Insomnia is common in the elderly, probably due to degeneration of the sleep-regulating mechanisms, but these effects are exaggerated in dementia due to degenerative disorders, particularly Alzheimer's disease , and in multi-infarct dementia. The clinical features vary according to the distribution of the degeneration, and in particular whether the homeo-static sleep control mechanisms or other regions of the brain are mainly involved. The suprachiasmatic nuclei appear to be normal and the diurnal pattern of cortisol secretion is normal.
There is a loss of the sleep-wake cycle and of the normal sleep architecture (see Table 7.10). Daytime naps are frequent and occasionally there is sleep reversal if the dementia is severe. 'Sundowning' is a characteristic particularly of Alzheimer's disease, in which there is agitation, particularly in the afternoon or early evening, associated with wandering and an exacerbation of any behavioural abnormalities. It may be more frequent in the winter when there is less exposure to light, and improves with light therapy of 5000-10 000 lux in the evenings, which causes a sleep phase delay.
An advanced sleep phase is common in dementia and the time spent awake at night is increased. Confusion and wandering at night are common and may be exacerbated by adverse environmental conditions and poor sleep hygiene.
Polysomnography is rarely required, but shows that the sleep efficiency may be reduced to around 60% by the frequent awakenings, which are often prolonged. The duration of stage 1 NREM sleep is lengthened and stages 3 and 4 are shortened. Sleep spindles are few in number, and slow in frequency, and there are few K-complexes. There is an increase in REM sleep latency and the duration of REM sleep is reduced. Periodic limb movements are common, possibly due to a reduction in dopamine and acetylcholine as neurotrans-mitters, and obstructive and central sleep apnoeas and Cheyne-Stokes respiration are frequently seen.
Table 7.10 Effects of dementia and depression on sleep.
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