Control of sweating during the human menstrual cycle

Thermoregulatory responses were studied in seven women during two separate experimental protocols in the follicular (F, days 4-7) phase and during the luteal (L, days 19-22) phase of the menstrual cycle. Continuous measurements of esophageal temperature (Tes), mean skin temperature (Tsk), oxygen uptake and forearm sweating (ms) were made during all experiments. Protocol I involved both passive heat exposure (3 h) and cycle exercise at approximately 80% VO2 peak during which the environmental chamber was controlled at Ta = 50.0 degrees C, rh = 14% (Pw = 1.7 kPa). In protocol II subjects were tested during thirty-five minutes of exercise at approximately 85% VO2 peak at Ta = 35 degrees C and rh = 25% (Pw = 1.4 kPa). The normal L increase in resting Tes (approximately 0.3 degrees C) occurred in all seven subjects. Tsk was higher during L than F in all experiments conducted at 50 degrees C. During exercise and passive heat exposure, the Tes threshold for sweating was higher in L, with no change in the thermosensitivity (slope) of ms to Tes between menstrual cycle phases. This rightward or upward shift in Tes threshold for initiation of sweating averaged 0.5 degrees C for all experiments. The data indicate the luteal phase modulation in the control of sweating in healthy women is also apparent during severe exercise and/or heat stress.     

Influence of menstrual cycle on thermoregulatory, metabolic, and heart rate responses to exercise at night

Ten women [mean maximal O2 uptake (VO2max), 2.81 l X min-1] exercised for 15 min on a cycle ergometer in the middle of the luteal phase (L) and in the early follicular phase (F) of the menstrual cycle at the same constant work rates (mean 122 W) and an ambient temperature of 18 degrees C. Serum progesterone averaged 44.7 nmol X l-1 in L and 0.7 nmol X l-1 in F. After a 4-h resting period, exercise was performed between 3 and 4 A.M., when the L-F core temperature difference is maximal. Preexercise esophageal (Tes), tympanic (Tty), and rectal (Tre) temperatures averaged 0.6 degrees C higher in L. During exercise Tes, Tty, and Tre averaged 0.5 degrees C higher. The thresholds for chest sweating and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were elevated in L by an average of 0.47 degrees C, related to mean body temperature (Tb(es) = 0.87Tes + 0.13Tskin), Tes, Tty, or Tre. The above-threshold chest sweat rate and cutaneous heat clearances were also increased in L. The mean exercise heart rate was 170.0 beats X min-1 in L and 163.8 beats X min-1 in F. The mean exercise VO2 in L (2.21 l X min-1) was 5.2% higher than in F (2.10 l X min-1), the metabolic rate was increased in L by 5.6%, but the net efficiency was 5.3% lower. No significant L-F differences in the respiratory exchange ratio and postexercise plasma lactate were demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decreased thermal conductance during the luteal phase of the menstrual cycle in women

To study the influence of the menstrual cycle on whole body thermal balance and on thermoregulatory mechanisms, metabolic heat production (M) was measured by indirect calorimetry and total heat losses (H) were measured by direct calorimetry in nine women during the follicular (F) and the luteal (L) phases of the menstrual cycle. The subjects were studied while exposed for 90 min to neutral environmental conditions (ambient temperature 28 degrees C, relative humidity 40%) in a direct calorimeter. The values of M and H were not modified by the phase of the menstrual cycle. Furthermore, in both phases the subjects were in thermal equilibrium because M was similar to H (69.7 +/- 1.8 and 72.1 +/- 1.8 W in F and 70.4 +/- 1.9 and 71.4 +/- 1.7 W in L phases, respectively). Tympanic temperature (Tty) was 0.24 +/- 0.07 degrees C higher in the L than in the F phase (P less than 0.05), whereas mean skin temperature (Tsk) was unchanged. Calculated skin thermal conductance (Ksk) was lower in the L (17.9 +/- 0.6 W.m-2.degrees C-1) than in the F phase (20.1 +/- 1.1 W.m-2.degrees C-1; P less than 0.05). Calculated skin blood flow (Fsk) was also lower in the L (0.101 +/- 0.008 l.min-1.m-2) than in the F phase (0.131 +/- 0.015 l.min-1.m-2; P less than 0.05). Differences in Tty, Ksk, and Fsk were not correlated with changes in plasma progesterone concentration. It is concluded that, during the L phase, a decreased thermal conductance in women exposed to a neutral environment allows the maintenance of a higher internal temperature.     

