Masking of the Circadian Rhythms of Heart Rate and Core Temperature by the Rest-Activity Cycle in Man

Abstract

Heart rate and core temperature are elevated by physical activity and reduced during rest and/or sleep. These masking effects may confound interpretation of rhythm waveforms, particularly in situations where the rest-activity rhythm has a different period from that of the core temperature rhythm. Such desynchronization often occurs temporarily as an individual adjusts to a new work shift or to a new time zone following rapid transmeridian travel, making it difficult to assess the impact of such schedule changes on the circadian system. The present experiments were designed to estimate the magnitude of these masking effects, by monitoring the heart rate, rectal temperature, and nondominant wrist activity (2-min samples) of 12 male subjects during 6 days of normal routine outside the lab and during 6 days of strict bedrest. Subjects also kept sleep, dietary, and exercise logs throughout the study. Average (20- min) waveforms were computed for each subject and each rhythm, at home and in bedrest. In addition, data were partitioned according to self-reported sleep and wake times and were analyzed separately for each state. Average waveform comparisons indicated that about 45% of the range of the circadian heart rate rhythm during normal routine was attributable to the masking effects of activity during wake, which also produced a 16% elevation in mean heart rate during wake and an 11% increase in mean heart rate overall. (Analysis of variance indicated that mean heart rate during sleep at home was not significantly different from the mean during sleep in bedrest.) On average, about 14% of the range of the circadian temperature rhythm during normal routine was attributable to the effects of activity masking. However, the change in range of the temperature rhythm, from home to bedrest, was very variable between subjects (—41% to +13%). This variability was not accounted for by age or by reported frequency of exercise at home. Normal activity during wake increased the mean temperature during wake by an average of 0.16° C and the overall mean by about 0.12° C. (Analysis of variance indicated that mean temperature during sleep at home was not significantly different from the mean during sleep in bedrest.) A 10-hr "night" (lights-off from 2200 to 0800 hr) was provided during bedrest, within which subjects could select their own sleep times. Times of sleep onset and wake onset were not significantly different between home and bedrest. Nevertheless, most subjects showed small phase shifts in their heart rate and temperature rhythms, which were not significantly correlated with the small changes in sleep times, but were significantly correlated with subjects scores on the Horne and Ostberg (1976) morningness-eveningness questionnaire. These shifts in the heart rate and core temperature rhythms were probably attributable to masking-induced changes in waveshape, rather than to real phase shifts of the circadian timing system. These data indicate that normal wake-time activities produce major masking effects on the circadian heart rate rhythm and somewhat smaller effects on the temperature rhythm. This may be partially attributable to core temperature's being more rigorously conserved than heart rate, at least during moderate exercise.