Seasonal changes in thermal responses of urban residents to cold exposure

To determine whether urban circumpolar residents show seasonal acclimatisation to cold, thermoregulatory responses and thermal perception during cold exposure were examined in young men during January-March (n=7) and August-September (n=8). Subjects were exposed for 24 h to 22 and to 10 degrees C. Rectal (T(rect)) and skin temperatures were measured throughout the exposure. Oxygen consumption (VO(2)), finger skin blood flow (Q(f)), shivering and cold (CDT) and warm detection thresholds (WDT) were assessed four times during the exposure. Ratings of thermal sensations, comfort and tolerance were recorded using subjective judgement scales at 1-h intervals. During winter, subjects had a significantly higher mean skin temperature at both 22 and 10 degrees C compared with summer. However, skin temperatures decreased more at 10 degrees C in winter and remained higher only in the trunk. Finger skin temperature was higher at 22 degrees C, but lower at 10 degrees C in the winter suggesting an enhanced cold-induced vasoconstriction. Similarly, Q(f) decreased more in winter. The cold detection threshold of the hand was shifted to a lower level in the cold, and more substantially in the winter, which was related to lower skin temperatures in winter. Thermal sensations showed only slight seasonal variation. The observed seasonal differences in thermal responses suggest increased preservation of heat especially in the peripheral areas in winter. Blunted vasomotor and skin temperature responses, which are typical for habituation to cold, were not observed in winter. Instead, the responses in winter resemble aggravated reactions of non-cold acclimatised subjects.     

Effects of bathing and hot footbath on sleep in winter

The effects of daily bathing and hot footbath (immersion of feet in hot water) in winter on the sleep behavior of nine healthy female volunteers were studied. Subjects were assigned to three sleep conditions: sleep after bathing (Condition B), sleep after hot footbath (Condition F), and sleep without either treatment (Control). Polysomnograms (consisting of electroencephalograph, electrooculograph, and electromyograph) were obtained, and body movements during sleep were measured while monitoring both the rectal and skin temperatures of subjects. In addition, subjective sleep sensations were obtained with a questionnaire answered immediately by the subjects on awakening. The rectal temperature increased by approximately 1.0 degree C under Condition B, but this elevation was not observed under Condition F compared with Control. In contrast, the respective increases in the mean skin temperature of participants subjected to bathing and hot footbath were greater than those of Control, although these temperature differences became negligible 2 h after subjects went to bed. The sleep onset latency was shortened under both conditions compared with Control. Body movements during the first 30 min of sleep in Control were greater than under the other conditions. Rapid eye movement (REM) sleep decreased under Condition B compared with Condition F, and stage 3 was greater under the latter condition compared with Control. As such, the subjective sleep sensations were better under the two treatment conditions. These results suggest that both daily bathing and hot footbath before sleeping facilitates earlier sleep onset. A hot footbath is especially recommendable for the handicapped, elderly, and disabled, who are unable to enjoy regular baths easily and safely.     

Bathing before sleep in the young and in the elderly

In this study we investigated the effects of bathing on the quality of sleep in 30 elderly people (ages 65-83 years) and in 30 young people (ages 17-22 years) in their homes. Room temperature did not vary significantly during the nights that data were acquired, ranging from 8 to 12 degrees C. After bathing and at the beginning of sleep, the mean (SE) rectal temperatures of the young and the elderly were 37.8 (0.08) and 37.5 (0.07) degrees C, respectively, and were higher by 0.7 (0.13) and 0.6 (0.07) degrees C, respectively, than when the subjects had not bathed. At the beginning of the sleep after bathing in the young subjects, skin temperature was 32.5 (0.24) and 1.5 (0.34) degrees C higher than when those subjects had not bathed. In the elderly, however, there were no significant differences in skin temperature with and without prior bathing because they used electric blankets during sleep. After bathing, the young people reported "warmth" in their hands and/or legs, while the elderly more often reported "good sleep" or "quickness of falling asleep". During the first 3 h of sleep, body movements were less frequent after bathing for both the young and the elderly subjects. The results suggest that a bath before sleep enhances the quality of sleep, particularly in the elderly.     

