Effects of a Nocturnal Environment Perceived as Warm on Subsequent Daytime Sleep in Humans

Abstract

BED-TIME present alterations in their nocturnal sleep. Several adverse effects, which are known to reduce the subsequent vigilance level, have been determined, such as partial sleep deprivation and/or sleep fragmentation. Surprisingly, the influence of thermal stressors on subsequent daytime recovery sleep remains unknown, as only one such study has been made, 5 but it did not provide the magnitude of sleep changes. Moreover, the hypnic alterations provoked by thermal stressors may be enhanced by the unusual circadian placement and associated sleep deprivation.In the present study, we analyzed the daytime sleep patterns of individuals kept awake and exposed to a moderate warm stress during the preceding night. Because our interest was focused strictly on the influence of the thermal factor, we controlled the circadian placement and the duration of the diurnal sleep session, and thus also the sleepdeprived state of the volunteers participating in this study. MATERIALS AND METHODSSixteen young male subjects (20-25 years old) gave their informed consent to participate in the study, which had been approved by the local ethics committee. These volunteers were good sleepers, with sleeping habits intermediary between morning and evening types. 28 They had not been exposed and/or naturally adapted to a warm climate during the six months prior to the experiment.The subjects were randomly assigned to two equal independent groups (control and experimental) and were then studied in pairs. One pair was studied per week, with control pairs alternating with experimental pairs. Before starting the recordings, they were habituated to the experimental and sleep (00:00 -08:00) conditions for two successive nights and days. The habituation period was spent in a living room controlled in temperature. During the third night each subject went to an individual climatic (soundproof) chamber adjacent to the living room and was kept awake (23:00 -08:00). A light breakfast was served at 06:00 (<100kcal). Daytime sleep is usually shorter than nighttime sleep. In a previous experiment, thermally comparable to the present control condition, 10 subjects (also awake at night) slept ad lib (from 08:00) an average of 6.2hours (unpublished data). We thus limited the sleep duration in the present study to six hours (light switched off at 08:00). Throughout the experiment, subjects always slept partially Abstract: We studied the influence of a nocturnal environment perceived as warm on the subsequent daytime sleep of healthy human subjects (20-25 years old). From 00:00 to 8:00, they were kept awake and exposed to either a thermoneutral and comfortable (CN) or a warm and uncomfortable (EW) environment, as assessed by the predicted mean vote/percentage of persons dissatisfied questionnaire (PMV/PPD). The subjects then slept from 8:00 to 14:00 in a thermoneutral environment. Sleep was scored according to the Rechtschaffen and Kales criteria. Rectal temperature was recorded from 22:00 to the end of the sleep period. Compared to CN, a significant but mod...

Effects of a moderate nocturnal cold stress on daytime sleep in humans

“…This invariance was also found in young adults exposed to a moderate warm stress at night (Bradley 1994;Dewasmes et al 2000). Eventually, bearing in mind that there are limited details available in the Bradley paper, it appears that SWS occurring during a morning sleep is relatively insensitive to prior moderate nocturnal passive warming or cooling.…”

“…This hypothesis was notably corroborated by the fact that an active thermal stress (heat storage due to exercise) can also diachronically increase the nocturnal REM sleep, on the condition that the imposed thermal stress remains modest, thus enabling the brain to deal with it (Buguet et al 1998). REM sleep being involved in memory processing (Dujardin et al 1990;Hennevin et al 1995;Smith 1995), we also assumed that ''consciously felt thermophysiological changes'' could be potent sensory informations that may be open to retention during subsequent REM sleep (Dewasmes et al 2000). Eventually, it may be that the REM-sleep increase observed helps an individual to better tolerate moderate thermal stresses, especially if they are repeated.…”

“…In her pioneer study, Bradley (1994) mentioned that the morning sleep patterns of individuals exposed to short-term heating at night were altered, but she did not provide the magnitude of the changes. Recently, we clearly showed that the REM-sleep quotas of young adults exposed to a moderate nocturnal warm stress were strikingly augmented during subsequent morning sleep (Dewasmes et al 2000). Because this REM-sleep increase could not be ascribed to sleep deprivation per se (a controlled effect), we assumed that it was due to the stress associated with the thermal discomfort consciously perceived by the subject (Dewasmes et al 2000).…”

“…Recently, we clearly showed that the REM-sleep quotas of young adults exposed to a moderate nocturnal warm stress were strikingly augmented during subsequent morning sleep (Dewasmes et al 2000). Because this REM-sleep increase could not be ascribed to sleep deprivation per se (a controlled effect), we assumed that it was due to the stress associated with the thermal discomfort consciously perceived by the subject (Dewasmes et al 2000). This hypothesis was notably corroborated by the fact that an active thermal stress (heat storage due to exercise) can also diachronically increase the nocturnal REM sleep, on the condition that the imposed thermal stress remains modest, thus enabling the brain to deal with it (Buguet et al 1998).…”

“…The sensitivity of human nighttime sleep to passive thermal challenges has been extensively explored (Buguet et al 1976;Bunnell et al1988;Candas et al 1982;Di Nisi et al 1989;Haskell et al 1981;Horne and Shackell 1987;Jordan and Montgomery 1990;Karacan et al 1978;Libert et al 1988;Muzet et al 1983), but not that of daytime sleep which remains anecdotally investigated (Bradley 1994 ;Dewasmes et al 2000). In her pioneer study, Bradley (1994) mentioned that the morning sleep patterns of individuals exposed to short-term heating at night were altered, but she did not provide the magnitude of the changes.…”