Influence of menstrual cycle on shivering, skin blood flow, and sweating responses measured at night

In 10 women, external cold and heat exposures were performed both in the middle of luteal phase (L) and in the early follicular phase (F) of the menstrual cycle. Serum progesterone concentrations in L and F averaged 46.0 and 0.9 nmol X l-1, respectively. The experiments took place between 3:00 and 4:30 A.M., when the L-F core temperature difference is maximal. At neutral ambient temperature, esophageal (Tes), tympanic (Tty), rectal (Tre), and mean skin (Tsk) temperatures averaged 0.59 degrees C higher in L than in F. The thresholds for shivering, chest sweating, and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were increased in L by an average of 0.47 degrees C, related to mean body temperature [Tb(es) = 0.87Tes + 0.13 Tsk] and to Tes, Tty, Tre, or Tsk. The above-threshold chest sweat rate and cutaneous heat clearances at the thumb and forearm were also enhanced in L, when related to Tb(es) or time. The metabolic rate, arm blood flow, and heart rate at thermoneutral conditions were increased in L by 5.0%, 1.1 ml X 100 ml-1 X min-1, and 4.6 beats X min-1, respectively. The concomitant increase in threshold temperatures for all autonomic thermoregulatory responses in L supports the concept of a resetting of the set point underlying the basal body temperature elevation in L. The effects of the increased threshold temperatures are counteracted by enhanced heat loss responses.     

Thermoregulation, metabolism, and stages of sleep in cold-exposed men

Four naked men, selected for their ability to sleep in the cold, were exposed to an ambient temperature (Ta) of 21 degrees C for five consecutive nights. Electrophysiological stages of sleep, O2 consumption (VO2), and skin (Tsk), rectal (Tre), and tympanic (Tty) temperatures were recorded. Compared with five nights at a thermoneutral Ta of 29 degrees C, cold induced increased wakefulness and decreased stage 2 sleep, without significantly affecting other stages. Tre and Tty declined during each condition. The decrease in Tre was greater at 21 degrees C than at 29 degrees C, whereas Tty did not differ significantly between conditions. Increases in Tty following REM sleep onset at 21 degrees C were negatively correlated with absolute Tty. VO2 and forehead Tsk also increased during REM sleep at both TaS, whereas Tsk of the limb extremities declined at 21 degrees C. Unsuppressed REM sleep in association with peripheral vasoconstriction and increased Tty and VO2 in cold-exposed humans, do not signify an inhibition of thermoregulation during this sleep stage as has been observed in other mammals.     

Menstrual cycle phase affects temperature regulation during endurance exercise.

We investigated whether menstrual cycle phase would affect temperature regulation during an endurance exercise bout performed at room temperature (Ta) of 22 degrees C and 60% relative humidity. Nine eumenorrheic women [age 27.2 +/- 3.7 yr, peak O2 uptake (VO2) 2.52 +/- 0.35 l/min] performed 60 min of cycle exercise at 65% of peak VO2. Subjects were tested in both midfollicular (F) and midluteal (L) phases, although one woman did not show a rise in serum progesterone (P4) that is typically evident 1 wk after ovulation. VO2, rectal (Tre) and skin (Tsk) temperatures, heart rates (HR), and ratings of perceived exertion (RPE) were measured throughout exercise. Sweat loss (SL) was estimated from pre- and postexercise body weight differences. VO2, SL, and Tsk were not affected by menstrual cycle phase. Preexercise Tre was 0.3 degrees C higher during L than during F conditions, and this difference increased to 0.6 degrees C by the end of exercise (P less than 0.01). Compared with F, HRs during L were approximately 10 beats/min greater (P less than 0.001) at all times, whereas RPE responses were significantly greater (P less than 0.01) by 50 min of cycling. No differences in any measured values were found in the subject whose P4 was low in both test conditions. Results indicate that thermoregulation (specifically, regulation of Tre), as well as cardiovascular strain and perception of exercise, was adversely affected during the L phase.     

