Soporifics Hypnotics

During sleep, the brain generates a patterned rhythmic activity that can be monitored by means of the electroencephalogram (EEG). Internal sleep cycles recur 4 to 5 times per night, each cycle being interrupted by a Rapid Eye Movement (REM) sleep phase (A). The REM stage is characterized by EEG activity similar to that seen in the waking state, rapid eye movements, vivid dreams, and occasional twitches of individual muscle groups against a background of generalized atonia of skeletal musculature. Normally, the REM stage is entered only after a preceding non-REM cycle. Frequent interruption of sleep will, therefore, decrease the REM portion. Shortening of REM sleep (normally approx. 25% of total sleep duration) results in increased irritability and restlessness during the daytime. With undisturbed night rest, REM deficits are compensated by increased REM sleep on subsequent nights (B).

Hypnotics fall into different categories, including the benzodiazepines (e.g., triazolam, temazepam, clotiaze-pam, nitrazepam), barbiturates (e.g., hexobarbital, pentobarbital), chloral hydrate, and ^-antihistamines with sedative activity (p. 114). Benzodiazepines act at specific receptors (p. 226). The site and mechanism of action of barbiturates, antihistamines, and chloral hydrate are incompletely understood.

All hypnotics shorten the time spent in the REM stages (B). With repeated ingestion of a hypnotic on several successive days, the proportion of time spent in REM vs. non-REM sleep returns to normal despite continued drug intake. Withdrawal of the hypnotic drug results in REM rebound, which tapers off only over many days (B). Since REM stages are associated with vivid dreaming, sleep with excessively long REM episodes is experienced as unre-freshing. Thus, the attempt to discontinue use of hypnotics may result in the impression that refreshing sleep calls for a hypnotic, probably promoting hypnotic drug dependence.

Depending on their blood levels, both benzodiazepines and barbiturates produce calming and sedative effects, the former group also being anxiolytic. At higher dosage, both groups promote the onset of sleep or induce it (C).

Unlike barbiturates, benzodiaze-pine derivatives administered orally lack a general anesthetic action; cerebral activity is not globally inhibited (respiratory paralysis is virtually impossible) and autonomic functions, such as blood pressure, heart rate, or body temperature, are unimpaired. Thus, benzo-diazepines possess a therapeutic margin considerably wider than that of barbiturates.

Zolpidem (an imidazopyridine) and zopiclone (a cyclopyrrolone) are hypnotics that, despite their different chemical structure, possess agonist activity at the benzodiazepine receptor (p. 226).

Due to their narrower margin of safety (risk of misuse for suicide) and their potential to produce physical dependence, barbiturates are no longer or only rarely used as hypnotics. Dependence on them has all the characteristics of an addiction (p. 210).

Because of rapidly developing tolerance, choral hydrate is suitable only for short-term use.

Antihistamines are popular as nonprescription (over-the-counter) sleep remedies (e.g., diphenhydramine, doxylamine, p. 114), in which case their sedative side effect is used as the principal effect.

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