A rest–activity pattern is detectable in almost all organisms and this may be related to the presence of clock genes which have been largely conserved throughout evolution. Whether or not these changes in activity are recognizable as sleep and wakefulness depends on the level of organization of the central nervous system of the organism.

In invertebrates the heart rate is slower during ‘behavioural sleep’ than when they are more active, and there appears to be a homeostatic mechanism for rest and activity, such that if activity is prolonged the subsequent rest period is also longer.

All vertebrates have NREM-like sleep (Fig. 1.2), but stages 3 and 4 NREM sleep have not been detected in fish, amphibia or reptiles. REM sleep has not been definitely established in these groups either and, like stages 3 and 4 NREM sleep, its evolution appears to be related to the extent of the development of the telencephalon. In birds, stages 3 and 4 NREM sleep may be present in only one hemisphere, leading to contralateral eye closure. The albatross, for instance, can fly with one hemisphere asleep. Birds who are sleeping on the edge of a flock often have unihemispheric sleep, with the eye facing outwards open in order to detect predators. REM sleep in birds is characteristically brief, often only 10–30s, and the total duration of REM sleep is only about 25% that of mammals.

All mammals have both stages 3 and 4 NREM sleep and REM sleep. Monotremes, such as echidna and the platypus, appear to have REM sleep which is accompanied by slow waves on the electroencephalogram rather than the high-frequency waves seen in other mammals.

The duration of sleep varies widely among mammals and is not related to their evolutionary relationships as shown in the Linnaean classification, or the size of the cerebral cortex. Smaller mammals sleep for longer each day and they also have a higher metabolic rate.

The metabolic rate within the brain is also increased, and the consequences of this may include increased glucose utilization and free radical generation.

In contrast, the duration of REM sleep in adult mammals is closely correlated with the degree of immaturity of the offspring at birth and is inversely related to birthweight. Mammalian neonates are more vulnerable than those of reptiles and birds which immediately lead an independent life. In mammals there is an urgent need to develop behavioural patterns, such as maternal bonding, which increase the chance of survival. The prolonged duration of REM sleep may have an important neurodevelopmental role soon after birth, but also persists throughout life.

In primates sleep is usually monophasic, although in most other mammals it is usually polyphasic, with episodes of sleep both during the day and at night.

NREM sleep occurs in both the standing and lying positions, but REM sleep only appears when the animal is lying on the ground, probably because of the motor inhibition which is characteristic of this state.

In aquatic mammals, such as the dolphin, NREM sleep is unihemispheric. There is unihemispheric sleep rebound after sleep deprivation, emphasizing that sleep-related processes can be localized as well as generalized within the brain.

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