Characteristics of body heat balance of paraplegics during exercise in a hot environment

The purpose of this investigation was to clarify the characteristics of body temperature regulation in paraplegics due to spinal cord injury (SCI) during an arm cranking exercise in a hot environment. Twelve paraplegics with lesions located between Th3 and L1,2 and seven able-bodied subjects (AB) participated in this study. The subjects were exposed to a hot (33 degrees C) or a moderate temperature (25 degrees C) environment for one hour and during the last 10 min of the exposure, the subjects performed arm cranking exercises at an exercise intensity of 40 W. The skin temperatures at the chest, the upper arm, the thigh and the calf, the tympanic membrane temperature (Tty), and the skin blood flow of the thigh (SBFT) were continuously monitored during the experiment. Although no systematical variation was found in the Tty at 25 degrees C, the Tty at 33 degrees C in paraplegics during exercise was significantly greater than that at rest (P < 0.01), which indicated a pronounced heat stress for paraplegics at 33 degrees C. SBFT of paraplegics with high lesions of the SCI remained unchanged during the experiment at 25 degrees C and 33 degrees C, while paraplegics with low lesions in this study showed consecutive increases in SBFT during exercise in both environmental conditions similar to AB. The increased core temperature in paraplegics with high lesions was considered to be due to a lack of sweat response and vasomotor activity in the paralyzed area. On the basis of the findings in this study, it can be suggested that high core temperature without any increment of SBFT may be characterized as body heat balance of paraplegics with high lesions during exercise in a hot environment.     

Circadian rhythm changes in core temperature over the menstrual cycle: method for noninvasive monitoring

The purpose of this study was to determine whether core temperature (T(c)) telemetry could be used in ambulatory women to track changes in the circadian T(c) rhythm during different phases of the menstrual cycle and, more specifically, to detect impending ovulation. T(c) was measured in four women who ingested a series of disposable temperature sensors. Data were collected each minute for 2-7 days and analyzed in 36-h segments by automated cosinor analysis to determine the mesor (mean temperature), amplitude, period, acrophase (time of peak temperature), and predicted circadian minimum core temperature (T(c-min)) for each cycle. The T(c) mesor was higher (P < or = 0.001) in the luteal (L) phase (37.39 +/-0.13 degrees C) and lower in the preovulatory (P) phase (36.91 +/-0.11 degrees C) compared with the follicular (F) phase (37.08 +/-0.13 degrees C). The predicted T(c-min) was also greater in L (37.06 +/- 0.14 degrees C) than in menses (M; 36.69 +/- 0.13 degrees C), F (36. 6 +/- 0.16 degrees C), and P (36.38 +/- 0.08 degrees C) (P < or = 0. 0001). During P, the predicted T(c-min) was significantly decreased compared with M and F (P < or = 0.0001). The amplitude of the T(c) rhythm was significantly reduced in L compared with all other phases (P < or = 0.005). Neither the period nor acrophase was affected by menstrual cycle phase in ambulatory subjects. The use of an ingestible temperature sensor in conjunction with fast and accurate cosinor analysis provides a noninvasive method to mark menstrual phases, including the critical preovulatory period.     

