Sleep on a high heat capacity mattress increases conductive body heat loss and slow wave sleep

Environmental temperature can strongly affect sleep. The habitual sleep phase is usually located between evening decline and morning rise of the circadian rhythm of core body temperature (CBT). However, the thermophysiological mechanisms promoting or disturbing sleep are not yet fully understood. The purpose of this study was to examine the effects of a high heat capacity mattress (HHCM) on CBT, skin temperatures and sleep in comparison to a conventional low heat capacity mattress (LHCM). Based on the higher heat capacity of HHCM an increase in conductive body heat loss enhances the nocturnal decline in CBT can be expected. Based on previous findings this may then be accompanied by an increase in slow wave sleep (SWS). The mattresses were studied in a randomized single-blind crossover design in fifteen healthy young men (Age: 26.9±2.1yr, BMI: 22.2±0.4kg/m²) by overnight in laboratory standard video-polysomnography in a temperature stabilized setting. CBT, room temperature, and skin and mattress surface temperatures were continuously recorded in order to get information about inner and outer body heat flow. Additionally, subjective sleep quality was estimated by visual analogue scale. In comparison to LHCM sleep on HHCM exhibited a selective increase in SWS (16%, p<0.05), increased subjective sleep quality and sleep stability [reduced cyclic alternating pattern (CAP) rate; 5.3%, p<0.01]. Additionally, analyses of the sleep stages showed in the second part of the night a significant increase in SWS and a decrease in REMS. In addition, HHCM induced a greater reduction in CBT (maximally by -0.28°C), reduced the increase in proximal skin temperatures on the back (PROBA; maximally by -0.98°C), and delayed the increase in mattress surface temperature (maximal difference LHCM-HHCM: 6.12°C). Thus, the CBT reduction can be explained by an increase in conductive heat loss to the mattress via proximal back skin regions. Regression analysis identified PROBA as the critical variable to predict inner conductive heat transfer from core to shell and SWS. In conclusion, the study expands the previous findings that a steeper nocturnal decline in CBT increases SWS and subjective sleep quality, whereas inner conductive heat transfer could be identified as the crucial thermophysiological variable, and not CBT.