory is disrupted in schizophrenia. This is illustrated in Figure 14.4.

Figure 14.4 • Sleep-dependent consolidation of memory.

Fourteen healthy controls (open squares) and 20 schizophrenia patients (closed triangles) were asked to learn a motor skill (typing a sequence of digits). The vertical axes represent the number of correct sequences typed in each 30-second trial. Note that the these axes are scaled separately for control subjects (left) and patients (right) to illustrate better the qualitative similarity of learning curves on day 1 and the failure of overnight improvement in the schizophrenia group only. The dashed line is positioned at the mean value of the last three training trials for both the control and patient groups. The shaded bar represents the passage of 24 hours, including a night of sleep. Patients and control subjects did not differ in the amount of learning during training, but only control subjects showed significant overnight improvement.

(Reproduced with permission from Manoach DS et al 2004 Biological Psychiatry 56:951–956.)

Length of time asleep

Humans spend, on average, 25% of their lives asleep. There is variation in sleep time, both in individuals during the course of development and the life cycle, and also between individuals (Figure 14.4). The individual differences within and between subjects are particularly noticeable in infancy. The mean daily sleep length in the first week of life is 16 hours (SD two hours). There is a gradual reduction in mean daily sleep length throughout infancy and middle childhood (Figure 14.5). Most infants wake at least once each night but, apart from babies who need feeding (more often those who are breastfed), they usually drop off to sleep again by themselves. Parents are not normally aware of this unless they themselves are having problems sleeping. Adults will also wake briefly during the night but often will not remember. Frequency of waking increases with age.

Figure 14.5 • Changes with age in total amounts of daily sleep and daily REM sleep.

(Reproduced and modified with permission from Roffwarg et al. 1966 Science 152: 604–619.)

Normal sleep

• Normal sleep consists of a series of phases 1–4 of increasing depth, interspersed with short periods close to wakefulness during which there are rapid eye movements (REM).

• Each phase of sleep has a characteristic EEG.

• There is a wide range of individual differences in sleep length but there is generally a decrease with age.

Classification of sleep disorders

Sleep disorders may be divided into those primarily of emotional origin and those with an organic basis. The ICD-10 and DSM-IV-TR categorizations of sleep disorders are shown in Tables 14.2 and 14.3 respectively. This chapter will cover predominantly those sleep disorders with a psychiatric basis. However, sleep disorders with organic causes are important differential diagnoses and interactions with psychological factors are usual. Also, sleep disorders are often a component or complication of other psychiatric disorders.

Table 14.2 ICD-10 classification: F51 Non-organic sleep disorders

Table 14.3 DSM-IV-TR Sleep disorders

Insomnia

Insomnia is a disorder in which there is insufficient quantity or quality of sleep. The diagnosis may be used at any age (previously sleep disorder in childhood was classified separately). Insomnia may be transient (a history of only a few days or weeks) or chronic (an unremitting history over months or years).

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