Physiological responses and thermal sensations of the elderly in cold and hot environments

1. 1. 10 elderly and 10 college-aged females served as subjects in cold and heat environments. The subjects changed into the standard clothing (0.63 clo), and stayed in the neutral environment (25°C) for 23 min, thereafter they were exposed to the cold (10°C) or hot (35°C) environment for 49 min.

2. 2. Then they returned to the neutral environment, and stayed there for 47 min. Oral temperature, skin temperatures at 10 sites, blood pressure and thermal sensation were measured during the experiments.

3. 3. In the cold environment, the elderly could not reduce heat loss by vasoconstriction as did young people, and their blood pressures increased more rapidly than in young people. In the hot environment, the elderly could not promote heat loss by vasodilation as did young people. Moreover, there is a delayed sensitivity to cold for the elderly. Therefore, in the houses of the elderly, it is important to have heating and cooling systems which also includes the areas where the people do not stay for a long period of time (e.g. toilet, passageways).

Is there modal opponency in the thermal senses?

1. 1.The sensations evoked by pairs of distinct thermal stimuli applied to the back of the hand were studied in 17 volunteer subjects. Four stimulus combinations were used; neutral-cold (NC), neutral-neutral (NN), neutral-warm (NW), and cold-warm (CW).

2. 2.The subjects were first asked to estimate the magnitude of the thermal sensations evoked by the thermal stimuli. On average, the four pairs were reported as increasing magnitude in the following order: NC, CW, NN, and NW, seeming to suggest that the subjects experienced the cold-warm combination as a composite sensation of cold and warmth intermediate between pure cold and pure warmth.

3. 3.When asked only to detect the presence of a cold stimulus, the subjects performed as well for the CW combination as for the CN combination. This second result indicates that the reported composite magnitude of CW does not result from a true opponency of cold and warmth but from a cognitive combination of distinct sensations of cold and warmth.

A comparison of head-out mist bathing,with or without facial fanning,with head-out half-body low-water level bathing in humans—a pilot study

To reduce the risks of Japanese-style bathing, half-body bathing (HBLB) has been recommended in Japan, but discomfort due to the cold environment in winter prevents its widespread adoption. The development of the mist sauna, which causes a gradual core temperature rise with sufficient thermal comfort, has reduced the demerits of HBLB. We examined head-out 42 °C mist bathing with 38 °C HBLB up to the navel to see if it could improve thermal comfort without detracting from the merits of HBLB, with and without the effects of facial fanning (FF). The subjects were seven healthy males aged 22–25 years. The following bathing styles were provided: (1) HBLB—head-out half-body low bathing of 38 °C up to the navel (20 min); (2) HOMB—head-out mist bathing of 42 °C and HBLB of 38 °C (20 min); and (3) HOMBFF—HOMB with FF (20 min). HOMB raised the core temperature gradually. HOMBFF suppressed the core temperature rise in a similar fashion to HOMB. Increases in blood pressure and heart rate usually observed in Japanese traditional-style bathing were less marked in HOMBs with no significant difference with and without FF. The greatest body weight loss was observed after Japanese traditional-style bathing, with only one-third of this amount lost after mist bathing, and one-sixth after HBLB. HOMB increased thermal sensation, and FF also enhanced post-bathing invigoration. We conclude that HOMB reduces the risks of Japanese traditional style bathing by mitigating marked changes in the core temperature and hemodynamics, and FF provides thermal comfort and invigoration.     

A study on body temperature regulation and residential thermal environments of the elderly—II. residential thermal environments at the kansai district

1. 1. An investigation was carried out on 3 male and 4 female elderly people, from 65 to 78 years ol.d The first survey was made in August and September of 1990 (in summer) and the second in January and February of 1991 (in winter).

2. 2. Matters for investigation were human subjects and thermal environmental elements of houses which were recorded by a portable thermo-recorder and a vinyl resin globe thermometer. Dry bulb temperatures at a height of 0.1 and 1.2 m and wet bulb and globe temperatures at 1.2 m high were recorded in the living room, bedroom, toilet etc., the thermo-recorders which measured air and wet bulb temperatures were kept out of the sun.

