Everyone, of course, has had a great deal of personal experience with sleep, or lack of it, and has observed people in sleep, so it requires no special knowledge to understand something about this condition or to appreciate its importance to health and well-being. Sleep, that familiar yet inexplicable condition of repose in which consciousness is in abeyance, is obviously not abnormal, yet it is connected with a number of interesting and common irregularities, some of which approach serious extremes. Furthermore, certain neurological conditions have special types of sleep disorders as common features. The psychologic and physiologic benefits of sleep are of paramount importance, and it is increasingly recognized that disruption of sleep increases the risks for several diseases, including stroke, hypertension, and coronary disease.
Physicians are frequently consulted by patients who suffer from some derangement of sleep. Most often, the problem is one of sleeplessness, but sometimes it concerns excessive sleepiness or some peculiar phenomenon occurring in connection with sleep. Certain points concerning normal sleep and the sleep–wake mechanisms are worth reviewing, as familiarity with them is necessary for an understanding of disorders of sleep. A great deal of information about sleep and sleep abnormalities is now available as a result of the development of the subspecialty of sleep medicine and the existence of centers for the diagnosis and treatment of sleep disorders.
Most disorders of sleep can be readily recognized if one attends closely to the patient’s description of the disturbance. Cases requiring the documentation of apneic episodes or those with more complex disorders such as seizures and other motor symptoms during sleep, benefit from attention in sleep laboratories.
PHYSIOLOGY OF SLEEP AND SLEEP–WAKE MECHANISMS
Sleep represents one of the basic 24-h (circadian) rhythms, traceable through all mammalian, avian, and reptilian species. The neural control of circadian rhythms is thought to reside in the ventral-anterior region of the hypothalamus, more specifically, in the suprachiasmatic nuclei. The intrinsic circadian rhythm of about 25 hours exists independent of light entrainment but is altered to conform to the day by light. Lesions in these nuclei result in a disorganization of the sleep–wake cycles as well as of the rest–activity, temperature, and feeding rhythms. Chapter 26 describes the ancillary role of melatonin and the pineal body in modulating this cyclic activity. There is also an important dimension of a homeostatic drive to sleep as the day wears on.
Observations of the human sleep–wake cycle show it to be closely age linked. The newborn baby sleeps from 16 to 20 h a day, and the child, 10 to 12 h. Total sleep time drops to 9 to 10 h by mid-adolescence and to about 7 to 7.5 h during young adulthood. A gradual decline to about 6.5 h develops in late adult life. However, there are wide individual differences in the length ...