Circadian modulation of sequence learning under high and low sleep pressure conditions.

Humans are able to learn complex sequences even without conscious awareness. We have studied the repercussions of circadian phase and sleep pressure on the ability to learn structured sequences using a serial reaction time task (SRT). Sixteen young healthy volunteers were studied in a 40-h "constant posture protocol" under high sleep pressure (i.e. sleep deprivation) and low sleep pressure conditions (i.e. sleep satiation attained by multiple naps). Here we show that learning of different sequence structures improved after multiple naps, in particular after naps that followed the circadian peak of rapid-eye-movement (REM) sleep. This situation following sleep contrasted with the lack of learning without sleep. We have evidenced that the observed amelioration of learning new sequences came about by memorizing short sub-fragments ("chunks") of the sequence train. However, SRT performance did not deteriorate under high sleep pressure, despite the high level of sleepiness. Our data indicate that sequence learning is modulated by circadian phase, and the neurophysiological medium required for this type of learning is related to sleep.