Calorimetric study of the effect of salicylate in man during heat exposure and exercise (author's transl)]

1. The effect of sodium acetylo-salicylate (2 g per os) on the thermoregulatory responses of 10 male subjects was studied by direct and indirect calorimetry during two tests : heat exposure at 37 degrees C and exercise (50 W) at 25 degrees C. Both test were performed twice : with salicylate treatment and with a placebo. 2. During heat exposure at 37 degrees C for 75 min, the rise in tympanic temperature (Tty) and in mean skin temperature Ts, the time course of heat losses by radiation (R), convection (C) and evaporation (E), and the metabolic rate (M), measured by oxygen consumption, were not altered by salicylate treatment. 3. During exercise, salicylate treatment did not affect the time course of Tty and Ts, (R + C) and M. However, salicylate treatment decreased the delay for triggering the evaporative response (E) to the thermal load; similarly, the increase in cutaneous blood flow was triggered sooner in subjected receiving salicylate than in controls. 4. In conclusion, these results suggest that, during exercise, the thermal controller triggers thermoregulatory responses during passive hyperthermia by heat exposure.     

Water loss distribution in exercising men under hot conditions

Dynamics of sweating and water loss distribution were studied in 7 exercising men under thermoneutral conditions (Ta, 25 degrees C; Tw, 24 degrees C and RH, 54%) and during moderate heat exposure (Ta, 30 degrees C; Tw, 30 degrees C; RH, 54%). The subjects performed bicycle exercise at intensity of 50% V O2 max. Dynamics of sweating was greater after heat exposure (delay in onset of sweating 3.6 and 1.4 min, p less than 0.05; time constant 10.1 and 7.3 min, p less than 0.02). The dynamics of sweating was related to the net body heat load (r = -0.80, p less than 0.001). Sweat evaporation from the skin (Esk) was significantly higher in heat exposed exercising subjects while dripping sweat (mdrip) did not differ significantly. Water loss distribution in relation to total water loss during control exercise was as follows: (Ediff + Eres) 14.8% (Esk) 59.6%; and (mdrip) 25.6%. During exercise under heat exposure (Ediff + Eres) was 12.1%; (Esk) was 67.5%; and (mdrip) was 20.4%. It is concluded that moderate heat exposure accelerate sweating reaction but does not change significantly water loss distribution in exercising subjects. Dripping sweat seems to be an attribute of sweating not only in hot humid conditions but also under temperate temperature and air humidity.     

Finger vasodilation correlates better with tympanic than esophageal temperature

The relationship of finger blood flow (FBF) measured by venous occlusion plethysmography to tympanic temperature (Tty) was compared with that of FBF to esophageal temperature (Tes) during exercise at 50% VO2max for 40 min at an ambient temperature of 25 degrees C. The relationship of FBF to Tes showed an inflexion as Tes increased during exercise. The slope of the regression line showing the relationship between FBF and Tes was initially moderate, and then suddenly became steeper at the inflexion point. The relationship of FBF to Tty, however, was linear, without an inflexion. The results suggest that finger vasodilation during moderate exercise correlates better with tympanic than esophageal temperature.     

Efficiency of thermoregulatory system in man under endogenous and exogenous heat loads

