Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness

We studied the relationship between the phase and the amplitude of the circadian temperature rhythm using questionnaires that measure individual differences in personality variables, variables that relate to circadian rhythms, age and sex. The ambulatory core body temperature of 101 young men and 71 young women was recorded continuously over 6 days. The temperature minimum (T-min) and amplitude (T-min) were derived by fitting a complex cosine curve to each day's data for each subject. Participants completed the Horne-Ostberg Morningness-Eveningness Questionnaire (MEQ), the Circadian Type Inventory (CTI) and the MMPI-2, scored for the Psychopathology-5 (PSY-5) personality variables. We found that the average T-min occurred at 03.50 h for morning-types (M-types), 05.02 h for the neither-types and 06.01 h for evening-types (E-types). Figures were presented that could provide an estimate of T-min given an individual's morningness-eveningness score or weekend wake time. The T-min occurred at approximately the middle of the 8-h sleep period, but it occurred closer to wake in subjects with later T-min values and increasing eveningness. In other words, E-types slept on an earlier part of their temperature cycle than M-types. This difference in the phase-relationship between temperature and sleep may explain why E-types are more alert at bedtime and sleepier after waking than M-types. The T-min occurred about a half-hour later for men than women. Another interesting finding included an association between circadian rhythm temperature phase and amplitude, in that subjects with more delayed phases had larger amplitudes. The greater amplitude was due to lower nocturnal temperature.