Sleep and Memory

Abstract

Generally sleep is considered a time of amnesia. It is not uncommon for an individual to experience 8 h of sleep and have no memory for events during that time. Similarly, a substantial proportion of the population has no memory for dreams that occurred during the night, despite the fact that the literature on awakening during rapid eye movement (REM) sleep clearly shows that individuals normally have four to six “dream experiences” a night. Research on this issue seems to indicate that the lack of memory cannot be explained by the organisms’ inability to perceive stimuli. The data indicate that although perceptual thresholds are elevated, organisms can clearly perceive stimuli, and, in fact, can discriminate between them during sleep. The amnesia also cannot be explained by a defect in long-term memory, as studies have indicated that stimuli put into the memory during wakefulness are more efficiently retrieved after a sleep period than after a comparable period of wakefulness. The most likely explanation for the amnestic property of sleep seems to be the inability of organisms to transfer information from short-term memory to long-term memory during sleep. There are several sources of evidence to support this hypothesis. First, the probability of remembering a stimulus given during wakefulness is related to the proximity of sleep onset to the stimulus. Generally, information put into the system within 5 min of sleep onset is lost from memory. Secondly, disorders of excessive daytime somnolence which cause individuals to have frequent microsleeps are often associated with complaints of memory problems. When these patients are kept fully alert, or if their condition is reversed, they show perfectly normal memory functioning. Finally, anterograde amnesia is often associated with drugs that produce sleepiness. A good example of such a class of drugs is the benzodiazepines. The effect of benzodiazepines on anterograde amnesia is dose dependent and related to their hypnotic activity. Generally, when the drug produces sleep onset within 5 min of the stimulus it is likely to produce amnesia for the event. In summary, the process of memory consolidation is inhibited during sleep. This factor must be considered in all studies evaluating the direct effect of benzodiazepines on memory processes.