Sleep 1993 Oct;16(7):638-40
When the human circadian system is caught napping: evidence for endogenous rhythms close to 24 hours.
Campbell SS, Dawson D, Zulley J
Department of Psychiatry, Cornell University Medical College, White Plains, New York 10605.
It is now well acknowledged that napping constitutes an inherent component of the human circadian system. To date, however, few studies have examined the effects of spontaneous napping on human free-running rhythms. This study investigated the free-running circadian periods of rest/activity and body core temperature in a group of young subjects who were permitted to nap during their time in isolation. Based on the frequency of self-reported sleep bouts, subjects were classified as Nappers or Nonnappers. Nappers exhibited free-running rhythms in both rest/activity and body core temperature that were not significantly different from 24 hours. Nappers showed a tendency for shorter free-running periods in both variables, when compared with Nonnappers. These findings emphasize the need for careful reassessment of data obtained from traditional free-run protocols. PMID: 8290857, UI: 94120298
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Psychophysiology 1989 Sep;26(5):580-5 Evidence for circadian influence on human slow wave sleep during daytime sleep episodes.
Campbell SS, Zulley J
The occurrence of slow wave sleep within spontaneously initiated daytime sleep episodes was studied to examine hypothesized associations with prior wakefulness and circadian factors. There was a strong relationship between measures of slow wave sleep and the proximity of sleep episodes to the maximum of body core temperature. Those sleep episodes that began within 4 hours of the maximum in body core temperature contained significantly more slow wave sleep than did all other daytime sleep periods, approximating proportions typical of nocturnal sleep. Multiple regression analysis revealed no relationship between measures of slow wave sleep and prior wakefulness. These findings are consistent with an hypothesized approximately-12-hour rhythm in the occurrence of slow wave sleep and they underscore the influence imposed on human sleep by the endogenous circadian timing system.
PMID: 2616705, UI: 90139376
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Biol Psychiatry 1987 Mar;22(3):389-92 The influence of daytime naps on the therapeutic effect of sleep deprivation.
Wiegand M, Berger M, Zulley J, Lauer C, von Zerssen D PMID: 3814687, UI: 87129158
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Hum Neurobiol 1985;4(2):123-6 Napping behavior during "spontaneous internal desynchronization": sleep remains in synchrony with body temperature. Zulley J, Campbell SS Patterns of sleep and wakefulness exhibited in an environment without time cues are generally considered to be monophasic, with a distinct relationship between sleep episodes and the minimum of body core temperature. In some cases this relationship between major sleep episodes and temperature becomes replaced by an apparently varying phase relationship between the two variables called "spontaneous internal desynchronization". In the present study the sleep-wake and temperature data of six subjects living in an environment without time cues and exhibiting internal desynchronization were reanalyzed to include subjectively designated naps. Two groups of naps were identified based on their phase positions relative to temperature, with one group occurring around the temperature minimum and another group clustering approximately halfway between successive minima. The results support the suggestion that neither monophasic sleep placement nor sleep patterns typically associated with spontaneous internal desynchronization reflect biological sleep tendency. Rather, sleep tendency is reflected more accurately by the bimodal sleep patterns exhibited by subjects who are allowed to time their sleep and waking with no restrictions. PMID: 4030422, UI: 85288452