Effect of 21 days of horizontal bed rest on behavioural thermoregulation

The present study investigated the effect of 21 days of horizontal bed rest on cutaneous cold and warm sensitivity, and on behavioural temperature regulation. Healthy male subjects (N = 10) were accommodated in a hospital ward for the duration of the study and were under 24-h medical care. All activities (eating, drinking, hygiene, etc.) were conducted in the horizontal position. On the 1st and 22nd day of bed rest, cutaneous temperature sensitivity was tested by applying cold and warm stimuli of different magnitudes to the volar region of the forearm via a Peltier element thermode. Behavioural thermoregulation was assessed by having the subjects regulate the temperature of the water within a water-perfused suit (T wps) they were wearing. A control unit established a sinusoidal change in T wps, such that it varied from 27 to 42°C. The subjects could alter the direction of the change of T wps, when they perceived it as thermally uncomfortable. The magnitude of the oscillations towards the end of the trial was assumed to represent the upper and lower boundaries of the thermal comfort zone. The cutaneous threshold for detecting cold stimulus decreased (P < 0.05) from 1.6 (1.0)°C on day 1 to 1.0 (0.3)°C on day 22. No effect was observed on the ability to detect warm stimuli or on the regulated T wps. We conclude that although cold sensitivity increased after bed rest, it was not of sufficient magnitude to cause any alteration in behavioural thermoregulatory responses.