3. 3. Data was recorded constantly for 7 days at 20 min intervals. Plans of houses and furniture arrangement which influence the thermal conditions were drawn from video recordings. Physical and psychological surveys of individuals were carried out over 2 days of daily life.

4. 4. Subjects carried the thermo-recorders (YM1, YM2) on their side. Rectal, back of hand, sole of foot and ambient temperatures were measured every 2 min and with thermistor sensors. At the same time, behaviour and clothes, assessment of thermal sensation, comfort sensation and sensation of estimated room temperature were reported.

5. 5. Thermal radiation was measured with thermographs during the winter. Human activity was recorded every minute for 33 h. This report is the result of surveys in summer and winter.

6. 6. The following results were obtained: (i) the air conditioning is used sometimes in the houses of the elderly; (ii) thermal sensation range reported is narrow; (iii) skin surface temperatures of the elderly are relatively high and their range of change is narrow, and the range of rectal temperature in a day is narrow; (iv) high activity and excessive heating cause a rise of rectal temperature and the rectal temperatures are lower at rest time.

Individual differences in temperature perception: evidence of common processing of sensation intensity of warmth and cold

The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29 degrees C) or heating (37 degrees C) local regions of the forearm. Stimuli were delivered via a 4 x 4 array of 8 mm x 8 mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. Stimulation of the full array showed that perception of warmth and cold were highly correlated (Pearson r = 0.83, p < 0.05). Ratings of nonpainful nociceptive sensations produced by the two temperatures were also correlated, but to a lesser degree (r = 0.44), and the associations between nociceptive and thermal sensations (r = 0.35 and 0.22 for 37 and 29 degrees C, respectively) were not significant after correction for multiple statistical tests. Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.     

Cutaneous thermal thresholds of tropical indigenes residing in Japan

The purpose of this study was to examine the deacclimatization of the cutaneous thermal sensations of tropical indigenes residing in temperate climates. Tropical indigenes (n=13) who were born and raised in tropics but had resided in Japan for 5–61 months participated in this study, along with temperate indigenes (n=11). Their cutaneous thermal thresholds for warm, cool, hot, and cold sensations were measured in 12 body regions using a thermal stimulator controlled by a Peltier element and a push button switch. Subjects pressed the button-switch as soon as they perceived a feeling of being ‘slightly warm’, ‘slightly cool’, ‘hot’, or ‘cold’ from a neutral thermal state. Our results showed that: (1) among the tropical indigenes, no significant relationship was found between the duration of their stay in Japan and their cutaneous thermal thresholds; (2) the tropical indigenes were, on average, 3.3, 3.5, 4.2, and 7.3 °C less sensitive to warm, hot, cool, and cold sensations, respectively, than the temperate indigenes (P<0.05); and (3) the inter-threshold sensory zones between cutaneous warmth and coolness, and hot and cold sensations were wider among the tropical indigenes than among the temperate indigenes. It was concluded that the nature of the heat acclimatization of the cutaneous thermal thresholds for the tropical indigenes was retained despite their residence in a temperate climate for up to 61 months, indicating that they had more blunted perceptions of both warming and cooling than the temperate indigenes.     

Individual differences in temperature perception: Evidence of common processing of sensation intensity of warmth and cold

The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29°C) or heating (37°C) local regions of the forearm. Stimuli were delivered via a 4?×?4 array of 8 mm?×?8?mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. Stimulation of the full array showed that perception of warmth and cold were highly correlated (Pearson r?=?0.83, p?<?0.05). Ratings of nonpainful nociceptive sensations produced by the two temperatures were also correlated, but to a lesser degree (r?=?0.44), and the associations between nociceptive and thermal sensations (r?=?0.35 and 0.22 for 37 and 29°C, respectively) were not significant after correction for multiple statistical tests. Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.     

