Reappearance of electroencephalogram slow waves in extended sleep with delayed bedtime

A three-part study using prolonged nights of sleep was undertaken to verify Broughton's hypothesis of an approximate 12-h ultradrian rhythm of human slow wave sleep (SWS). Part I consisted of 2 8-h adaptation nights followed by a prolonged 15-h night of sleep with bedtime at midnight. A significant return of SWS occurred 12 h and 32 min after the first appearance of SWS. In part II, after 1 adaptation night, subjects were asked to sleep for 15 h but bedtime was delayed until 0400 h. A two-peak return of SWS was observed with a first significant return at 1228 h and a second significant return at 1745 h (i.e., 13 h and 32 min after the first appearance of SWS). In part III, bedtime was again delayed to 0400 h; but subjects were given 3 nights to adapt before the 15 h extended sleep. A single significant return of SWS was then observed at 1656 h, i.e., 12 h and 24 min after the first appearance with no peaks around 1200 h, thus exhibiting the same pattern as in part I. These results suggest that the return of SWS seen normally between 1200 and 1500 h is relatively well entrenched since it remained present in the extended night following sudden bedtime delay. It appears, moreover, that SWS does follow a bimodal 12-h rhythm, which is seen immediately upon extended delayed sleep and can be fully phase-shifted with habituation.