Sleep and wakefulness are not totally distinct states.

The constant flux in the activity of many parts of the brain may lead at any one time to either an over-whelming balance of activity in favour of one state or the other, or to features of both appearing simultaneously. The fluctuations between NREM and REM sleep are also much more dynamic and complex than is usually appreciated. The instability of these states underlies the appearance of a range of mixed or incomplete states of wakefulness, NREM and REM sleep (

Table 1.3), and arousals.

The transitions between sleep and wakefulness may also be gradual, often lasting several minutes, rather than instantaneous. Consciousness and the awareness of activities, for example, can become separated from the performance of movements. The spinal cord can coordinate repetitive flexion and extension movements, such as those seen in walking, without higher control, and even complex activities can be organized by the basal ganglia and brainstem without the type of cerebral cortical involvement seen in wakefulness.

The characteristics of sleep are partially inherited and partially due to environmental factors. Most indices of the duration and type of sleep appear to be roughly equally genetically and environmentally determined, with a polygenic type of inheritance.

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