Estimates of the daily phase and amplitude of the endogenous component of the circadian rhythm of core temperature in sedentary humans living nychthemerally
J. Waterhouse et al., Estimates of the daily phase and amplitude of the endogenous component of the circadian rhythm of core temperature in sedentary humans living nychthemerally, BIOL RH RES, 31(1), 2000, pp. 88-107
Fifteen healthy female subjects were studied for eight days while living co
nventionally. Subjects were free to choose the ways they spent their time w
ithin a framework of regular times of retiring and rising; in practice, muc
h of the waking time was spent in sedentary activities. Nine of the subject
s were aware of the natural light-dark cycle, this approximating to a 12: 1
2 L:D schedule at the time of year when the study took place. Before the st
udy, subjects were assessed for their degree of "morningness" by questionna
ire; throughout the study, they wore a rectal probe, and an activity meter
on their nondominant wrist. The timing (phase) and amplitude of the circadi
an rectal temperature rhythm were assessed on each day by cosinor analysis
as well as by a method based on visual inspection of the data. These two pa
rameters were also assessed after the temperature data for each day had bee
n "purified" by a number of methods. From these results it was possible to
investigate the effect of purification upon the amplitude of the circadian
rhythm of temperature. Also, the day-by-day variability of phase, and the r
elationship between morningness and phase, were compared using these method
s of phase estimation, and using cross-correlation between data sets from a
djacent days; in all cases, raw and purified temperature data were used. Th
ere was a significantly greater amount of daily variation in phase using pu
rified rather than raw data sets, and this difference was present with all
methods of purification as well as with all methods for estimating phase. P
urification decreased the amplitude of the circadian temperature rhythm by
about 30%. Finally, there was a significant correlation between the morning
ness score of the subjects and the phase of the circadian temperature rhyth
m, the phase becoming earlier with increasing morningness; when this relati
onship was re-examined using purified data, it became more marked. These re
sults reflect the masking effects exerted upon raw temperature data by life
style. The extent to which the purification methods enable the endogenous c
omponent of a circadian rhythm - and, by implication, the output of the end
ogenous circadian oscillator - to be estimated in subjects living normally
is addressed.