The aim of the present work was to estimate the dynamics and efficiency (eta sw) of sweating, and thermoregulatory index (TI) defined as a ratio of heat loaded the body to the heat removed to the environment. In the first part of this work 22 men exercised with an intensity of 50% VO2 max. in 22 degrees C, 16 men were exposed to 40 degrees C at rest, and 9 men exercised at the level of 50% VO2 max. at 30 degrees C. In the second part, 8 men and 8 women were exposed to 40 degrees C before and after dehydration (1% of body mass, approximately), 8 men exercised at 23 degrees C before and after hyperhydration (35 ml/kg of body mass) and 22 men exercised before and after 3 months of endurance training. Body heat balance, rectal (Tre), tympanic (Tty) and mean skin (Tsk) temperatures were measured in all subjects. TI was greater during simultaneous (0.84) than during separate endo- (0.76, p less than 0.01) or exogenous (0.67, p less than 0.001) heat loads. The respective values of eta sw were 0.82; 0.57 (p less than 0.001) and 0.78 (p less than 0.001). No difference in TI was found between men and women. Dynamics of sweating was greater in men but efficiency of sweating was greater in women. Dehydration before heat exposure decreased both dynamics of sweating and TI but it increased eta sw in men. As a result Tre was greater in dehydrated (0.45 degrees C) than in normally hydrated men (0.31 degrees C, p less than 0.002). Dehydration did not affect the measured variables in women. Hyperhydration of exercising men caused an increase in TI from 0.72 to 0.82 (p less than 0.05) and in eta sw from 0.57 to 0.81 (p less than 0.01). In men exercising after endurance training the onset of sweating was shortened from 4.0 to 0.9 min (p less than 0.002). TI increased from 0.76 to 0.89 (p less than 0.001), eta sw increased from 0.57 to 0.74 (p less than 0.02) whereas Tty was lower (1.10 and 0.58 degrees C, p less than 0.001, respectively). It is concluded that dynamics and efficiency of sweating, as well as the thermoregulatory index depend on the type of heat load. Men and women tolerate dry heat equally well. Dehydration changes thermoregulatory function in men but not in women. Hyperhydration before exercise and particularly endurance training increase tolerance of endogenous heat.(ABSTRACT TRUNCATED AT 400 WORDS)     

Estrogen modifies the temperature effects of progesterone.

To test the hypothesis that progestin-mediated increases in resting core temperature and the core temperature threshold for sweating onset are counteracted by estrogen, we studied eight women (24 +/- 2 yr) at 27 degrees C rest, during 20 min of passive heating (35 degrees C), and during 40 min of exercise at 35 degrees C. Subjects were tested four times, during the early follicular and midluteal menstrual phases, after 4 wk of combined estradiol-norethindrone (progestin) oral contraceptive administration (OC E+P), and after 4 wk of progestin-only oral contraceptive administration (OC P). The order of the OC P and OC E+P were randomized. Baseline esophageal temperature (T(es)) at 27 degrees C was higher (P < 0.05) in the luteal phase (37.08 +/- 0.21 degrees C) and in OC P (37.60 +/- 0.31 degrees C) but not during OC E+P (37.04 +/- 0.23 degrees C) compared with the follicular phase (36.66 +/- 0.21 degrees C). T(es) remained above follicular phase levels throughout passive heating and exercise during OC P, whereas T(es) in the luteal phase was greater than in the follicular phase throughout exercise (P < 0.05). The T(es) threshold for sweating was also greater in the luteal phase (38.02 +/- 0.28 degrees C) and OC P (38.07 +/- 0.17 degrees C) compared with the follicular phase (37.32 +/- 0.11 degrees C) and OC E+P (37.46 +/- 0.18 degrees C). Progestin administration raised the T(es) threshold for sweating during OC P, but this effect was not present when estrogen was administered with progestin, suggesting that estrogen modifies progestin-related changes in temperature regulation. These data are also consistent with previous findings that estrogen lowers the thermoregulatory operating point.     

The effect of face fanning during recovery from exercise hyperthermia

Hyperthermia was induced in nine subjects on two separate occasions by a progressive treadmill run, which resulted in an average esophageal temperature (Tes) of 39.77 +/- 0.07 degree C after 30-57 min. Fanning the face during exercise to simulate conditions during running (wind at 3.75 m X s-1) maintained a tympanic temperature (Tty) that was lower than Tes; the difference was 1.5 degrees C at the end of exercise. In one session, face fanning was interrupted at the end of running, whereas in the other it was maintained for 15 min after exercise stopped. Face fanning had no significant influence on the fall of Tes during recovery, but it markedly influenced the course of Tty during this period. When face fanning was stopped at the end of the run, Tty rose by nearly 0.5 degree C, peaked after 4.5 min, and thereafter decreased slowly to a value close to Tes. In contrast, when face fanning was maintained throughout the recovery period, Tty rose only slightly (0.1 degree C) and remained significantly lower than Tes at all times. The results suggest that following hyperthermic exercise, face fanning could be helpful in preventing acute cerebral hyperthermia.     

