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Naps and Circadian Rhythms in Postmenopausal Women

Departments of ¹ Psychiatry
² Family and Preventive Medicine, and Sam and Rose Stein Institute for Research on Aging, University of California, San Diego.

Address correspondence to Daniel F. Kripke, MD, Department of Psychiatry, 0667, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0667. E-mail: dkripke{at}ucsd.edu

Background. Napping patterns and relationships among naps, circadian rhythms, and nocturnal sleep were evaluated in postmenopausal women. Naps and nocturnal sleep were compared between depressed women and healthy volunteers.

Methods. The study included 436 postmenopausal women between the ages of 50 and 81 years. Psychiatric interviews were conducted using the Structured Clinical Interview for DSM-IV Axis I Disorders. Each participant wore a wrist-activity monitor and kept a daily sleep log at home for 1 week. The major urinary melatonin metabolite (6-sulfatoxymelatonin [aMT6s]) was measured for two 24-hour periods at home.

Results. There was no significant difference in naps and nocturnal sleep patterns between depressed participants (n = 30) and those with no history of mental disorder (n = 222). Three peaks occurred in the timing of napping: approximately 1 hour before bedtime, 8 to 9 hours after wake-up time, and 2 hours after waking, in descending order of magnitude. Significant inverse correlations were observed between evening nap duration and both wake-up time and aMT6s acrophase. The onset time of aMT6s excretion was advanced by 32 minutes in evening nappers.

Conclusions. Evening napping, a characteristic of these postmenopausal women, may be related to an advance of circadian rhythms in this age group. Relationships between evening napping and earlier wake-up time may be a common manifestation of advanced circadian rhythms or may be a homeostatic effect.