Characteristics of spontaneous sleep with varying NREMS episodes in healthy men: Implication for delta activity homeostasis

Study Objectives: We investigated the effects of the variation in the numbe r of NREMS episodes on human sleep, in particular on delta activity distrib ution. Participants: Forty-one medication-free healthy men without personal or fam ily history of psychiatric disorders. Interventions: Subjects slept four consecutive nights, the last three were analyzed. Sleep was undisturbed and uninterrupted following self-selected s chedule. Results: With the exception of one night out of 123, subjects slept at leas t four NREMS episodes with 80% presenting five or six episodes per night. Y et, only 24% slept the same number of NREMS episodes across three nights. S horter first NREMS episode was associated with greater number of NREMS epis odes, and the total number of NREMS episodes was significantly predicted :b y the duration of the first one. Whether subjects slept four, five or six N REMS episodes, the proportion of TST spent in REMS, total : delta power, to tal spectral power and the rate of delta power in the first NREMS episode d id not differ between subjects. In each night, the distribution of delta ac tivity across the first four NREMS episodes was not modified by the total n umber of NREMS episodes. Conclusion: When spontaneous sleep is allowed, healthy men mostly sleep bey ond the fourth NREMS episode. Based on delta activity distribution across t he first four episodes, we have demonstrated that the number of NREMS episo des does not modify delta sleep homeostasis. Only duration of first NREMS e pisode predicts the number of NREMS episodes slept. These results suggest t hat sleep homeostasis and NREMS-REMS alternation may be two independent pro cesses.