Tympanic temperatures during hemiface cooling

In adult men the left half of the head was covered with thick heat insulation, and the right hemiface was cooled by spraying a mist of water, and vigorous fanning. The subjects were immersed up to the waist in warm water (42 degrees) to achieve hyperthermia. In control sessions the subjects were rendered slightly hypothermic by preliminary exposure to cold. Under the hypothermic condition during right skin cooling, the right Tty remained low as compared with oesophageal temperature, while the left Tty was raised. Under the hyperthermic condition right hemiface cooling maintained not only the right Tty lower than oesophageal but also, to a lesser extent the left Tty, while the skin on the left side was close to core temperature. This latter result cannot be explained by conductive cooling from the skin to the tympanic membrane and implies a vascular cooling of the left Tty originating from the other side of the head. It is concluded that selective cooling of the brain takes place during hyperthermia. The main mechanism is forced vascular convection, but conductive cooling also occurs.     

Different thermal sensitivity to exercise and cold in men and women

Rectal temperature gain for sweating and shivering was investigated in men (M) and women during exercise and cold exposure. Women, taking and not taking oral contraceptives, were tested during follicular (F) and luteal (L) phases of the menstrual cycle. The gains for sweating (M, 4.85; F, 5.00; L, 12.25 kPa °C⁻¹) and shivering (M, 6.50; F, 2.75; L, 2.00 ml kg min⁻¹ °C⁻¹) differed between sexes, as well as between F and L, and were affected by the oral contraceptives (5.45 and 8.00 kPa °C⁻¹, P<0.01, and 6.00 and 4.25 ml kg min⁻¹ °C⁻¹, P<0.05, respectively).     

Ambient temperature affects glabrous skin vasculature and sweating responses to mental task in humans

We compared responses in heart rate (HR), mean blood pressure (MAP), sweating rate (SR), sweating expulsion (SwE), and skin vascular conductance (VC) to mental task among different ambient temperature (Ta) conditions, i.e., 12, 16, 20, and 24 degrees C. Seven subjects (27+/-5 yrs, 64+/-14 kg) underwent a 2-min color word conflict test (CWT) after 2 mins of baseline data acquisition following a 20-min resting period. All subjects wore long sleeve shirts and long pants. The skin blood flow was measured with a laser Doppler probe on the left index finger pulp to calculate skin VC, and the SR and sweating expulsion (SwE) were measured with a ventilated capsule on the left thenar. CWT significantly increased the HR and MAP, while there was no significant effect of Ta on the magnitudes of these responses. CWT significantly decreased the skin VC when the Ta was 24 degrees C, whereas it significantly increased the skin VC when the Ta was 12 or 16 degrees C. CWT significantly increased SR and SwE in all Ta conditions, and the SwE was greater in warmer conditions. These findings suggest that different ambient temperatures induce different responses in finger skin vasculature to mental task, implying the independent response of cutaneous vasomotor tone and sweat glands in glabrous skin to mental task.     

Thermoregulatory responses of the immature rat following repeated postnatal exposures to 2,450-MHz microwaves

