Chronobiological rhythm constraints of memory processes.

Memory performances are not constant but undergo fluctuations whose periodicity may be analyzed. Low frequency infradian rhythms (periods longer than 28 hours), circadian rhythms (periods between 20 and 28 hours) and ultradian rhythms (periods less than 20 hours) are involved in memory disorders. Paradoxical sleep represents a preferential period in which certain processes, such as the activation of the central nervous system for example, may facilitate learning process. Relations between sleep and memory lead to the discussion of the incidence of the cyclic nature of sleep (and notably the rhythm of 90 minutes' paradoxical sleep) on the daily course of cerebral activation and its effects on the variable aptitude of our capacity for remembering. The close interrelation between biological rhythms and cognition is demonstrated using two examples of mental disorders in which severe memory deficiencies are observed.