• D J Dijk, J F Duffy, and C A Czeisler.
• Department of Medicine, Harvard Medical School, Brigham and Women's Hospital, Boston, MA, USA. d.j.dijk@surrey.ac.uk
• Sleep. 2001 Aug 1; 24 (5): 565-77.

Study Objectives(1) To assess the circadian and sleep-dependent regulation of the frequency and duration of awakenings in young and older people; (2) to determine whether age-related deterioration of sleep consolidation is related to an increase in the frequency or duration of awakenings; (3) to determine whether pre-awakening sleep structure is preferentially enriched by REM sleep or nonREM sleep and (4) to determine whether sleep structure prior to awakenings is affected by age.DesignBetween age-group comparison of sleep consolidation and sleep structure preceding awakenings.SettingEnvironmental Scheduling Facility, General Clinical Research Center.ParticipantsEleven healthy young men (21-30 years) and 13 older healthy men (n=9) and women (n=4) (64-74 years).InterventionsForced desynchrony between the sleep-wake cycle and circadian rhythms by scheduling of the rest-activity cycle to 28-h for 21-25 cycles.Measurements And ResultsCircadian and sleep-dependent regulation of the frequency and duration of awakenings and of sleep structure preceding awakenings were assessed in 482 sleep episodes (9h 20 min each). The circadian modulation of wakefulness within sleep episodes was primarily related to a variation in the duration of awakenings. In contrast, the age-related reduction of sleep consolidation was primarily related to an increase in the frequency of awakenings. Whereas in both young and older subjects pre-awakening sleep contained more REM sleep than overall sleep, this enrichment of REM sleep (i.e., the gating of wakefulness by REM sleep) was diminished in older people. In older people, preawakening sleep contained more nonREM sleep and stage two sleep in particular, than in young people.ConclusionsAt all circadian phases, the age-related reduction of sleep consolidation is primarily related to a reduction in the consolidation of nonREM sleep.

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