Regional differences in temperature sensation and thermal comfort in humans

Sensations evoked by thermal stimulation (temperature-related sensations) can be divided into two categories, "temperature sensation" and "thermal comfort." Although several studies have investigated regional differences in temperature sensation, less is known about the sensitivity differences in thermal comfort for the various body regions. In the present study, we examined regional differences in temperature-related sensations with special attention to thermal comfort. Healthy male subjects sitting in an environment of mild heat or cold were locally cooled or warmed with water-perfused stimulators. Areas stimulated were the face, chest, abdomen, and thigh. Temperature sensation and thermal comfort of the stimulated areas were reported by the subjects, as was whole body thermal comfort. During mild heat exposure, facial cooling was most comfortable and facial warming was most uncomfortable. On the other hand, during mild cold exposure, neither warming nor cooling of the face had a major effect. The chest and abdomen had characteristics opposite to those of the face. Local warming of the chest and abdomen did produce a strong comfort sensation during whole body cold exposure. The thermal comfort seen in this study suggests that if given the chance, humans would preferentially cool the head in the heat, and they would maintain the warmth of the trunk areas in the cold. The qualitative differences seen in thermal comfort for the various areas cannot be explained solely by the density or properties of the peripheral thermal receptors and thus must reflect processing mechanisms in the central nervous system.     

Regional distribution of thermal sensitivity to cold at rest and during mild exercise in males

Although several studies have compared thermal sensitivity between body segments, little is known on regional variations within body segments. Furthermore, the effects of exercise on the thermal sensation resulting from a cold stimulus remain unclear. The current experiment therefore aimed to explore inter- and intra-segmental differences in thermal sensitivity to cold, at rest and during light exercise. Fourteen male participants (22.3±3.1 years; 181.6±6.2 cm; 73.7±10.3 kg) were tested at rest and whilst cycling at 30% VO_2 max. Sixteen body sites (front torso=6; back=6; arms=4) were stimulated in a balanced order, using a 20 °C thermal probe (25 cm²) applied onto the skin. Thermal sensations resulting from the stimuli were assessed using an 11-point cold sensation scale (0=not cold; 10=extremely cold). Variations were found within body segments, particularly at the abdomen and mid-back where the lateral regions were significantly more sensitive than the medial areas. Furthermore, thermal sensations were significantly colder at rest compared to exercise in 12 of the 16 body sites tested. Neural and hormonal factors were considered as potential mechanisms behind this reduction in thermal sensitivity. Interestingly, the distribution of cold sensations was more homogenous during exercise. The present data provides evidence that thermal sensitivity to cold varies within body segments, and it is significantly reduced in most areas during exercise.     

Oral temperatures of the elderly in nursing homes in summer and winter in relation to activities of daily living

 This study was conducted to clarify the seasonal difference in body temperature in summer and winter, and to document the thermal environment of the elderly living in nursing homes. The subjects were 57 healthy elderly people aged ≥63 years living in two nursing homes in Japan. One of the homes was characterized by subjects with low levels of activities of daily living (ADL). Oral temperatures were measured in the morning and afternoon, with simultaneous recording of ambient temperature and relative humidity. Oral temperatures in summer were higher than in winter, with statistically significant differences (P<0.05) of 0.25 (SD 0.61) °C in the morning and 0.24 (SD 0.50) °C in the afternoon. Differences between oral temperatures in summer and winter tended to be greater in subjects with low ADL scores, even when their room temperature was well-controlled. In conclusion, the oral temperatures of the elderly are lower in winter than summer, particularly in physically inactive people. It appears that those with low levels of ADL are more vulnerable to large changes in ambient temperature. Received: 28 March 1996 / Accepted: 12 November 1996     

A large-scale study on subjective perception of discomfort during 7 and 1.5 T MRI examinations

A study on subjective perception has been carried out in order to gain further insight into subjective discomfort and sensations experienced during 7 T magnetic resonance imaging (MRI). This study provides information about subjective acceptance, which is essential if 7 T MRI is to become a clinical diagnostic tool. Of 573 subjects who underwent 7 T MRI, 166 were also examined at 1.5 T, providing a means of discriminating field‐dependent discomfort. All subjects judged sources of discomfort and physiological sensations on an 11‐point scale (0 = no side effects, 10 = intolerable side effects) and scores were analyzed separately for exam phases, with and without table movement at each field strength. Results revealed that 7 T MRI was, in general, judged more uncomfortable than 1.5 T; however, most subjects rated the effects as being non‐critical (mean scores between 0.5 and 3.5). Significant differences were detected regarding vertigo and sweating between subjects positioned “head‐first” and “feet‐first” at 7 T (worse in “head‐first”) and between 7 and 1.5 T (worse at 7 T), with the effects being more pronounced in the moving compared to the stationary table position. The most unpleasant factor at 7 T was the extensive examination duration, while potentially field‐dependent sensations were rated less bothersome. In summary, our study indicates that although certain sensations increase at 7 T compared to 1.5 T, they are unlikely to hinder the use of 7 T MRI as a clinical diagnostic tool. Bioelectromagnetics. Bioelectromagnetics 32:610–619, 2011. © 2011 Wiley Periodicals, Inc.     

