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

doi:10.1016/j.physbeh.2017.12.014

Physiology & Behavior

Volume 185, 1 March 2018, Pages 23-30

https://doi.org/10.1016/j.physbeh.2017.12.014 Get rights and content

Highlights

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Sleep on high heat capacity mattress increases body heat loss and slow wave sleep.
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Increase in conductive body heat loss via the back decreases core body temperature.
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Increased inner conductive heat transfer is suggested as crucial to increase SWS.
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Sleep on a high heat capacity mattress increases sleep stability.
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The study findings are similar to a weak form of daily torpor in certain animals.

Abstract

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 ± 2.1 yr, BMI: 22.2 ± 0.4 kg/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.

Introduction

It is common knowledge that a comfortable environment positively influences quality of sleep. Positive factors favoring initiation and maintenance of human sleep are darkness, a quiet setting, a supine body position, a familiar environment and a comfortable ambient temperature.

It has been shown that habitual sleep is closely related to the circadian rhythm of core body temperature (CBT), which is a resultant of the relationship between heat production and heat loss [1], [2]. Sleep onset usually coincides with the maximal rate of decline of CBT [3], [4], [5] initiated by increased skin blood flow, skin warming and body heat loss [6], [7]. All the mentioned processes are not only governed by endogenous circadian clock(s) but also modulated by so-called masking effects [8]. For instance, the process of sleep initiation includes behaviors such as lying down, switching the lights off, relaxation of mind and muscles all increasing blood redistribution from the core to the shell (skin) and thereby declining CBT and increasing skin temperature (ST). The latter most pronounced in distal skin regions [6], [7]. Under real life situation all these processes occur together and are all supportive for this mechanism. Besides investigations describing habitual sleep in relation to thermophysiology, many studies explored the possibility to thermally influence sleep via air temperature, bed clothes, or bathing before sleep [9], [10], [11], [12]. However, this is rather a complex endeavor and studies applying these methods reported contradictory results [13], [14]. One consistent finding was that ambient temperatures outside the thermal comfort zone disturbs initiation and/or maintenance of sleep and prevents a conclusive interpretation of thermal interventions on sleep [7], [12], [15]. It is therefore crucial to select an intervention that thermally affects the body only mildly, without provoking a counter-regulation and yet strong enough to trigger specific effects on sleep [13], [14]. For this purpose we have chosen a strategy of slowly removing body heat via conductive heat transfer by means of a high heat capacity mattress (HHCM). In comparison to a conventional low heat capacity mattress (LHCM), a HHCM has a higher thermal capacity thanks to its much higher density of the surface layer (1006 kg/m³ vs. 80 kg/m³). This difference should lead to an enhanced overnight body heat loss and this mild “thermic intervention” could therefore be useful to test the hypothesis that reduced CBT during the night leads to increased slow wave sleep (SWS, = N3 sleep) [16], [17], [18].

The main aim of this study was to examine the effects of a HHCM vs. LHCM upon sleep in relation to their different thermic properties. Or more specifically, is CBT decline enhanced and SWS increased under gentle core body cooling on HHCM, as previously found with mild air-cooling during night sleep [14].

Section snippets

Study subjects

After giving written informed consent, 28 male subjects, aged between 25 and 30 yr (BMI 19-25 kg/m²), were screened by a board-certified sleep medicine physician. Exclusion criteria were major not stabilized medical illnesses, history of alcoholism, drug dependence or abuse, neurological disorders, head trauma and mental disorders according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-V, American Psychiatric Association, 2013), Mini Mental State Examination score < 26, CNS

Temperatures

In comparison to LHCM, sleep on HHCM significantly reduced CBT (top graph in Fig. 1, Table 1). The 8 h-mean value was significantly reduced by 0.163 ± 0.035 °C [main effect: LHCM vs. HHCM, F(1,14) = 35.93, p = 0.0001; Table 1]. The reduction developed steadily, was most pronounced in the middle (− 0.28 °C 5 h after lights off) and disappeared at the end of the sleep phase leading to a significant interaction term [TIME × MATTRESS, F(14,47) = 2.427, p < 0.0001]. The first significant difference in CBT between

Discussion

The main outcome of the study is that in comparison to a conventional low heat capacity mattress (LHCM) subjects sleeping on a high heat capacity mattress (HHCM) significantly reduced core body temperature (CBT), proximal skin temperatures on the back (PROBA) and mattress surface temperature, and significantly increased sleep stage N3. Regression analyses revealed a significant relationship selectively between increased CBT-PROBA (and reduced PROBA) with enhanced sleep stage N3.

Conclusions

The present controlled laboratory study provides evidence that sleep characteristics can be influenced by the thermal property of the mattress. In comparison to a conventional LHCM sleep on the HHCM significantly reduced PROBA and CBT, and increased SWS and sleep continuity. Detailed analyses with respect to time courses of body and mattress surface temperatures clearly indicate that CBT reduction by HHCM was induced by conductive heat loss via back skin regions to the mattress. These findings

Declaration of interest

The authors report no conflicts of interest; they alone are responsible for the content and writing of the paper.

Sources of funding

Technogel Italia S.R.L. (Vicenza, Italy) supported the study through an unconditional funding to Neuroscience Department of University of Torino (FE 3.04.24.62 & FS 7.07.0960), Italy.

Acknowledgments

We thank Andy Schötzau (eudox, Statistische Beratung, Malzgasse 9, Basel, Switzerland) for his advice in statistics.

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

Highlights

Abstract

Introduction

Section snippets

Study subjects

Temperatures

Discussion

Conclusions

Declaration of interest

Sources of funding

Acknowledgments

Sleep Med. Rev.

Prog. Brain Res.

Physiol. Behav.

Trends Neurosci.

Behav. Brain Res.

Sleep Med.

Sleep Med. Rev.

How is the circadian rhythm of core body temperature regulated?

Clin. Auton. Res.

Rapid decline in body temperature before sleep: fluffing the physiological pillow?

Chronobiol. Int.

The dependence of onset and duration of sleep on the circadian rhythm of rectal temperature

Pflugers Arch.

Interaction between the sleep-wake cycle and the rhythm of rectal temperature

The interrelationship between sleep regulation and thermoregulation

Front Biosci (Landmark Ed).

Exogenous and endogenous components in circadian rhythms

Cold Spring Harb. Symp. Quant. Biol.

The interrelationship between sleep regulation and thermoregulation

Front Biosci (Landmark Ed).