Sleep adaptation after phase shifts of the sleep--wakefulness rhythm in man.

The sleep of 15 young adult subjects was polygraphically recorded before and after 8 hr phase shifts of the sleep--wakefulness rhythm in an isolation unit in which the natural cues to the time of day were absent. Following a phase delay, there was a significant increase in the amount of REM and a substantial alteration in its distribution within the sleep episodes, with the amount in the first third doubled. There were differences in the rates of adaptation among subjects, but this was generally accomplished within a week; this finding was reinforced by a similar response to a second identical shift. After a phase advance there was a significant reduction in the amount of REM and similar alterations in its distribution to those obtained after the phase delay. However, adaptation was not as clear as with the phase delay, and a further identical shift gave indications that adaptation had not been fully achieved. These results provide further evidence for a circadian rhythm of REM, with a maximum propensity in the morning, which does not adapt immediately to a sudden 8 hr phase shift of the sleep--wakefulness rhythm. However, adaptation can be achieved within a week with phase delay, while this is more difficult to detect for phase advance.