Seasonal variations of physiological characteristics and thermal sensation under identical thermal conditions

Seasonal variations of human thermal characteristics were inspected in thermal comfort and when constantly indoors. Metabolic rate, tympanic temperature, skin temperature, body fat, body weight and thermal sensation were measured under identical thermal conditions in a chamber over the course of one year. Experiments were carried out for each subject in both summer and winter. Six subjects were measured 35 times in summer and 45 times in winter. one subject was measured weekly for 14 months. Measurements for analyses were taken 40-60 min after entrance into the chamber. Results revealed the following. 1) For all subjects, the metabolic rate, tympanic temperature and body fat were lower in summer than in winter; thigh skin temperatures were higher in summer than in winter. The averaged individual ratio of seasonal difference was 11.9% for metabolic rate, 14.9% for body fat, 1.8% for thigh temperature and 0.53% for tympanic temperature. Seasonal differences of about 10% in metabolic rate were maintained in this study. 2) Seasonal variations of the variables were examined for phase relationships against the outdoor temperature. 2-1) Metabolic rate, thermal sensation, body weight and body fat changed in reverse phase, whereas skin temperature was in-phase. 2-2) Skin temperature lagged by about one month in both summer and winter. Body fat also lagged by about one month in summer, but corresponded to the phase in winter. Metabolic rates were also in-phase in winter but led about three months in summer. Thermal sensations lagged by about three months in winter but were in-phase in summer. Body weight was in-phase in summer and winter. 2-3) Summer disorders were observed particularly in seasonal variations of metabolic rates, tympanic temperature, skin temperatures, and thermal sensation, thereby suggesting that the effect of temperature exposure was altered by air-conditioner use.     

Hypothermia in the elderly: scope for prevention.

Concern is growing about the number of elderly people dying of hypothermia. A register was compiled of patients over 75 on a general practitioner''s list who were identified from their medical records as being at risk of hypothermia, having two or more established risk factors. Twenty four patients from this register were visited early in winter by a doctor to discuss how hypothermia could be prevented. They were then revisited during very cold weather to see whether they had made any changes. Several improvements to heating arrangements were noted, but the median temperature in the bedrooms of houses with no central heating was 10 degrees C below the World Health Organisation''s recommended temperature. In addition, eight patients were not visited daily. Even with media publicity and visits from carers and a doctor, 17 of the 24 elderly people studied continued to live in an environment in which they were at risk of developing hypothermia.     

Environmental Conditions and Body Temperatures of Elderly Women Living Alone or in Local Authority Home

The environmental and body temperatures of two groups of elderly women have been measured. One group was living in a local authority home (L.A.H.) and the others in their own homes in North London. The L.A.H. provided a constant environmental temperature which was at all times higher than that of the private houses. In the latter group the ambient temperature was higher in bed-sitting rooms than in houses with separate living rooms and bedrooms.Body temperatures in summer were similar throughout both groups. In winter the skin and mouth temperatures of the subjects living independently were lower than those in the L.A.H.Four subjects who had low mouth temperatures measured during two consecutive winters did not prove to have lowered deep body temperatures. The diet of these four subjects was similar in respect of all nutrients to that found in other groups of subjects of the same age, and in relation to published dietary standards was adequate in all respects.     

