Sleep is the integrated product of two oscillatory processes. The first process, frequently referred to as the sleep homeostat, is an oscillation that stems from the accumulation and dissipation of sleep debt. The biological substrates encoding sleep debt are not known, although adenosine is emerging as a primary candidate. The second oscillatory process is governed by the circadian clock and controls a daily rhythm in sleep propensity or, conversely, arousal.

The circadian cycle in arousal (wakefulness) steadily increases throughout the day, reaching a maximum immediately before an increase in plasma melatonin. Arousal subsequently decreases to coincide with the circadian trough in core body temperature. Experiments imposing forced sleep schedules throughout the circadian day have shown that an uninterrupted 8-hour bout of sleep can only be obtained if sleep is initiated approximately 6 hours before the temperature nadir.

Age is one contribution to the sleep homeostat. Sleep deprivation studies have shown that the homeostat component of sleep is remarkably similar among individuals of similar age. There is a well-established age-dependent decline in sleep need.

Biological evidence exists for an adaptive system that allows humans to track the transition from day to night (dusk) and night to day (dawn). This system could function to maintain proper sleep patterns during changing photoperiods (changing day length associated with seasonality). Ill health (emotional or physical) can disrupt normal sleep physiology.

Nasal continuous positive airway pressure (nCPAP) is the treatment of choice for patients with obstructive sleep apnea. Other procedures include weight loss, nasal surgery, tracheostomy, and uvulopalatoplasty. Some medications may normalize sleep in patients with apnea. Selective serotonin reuptake inhibitors (SSRIs) and heterocyclic antidepressant drugs sometimes help treat patients with sleep apnea by decreasing the amount of time spent in REM sleep, the stage of sleep in which apneic episodes occur most often. In addition, theophylline has been shown to decrease the number of episodes of apnea; however, it may interfere with the overall quality of sleep, limiting its general utility. When apnea is established or suspected, patients must avoid using sedative medication, including alcohol, because it can considerably exacerbate the condition, which may then become life threatening.

A common finding is that a patient’s lifestyle leads to sleep disturbance. This is usually phrased as inadequate sleep hygiene, referring to a problem in following generally accepted practices to aid sleep. These practices include, for instance, keeping regular hours of bedtime and arousal, avoiding excessive caffeine, not eating heavy meals before bedtime, and getting adequate exercise (exercise during the day, not in the evening, because it may disturb sleep). In general, napping is discouraged except in elderly and debilitated patients. Two caveats are in order when helping to educate a patient about sleep hygiene. The first is that these are general principles and are not applicable to all patients. The second is that when trying to modify a patient’s behavior, it is usually better to focus on one or two changes at a time, rather than assaulting him or her with panoply of desired changes, which can come across as overwhelming.

Common polysomnographic measures are used to diagnose and describe sleep disorders. Sleep latency is the period of time from turning out the lights until the appearance of stage 2 sleep. The period of time from the onset of sleep until the first REM period is known as REM latency. Early morning awakening is defined as a time of being continuously awake from the last stage of the sleep until the end of the sleep record (usually at 7 am).

In a non–24-hour sleep–wake cycle, a patient may report intermittent insomnia that periodically recurs. It is usually found in blind individuals and results from a complete failure of the resetting mechanism of the pacemaker. The patient then begins to live with a propensity to have a sleep–wake rhythm with the inherent and uncorrected period of the internal pacemaker, approximately 24.15 hours. In a world in which day and night follow a 24-hour cycle, this means that the patient’s propensity for sleep is constantly shifting forward relative to what would be appropriate to his or her surroundings. The result is that he or she experiences insomnia, which is periodically exacerbated when his or her internal rhythm is most out of phase with the environment. Melatonin administration has been demonstrated to be useful in regulating sleep in these individuals. Delayed sleep phase syndrome is a persistent pattern of late sleep onset and late awakening times, with an inability to fall asleep and awaken at a desired earlier time. Advanced sleep phase syndrome, in contrast, is early sleep onset and early awakening, with an inability to fall asleep and awaken at a desired later time. Irregular sleep–wake rhythm is characterized by constantly shifting hours of sleepiness and wakefulness. It is generally a disorder disco aficionados and others who maintain a highly irregular schedule, although it is sometimes seen in persons who have had tumors or other pathology of the hypothalamus. Treatment is organized around altering behavior, encouraging the patient to keep regular hours of bedtime and arising and to avoid napping.

Nightmares are vivid dreams that become progressively more anxiety producing, ultimately resulting in an awakening. They can occur occasionally in as many as half of children in the range of 3 to 6 years of age. In contrast to night terrors, the child is not confused, does not exhibit the massive autonomic signs, and describes having had a scary dream. Nightmares in children are not associated with psychiatric illness; in contrast, approximately half of the roughly 1 percent of adults who experience frequent nightmares are found to have disorders, including borderline personality and schizophrenia. Nightmares are sometimes brought out by REM-suppressing drugs, in which case, the treatment is to gradually discontinue the medication, if possible. Some workers suggest that nightmares occur more frequently in persons with certain personality traits, including those with thin boundaries and more creative individuals. There is no widely accepted medical intervention.

Prolonged periods of sleep deprivation sometimes lead to ego disorganization, hallucinations, and delusions. Depriving persons of REM sleep by awakening them at the beginning of REM cycles increases the number of REM periods and the amount of REM sleep (rebound increase) when they are allowed to sleep without interruption. Studies have also shown that REM deprivation leads to a shortening of REM latency (not an increase). There has been no evidence of increased frequency of nighttime awakenings in studies of REM deprivation.

REM-deprived patients may exhibit irritability and lethargy. In studies with rats, sleep deprivation produces a syndrome that includes a debilitated appearance, skin lesions, increased food intake, weight loss, increased energy expenditure, decreased body temperature, and death. The neuroendocrine changes include increased plasma norepinephrine and decreased plasma thyroxine levels.

It has become apparent that the hypocretin system plays a critical role in narcolepsy. Narcolepsy is the prototypical example of sleepiness produced by a basic central nervous system dysfunction of sleep mechanisms. The etiology stems from a genetically triggered hypocretin dysfunction and deficit.