Ultrashort sleep-waking schedule. III. 'Gates' and 'forbidden zones' for sleep.

Three experiments which utilized an ultrashort sleep-waking cycle were conducted to investigate the 24 h structure of sleepiness after 1 night of sleep deprivation under 2 experimental conditions: instructing subjects to attempt to fall asleep or instructing subjects to attempt to resist sleep. Six subjects participated in experiment 1. At 19.00 h they started a 13 min waking-7 min sleep attempt, or 13 min waking-7 min resisting sleep, until 19.00 h on the next day. Eight subjects were tested in a similar way in experiment 2, which started at 07.00 h after a night of sleep deprivation and lasted for 24 h. Eight subjects were similarly tested in experiment 3 which started at 11.00 h after a night of sleep deprivation and lasted for 36 h until 23.00 h on the next day. The results showed that in spite of the significant between-group differences in total sleep, the temporal structure of sleepiness was very similar in the 3 experiments. In each there was a bimodal distribution of sleepiness: a major nocturnal sleepiness crest and a secondary mid-afternoon sleepiness peak. These were separated by a 'forbidden zone' for sleep centred at around 20.00-22.00 h. The onset of the nocturnal sleep period (the sleep gate) was found to be a discrete event occurring as an 'all or none' phenomenon. Its timing was stable over a 2 week period, and independent of the specific experimental demands; there were no significant differences between the AS and RS conditions with respect to total sleep time or any of the sleep stages. These results, which demonstrate structured variations in sleepiness across the nycthemeron are discussed in the light of the recent modelling of sleep along homeostatic principles.