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The effects of evening bright-light on human body temperature and sleep architecture


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Croft, Tobias Daniel 1996 , 'The effects of evening bright-light on human body temperature and sleep architecture', PhD thesis, University of Tasmania.

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A number of influential theories of sleep function argue for a close
relationship between body temperature and sleep architecture. The energy
conservation theory and thermoregulatory theories of sleep, including the central
nervous system restoration and protection theories, each predict increased Slow
Wave Sleep (SWS) following elevations in body temperature at sleep onset and
during sleep. It has recently become apparent that appropriately timed evening
bright-light (BL) (>2500 lux) may suppress melatonin, elevate core temperature,
and produce delays of the human Core Temperature Rhythm (CTR). While these
effects are being investigated for their potential within industry and to treat
various affective and sleep related disorders, the effects of evening BL on sleep
are unclear as the literature is small and differing results are reported. The effects
of BL on temperature suggest that enhanced SWS might follow evening BL, as
increased SWS has been empirically associated with elevations in temperature at
sleep onset and, to a lesser extent, with a phase-delay of the CTR. However, the
suppression of melatonin associated with evening BL suggests sleep might be
disrupted, as melatonin has been proposed to be a hypnotic agent.
In the present thesis three experiments are reported which were designed
to investigate the effects of evening BL on rectal temperature and sleep. In the
first experiment 11 male subjects were twice exposed to BL or Dim-Light (DL)
(normal room illumination) for 2hrs prior to habitual bedtime for two consecutive
nights in a crossover design. Rectal temperature was significantly elevated
during the first and second hours following BL, and significantly more Stage 3
sleep and SWS occurred with a trend for increased SWS in the fourth sleep cycle.
The second experiment was designed to assess the immediate effects of
evening BL on core temperature and sleep, independent from potential CTR
phase-shifting effects produced after multiple exposures to evening BL on consecutive evenings. 11 male subjects were run in two conditions. In the BL
condition Ss were exposed to BL for three consecutive evenings and to DL on the
fourth. In the dim-light condition Ss were exposed to DL on all four evenings. It
was anticipated that three consecutive exposures to evening BL would phasedelay
the CTR such that the effects of a temperature rhythm delay on sleep
could be assessed independent of BL itself on night 4. On night 1 no significant
elevation in rectal temperature was found but SWS was increased in the fourth
sleep cycle. On night 3 rectal temperature was significantly elevated during the
first two hours following BL, and SWS and Slow Wave Activity (SWA) (.25-3
Hz EEG activity) were significantly increased across the night, most noticeably in
the third sleep cycle. No differences in the position of the CTR were evident on
night 4, but temperature in the BL condition was significantly lower than in the
DL condition during the first and fifth hours following light exposure. In
addition, sleep onset latency (SOL) and amounts of Wake were increased in the
BL condition on this night. The results indicated that BL administered until
habitual bedtime over three consecutive nights may produce immediate effects on
core temperature without necessarily delaying the CTR. The SWS enhancing
effects of evening BL were found when, and only when, core temperature was
significantly elevated around the time of sleep onset. This experiment also
suggested that these effects are found more robustly following more than one
exposure to BL.
The third experiment was designed to test this hypothesis. A betweensubjects
design was utilised in which three groups of 12 male subjects were run
in three conditions. In the Dim-Dim (DD) condition subjects were exposed to
DL for two consecutive nights. In the Dim-Bright (DB) condition subjects were
exposed to DL on night 1 and BL on night 2 over consecutive nights. In the
Bright-Bright (BB) condition subjects were exposed to BL on both nights.
Rectal temperature during the first hour following light treatment was
significantly elevated in the BB condition and non-significantly elevated in the
, DB condition compared to the DD condition. SWA and Total EEG Power (TP)
(0.25 Hz to 50 Hz EEG activity) were enhanced in both the DB and BB conditions. In addition, significantly more SWS was found to occur in the fourth
sleep cycle in the DB and BB conditions compared with the DD condition. Thus,
evening BL elevated rectal temperature and enhanced SWS upon a single
exposure, but these effects were enhanced by exposure to the light on the
previous night.
Taken together these experiments indicate that evening bright light
elevates rectal temperature and SWS/SWA increases during subsequent sleep,
particularly late in the night. These results are consistent with other experimental
paradigms in which temperature is raised (e.g. exercise and passive heating) and
SWS is observed to increase. Interpreted this way the results add to the body of
evidence that suggests SWS and thermoregulation are intimately linked. Another
possible interpretation is that a rebound in melatonin following early suppression
by bright-light produced increased SWS later the same night. It is also possible
that both thermal and melatonin rebound effects occurred following evening
bright-light; the thermal effects maintained SWS levels under conditions of
melatonin suppression early in the sleep period while melatonin rebound later in the sleep period resulted in the most prominent increases in SWS. Further
research might examine this possibility by monitoring plasma melatonin levels,
temperature and SWS continuously following evening bright-light. The major
finding of this thesis is that bright-light administered until habitual bedtime
produces immediate elevations in temperature and increased SWS/SWA,
especially late in the night.

Item Type: Thesis - PhD
Authors/Creators:Croft, Tobias Daniel
Keywords: Sleep, Body temperature
Copyright Holders: The Author
Copyright Information:

Copyright 1996 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1996. Includes bibliographical references (leaves 123-147)

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