Fluid mechanics in dentinal microtubules provides mechanistic insights into the difference between hot and cold dental pain

Dental thermal pain is a significant health problem in daily life and dentistry. There is a long-standing question regarding the phenomenon that cold stimulation evokes sharper and more shooting pain sensations than hot stimulation. This phenomenon, however, outlives the well-known hydrodynamic theory used to explain dental thermal pain mechanism. Here, we present a mathematical model based on the hypothesis that hot or cold stimulation-induced different directions of dentinal fluid flow and the corresponding odontoblast movements in dentinal microtubules contribute to different dental pain responses. We coupled a computational fluid dynamics model, describing the fluid mechanics in dentinal microtubules, with a modified Hodgkin-Huxley model, describing the discharge behavior of intradental neuron. The simulated results agreed well with existing experimental measurements. We thence demonstrated theoretically that intradental mechano-sensitive nociceptors are not "equally sensitive" to inward (into the pulp) and outward (away from the pulp) fluid flows, providing mechanistic insights into the difference between hot and cold dental pain. The model developed here could enable better diagnosis in endodontics which requires an understanding of pulpal histology, neurology and physiology, as well as their dynamic response to the thermal stimulation used in dental practices.     

Effects of room temperature on physiological and subjective responses during whole-body bathing,half-body bathing and showering

The effects of bathroom thermal conditions on physiological and subjective responses were evaluated before, during, and after whole-body bath (W-bath), half-body bath (H-bath) and showering. The air temperature of the dressing room and bathroom was controlled at 10 degrees C, 17.5 degrees C, and 25 degrees C. Eight healthy males bathed for 10 min under nine conditions on separate days. The water temperature of the bathtub and shower was controlled at 40 degrees C and 41 degrees C, respectively. Rectal temperature (Tre), mean skin temperature (Tsk), blood pressure (BP), heart rate (HR), body weight loss and blood characteristics (hematocrit: Hct, hemoglobin: Hb) were evaluated. Also, thermal sensation (TS), thermal comfort (TC) and thermal acceptability (TA) were recorded. BP decreased rapidly during W-bath and H-bath compared to showering. HR during W-bath was significantly higher than for H-bath and showering (p < 0.01). The double products due to W-bath during bathing were also greater than for H-bath and showering (p < 0.05). There were no distinct differences in Hct and Hb among the nine conditions. However, significant differences in body weight loss were observed among the bathing methods: W-bath > H-bath > showering (p < 0.001). W-bath showed the largest increase in Tre and Tsk, followed by H-bath, and showering. Significant differences in Tre after bathing among the room temperatures were found only at H-bath. The changes in Tre after bathing for H-bath at 25 degrees C were similar to those for W-bath at 17.5 degrees C and 10 degrees C. TS and TC after bathing significantly differed for the three bathing methods at 17.5 degrees C and 10 degrees C (TS: p < 0.01 TC: p < 0.001). Especially, for showering, the largest number of subjects felt "cold" and "uncomfortable". Even though all of the subjects could accept the 10 degrees C condition after W-bath, such conditions were intolerable to half of them after showering. These results suggested that the physiological strains during H-bath and showering were smaller than during W-bath. However, colder room temperatures made it more difficult to retain body warmth after H-bath and created thermal discomfort after showering. It is particularly important for H-bath and showering to maintain an acceptable temperature in the dressing room and bathroom, in order to bathe comfortably and ensure warmth.     

Cold-induced bradycardia in man during sleep in Arctic winter nights

Two young male Caucasians volunteered for a study on the effects of cold exposure during night sleep in winter in the Arctic. The 14-day experiment was divided in three consecutive periods, baseline (2 nights), cold exposure (10 night) and recovery (2 nights). Both baseline and recovery data were obtained in neutral thermal conditions in a laboratory. The subjects slept in a sleeping bag under an unheated tent during the cold exposure. Apart from polysomnographic and body temperature recordings, electrocardiograms were taken through a telemetric system for safety purposes. Heart rates were noted at 5-min intervals and averaged hourly. In both environmental conditions, heart rate decreased within the first two hours of sleep. Comparison of the data obtained during cold exposure vs. thermal neutrality revealed lower values of heart rate in the cold, while body temperatures remained within normal range. This cold-induced bradycardia supervening during night sleep is discussed in terms of the occurrence of a vagal reflex preventing central blood pressure to rise.