This study was designed to determine the changes that occur in the thermoregulatory ability of the immature rat repeatedly exposed to low-level microwave radiation. Beginning at 6-7 days of age, previously untreated rats were exposed to 2,450-MHz continuous-wave microwaves at a power density of 5 mW/cm2 for 10 days (4 h/day). Microwave and sham (control) exposures were conducted at ambient temperatures (Ta) which represent different levels of cold stress for the immature rat (ie, "exposure" Ta = 20 and 30 degrees C). Physiological tests were conducted at 5-6 and 16-17 days of age, in the absence of microwaves, to determine pre- and postexposure responses, respectively. Measurements of metabolic rate, colonic temperature, and tail skin temperature were made at "test" Ta = 25.0, 30.0, 32.5, and 35.0 degrees C. Mean growth rates were lower for rats exposed to Ta = 20 degrees C than for those exposed to Ta = 30 degrees C, but microwave exposure exerted no effect at either exposure Ta. Metabolic rates and body temperatures of all exposure groups were similar to values for untreated animals at test Ta of 32.5 degrees C and 35.0 degrees C. Colonic temperatures of rats repeatedly exposed to sham or microwave conditions at exposure Ta = 20 degrees C or to sham conditions at exposure Ta = 30 degrees C were approximately 1 degrees C below the level for untreated animals at test Ta of 25.0 degrees C and 30.0 degrees C. However, when the exposure Ta was warmer, rats exhibited a higher colonic temperature at these cold test Ta, indicating that the effectiveness of low-level microwave treatment to alter thermoregulatory responses depends on the magnitude of the cold stress.     

Attenuated thermoregulatory sweating and cutaneous vasodilation after 14-day bed rest in humans.

We investigated the effect of head-down bed rest (HDBR) for 14 days on thermoregulatory sweating and cutaneous vasodilation in humans. Fluid intake was ad libitum during HDBR. We induced whole body heating by increasing skin temperature for 1 h with a water-perfused blanket through which hot water (42 degrees C) was circulated. The experimental room was air-conditioned (27 degrees C, 30-40% relative humidity). We measured skin blood flow (chest and forearm), skin temperatures (chest, upper arm, forearm, thigh, and calf), and tympanic temperature. We also measured sweat rate by the ventilated capsule method in which the skin area for measurement was drained by dry air conditioned at 27 degrees C under similar skin temperatures in both trials. We calculated cutaneous vascular conductance (CVC) from the ratio of skin blood flow to mean blood pressure. From tympanic temperature-sweat rate and -CVC relationships, we assessed the threshold temperature and sensitivity as the slope response of variables to a given change in tympanic temperature. HDBR increased the threshold temperature for sweating by 0.31 degrees C at the chest and 0.32 degrees C at the forearm, whereas it reduced sensitivity by 40% at the chest and 31% at the forearm. HDBR increased the threshold temperature for cutaneous vasodilation, whereas it decreased sensitivity. HDBR reduced plasma volume by 11%, whereas it did not change plasma osmolarity. The increase in the threshold temperature for sweating correlated with that for cutaneous vasodilation. In conclusion, HDBR attenuated thermoregulatory sweating and cutaneous vasodilation by increasing the threshold temperature and decreasing sensitivity. HDBR increased the threshold temperature for sweating and cutaneous vasodilation by similar magnitudes, whereas it decreased their sensitivity by different magnitudes.     

Effects of head cooling on human sleep stages and body temperature

The purpose of this study was to confirm the effect of head cooling on human sleep stages and body temperature. Nine healthy male volunteers with a mean age of 25 +/- 3.77 years served as subjects. The experiments were carried out under three different sets of conditions: 26 degrees C, relative humidity (RH) 50% (26/50); 32 degrees C, RH 80% (32/80); and 32 degrees C RH 80% with the use of a cooling pillow (32/80 HC). The subjects slept from 2300 hours to 0700 hours with a cotton blanket, wearing short-sleeved pyjamas and shorts on a bed, which was covered with a sheet. Electroencephalograms, electro-ouclogram, and mental electromyelograms were recorded through the night. Rectal temperature (Tre) and skin temperature (Tsk) were measured continuously. Whole-body sweat and the tympanic temperature (Tty) were measured before and after sleep. Wakefulness significantly increased at 32/80 than at 26/50; however, no significant difference was observed between 32/80 HC and 26/50. Tre and mean Tsk were higher both at 32/80 and 32/80 HC than at 26/50. The whole-body sweat loss was significantly greater and Tty in the morning was higher at 32/80 than 32/80 HC and 26/50. These results suggest that head cooling during sleep may help to decrease the whole-body sweat rate during sleep under humid heat conditions.