The return to sleep.

Six young adult subjects were awakened five to eight times per night from stage 2 sleep in a standardized manner for a series of at least 11 non-consecutive nights. After adaptation to the procedure, subjects received placebo, pentobarbital, or flurazepam on two random nights and caffeine on one night. The latency of the return to sleep after each awakening was measured. On placebo nights a characteristic U-shaped curve of latency as a function of time of night was found. Latencies were long shortly after sleep onset but decreased rapidly to about 50 sec before beginning an approximately linear logarithmic increase throughout the rest of the night. The drugs characteristically altered this time course. Pentobarbital decreased latencies in the first half of the night. Flurazepam decreased latencies throughout the night. Caffeine increased latencies during the first half of the night.