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.
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.
Effects of Age
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 and depth of sleep, apparently as a result of genetic factors, early life conditioning, the amount of physical activity, and psychologic states.
The pattern of sleeping, which is adjusted to the 24-h day, also varies in the different epochs of life. The circadian rhythm, with predominance of daytime wakefulness and nighttime sleep, begins to appear only after the first few weeks of postnatal life of the full-term infant; as the child matures, the morning nap is omitted, then the afternoon nap; by the fourth or fifth year, sleep becomes consolidated into a single long nocturnal period. (Actually, a large part of the world’s population continues to have an afternoon nap, or siesta, as a lifelong sleep–wake pattern.) Fragmentation of the sleep pattern begins in late adult life. Over ensuing years, night awakenings tend to increase in frequency, and the daytime waking period may be interrupted by episodic sleep lasting seconds to minutes (microsleep), as well as by longer naps. From about 35 years of age onward, women tend to sleep slightly more than men.
Stages of Sleep
Seminal contributions to our understanding of the physiology of sleep were made by Loomis and associates and by Aserinsky, Dement and Kleitman through electroencephalographic analysis and clinical observation. As a result of their studies, five stages of sleep have been defined. In each stage, the electrical activity of the brain occurs in organized and recurring cycles, referred to as the architecture of sleep. As the electrophysiologic stages of sleep progress, sleep becomes deeper, meaning that arousal requires a more intense stimulus. These findings put to rest the antiquated ideas that sleep is a purely passive state and reflects fatigue and reduction in environmental stimuli.