Two types of insomnia: too much waking or not enough sleep.

The stability of sleep was examined in two kinds of induced insomnia, namely after caffeine administration and after hypnotic drug withdrawal. The duration of each episode of any one sleep stage or any episode of intervening wakefulness plus drowsiness was determined. After caffeine there was an increase in longer episodes of intervening wakefulness plus drowsiness, but no significant change in the episode duration of any of the sleep stages. In the case of drug withdrawal there was no change in the episode duration of intervening wakefulness plus drowsiness, but there was a significant shortening of episode duration in sleep stages 2 and 3+4, with a similar trend for REM sleep episodes. Caffeine 'insomnia' thus seems characterized by increased stability of wakefulness, and hypnotic withdrawal 'insomnia' by decreased stability fo sleep. The type of analysis undertaken in this study could increase understanding of other types of insomnia.