Inter-individual differences in the effects of acute stress on the sleep-wakefulness cycle in the rat.

It has been described that an acute immobilization stress (IS) modifies subsequent paradoxical sleep (PS). However, its effects are complex because some subjects remain unaffected. This discrepancy might result from constitutive inter-individual psychobiological differences. In order to test this hypothesis, an inter-individual analysis of sleep patterns and their modifications after 60 min IS has been performed. Even though global analysis showed a PS increase after IS, inter-individual analysis evidenced different PS reactivity; subjects which had the least PS during control recordings were those with the largest PS increase. Unlike global analysis, an inter-individual study evidenced different modifications of wakefulness and slow wave sleep according to individuals. Subjects presenting the highest amount of wakefulness in control conditions (the lowest amount of slow wave sleep) decreased their wakefulness amount, while subjects with the lowest amount of wakefulness increased it. Thus, individual characteristics of the sleep-wakefulness cycle should be considered while studying its modifications induced by different treatments.