The Double Sensor—A non-invasive device to continuously monitor core temperature in humans on earth and in space

The objective of our study was to establish whether rectal temperature recordings in humans could be replaced by a non-invasive skin temperature sensor combined with a heat flux sensor (Double Sensor) located at the forehead to monitor core body temperature changes due to circadian rhythms. Rectal and Double Sensor data were collected continuously for 24 h in seven men undertaking strict head-down tilt bed-rest. Individual differences between the two techniques varied between −0.72 and +0.55 °C. Nonetheless, when temperature data were approximated by cosinor analysis in order to compare circadian rhythm profiles between methods, it was observed that there were no significant differences between mesor, amplitude, and acrophase (P > 0.310). It was therefore concluded that the Double Sensor technology is presently not accurate enough for performing single individual core body temperature measurements under resting conditions at normal ambient room temperature. Yet, it seems to be a valid, non-invasive alternative for monitoring circadian rhythm profiles.