Sleep is a necessary behaviour

Our lives are dominated by daily cycles of sleep and wake. The origin of these cycles begins with the earliest life on this planet. Life requires energy and the only available source of energy for the earliest life was the sun. Because of the cyclic availability of solar energy, prokaryotes evolved adaptations to use energy during the solar day, and to carry out other functions at night. With the evolution of nervous systems in primitive animals, this pattern of adaptation was maintained as rest–activity cycles. Recent studies indicate that, even in an invertebrate such as the fruit fly, Drosophila, the rest–activity cycles bear a striking resemblance to sleep–wake cycles in mammals. Sleep has long been recognized to have a restorative function and sleep is required to maintain life. Total deprivation of sleep results in death and even relatively brief periods of sleep deprivation, when repeated over several days, produce profound decrements in vigilance, psychomotor performance and mood. One of the commonest, transient forms of sleep disruption, that occurring with jet lag, can produce cognitive impairment and structural brain changes when it is chronic. Thus, sleep is necessary for life and successful adaptive, waking behaviour.

Sleep disorders are common and important

Loss of sleep is a major problem in our industrialized society, with an immense impact on health and productivity. This occurs as a conseqeunce of economic pressures and the pace of modern life but also results from environmental constraints that alter the normal pattern of the rest–activity cycle; shift work is an important example. Further, common medical and psychiatric illnesses impair sleep resulting in insomnia and chronic sleep deprivation. Finally, we now recognize that there are many primary sleep disorders that have a significant impact on health and normal function. In this chapter, we will review the neurobiology of sleep and important sleep disorders.

Neurobiology of sleep

Behaviour is divided into three states, waking,REM sleep and non-REM (NREM) sleep

With the development of electroencephalography (EEG) in the first half of the twentieth centry, it became possible to record brain activity continuously and to correlate the activity obtained from surface scalp electrodes with behavioural state. With this it was quickly recognized that the waking state is associated with desynchronized, low voltage EEG dominated by high frequency activity, predominantly in the 8–12 Hz range.