A comparison between rhythms in forehead skin and rectal (core) temperature in sedentary subjects living in a thermally neutral environment

Abstract

In human circadian rhythm research, rectal temperature is the traditional gold standard for estimating core temperature, but the measurement of rectal temperature is inconvenient and unacceptable to some subjects. This study examined whether the circadian rhythm of forehead skin temperature would be an alternative to that of rectal temperature in sedentary subjects living in a thermally neutral environment. Six healthy female volunteers participated as subjects for 37.5 h, from 21:30 h on day 0 to 06:30 h on day 2. They were tested twice while wearing two different types of clothing. One type of clothing covered the whole body except the head and hands (Type L); the other exposed the head and distal extremities (Type H). Subjects lived in a climatic chamber controlled at 23.8 ± 0.2 °C and 60 ± 5% RH with light intensity of 500 lux from 06:30 to 19:30 h, 100 lux from 19:30 to 22:30 h and 0 lux from 22:30 to 06:30 h. Rectal temperature and skin temperatures at seven sites were recorded continuously at 10 min intervals. The extent to which the circadian rhythms of forehead and hand skin temperature mirrored that of rectal temperature was determined by cross-correlation and cosinor analyses. Cross-correlation analysis of data from the group or individuals, and whether wearing Type L or Type H clothing, indicated that the profile of forehead skin temperature was similar to that of rectal temperature, with highly significant positive correlation coefficients (r = 0.9099–0.9744) and time lags of 0.17–0.33 h. By contrast, the profile of hand skin temperature was approximately in antiphase, with smaller correlation coefficients (though still highly significant: r = 0.6036–0.7526), and with time lags of 11.00–14.11 h. Cosinor analysis indicated that the acrophase of the circadian rhythm of forehead skin temperature for individuals was slightly, but not significantly, earlier than that of rectal temperature with both types of clothing (0.54 ± 0.68 and 0.35 ± 0.64 h with Type L and Type H clothing, respectively). The circadian rhythm of forehead skin temperature showed a significantly greater amplitude and lower mesor than rectal temperature, whether wearing Type L or Type H clothing. Individuals’ data gave 0.38 ± 0.09 and −2.92 ± 0.31 °C, respectively, with Type L clothing and 0.40 ± 0.26 and −2.86 ± 0.35 °C with Type H clothing; these differences were not significantly different between the two types of clothing. It is concluded that, in sedentary subjects living in a thermally neutral environment, forehead skin temperature gives information regarding phase and some aspects of amplitude that can act as substitutes for rectal temperature data.