Koh Mizuno

Effects of thermal environment on sleep and circadian rhythm

The thermal environment is one of the most important factors that can affect human sleep. The stereotypical effects of heat or cold exposure are increased wakefulness and decreased rapid eye movement sleep and slow wave sleep. These effects of the thermal environment on sleep stages are strongly linked to thermoregulation, which affects the mechanism regulating sleep. The effects on sleep stages also differ depending on the use of bedding and/or clothing. In semi-nude subjects, sleep stages are more affected by cold exposure than heat exposure. In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation. On the other hand, cold exposure does not affect sleep stages, though the use of beddings and clothing during sleep is critical in supporting thermoregulation and sleep in cold exposure. However, cold exposure affects cardiac autonomic response during sleep without affecting sleep stages and subjective sensations. These results indicate that the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.

Effects of partial humid heat exposure during different segments of sleep on human sleep stages and body temperature.

The effects of partial humid heat exposure applied at different segments of sleep on sleep stages and body temperature were examined. In the first experiment, eight male subjects slept under 26 degrees C 50% (26) and 26 degrees C for the first 3 h and 45 min followed by a 30-min transition to the conditions of 32 degrees C 80%, which was maintained for the final 3 h and 45 min (26-32). Wakefulness increased significantly over the last 4 h under 26-32 compared to 26. Mean skin temperature and clothing microclimate temperature (Tcm) were significantly higher during the last 3 h and 45 min, while rectal temperature (Tre) was higher during the last 3 h under 26-32 than in 26. In the second experiment, eight male subjects slept under 26 degrees C 50% (26) and 32 degrees C 80% for the first 3 h and 45 min followed by a 30-min transition to 26, which was then maintained for the last 3 h and 45 min (32-26). Wakefulness increased both in first and during the last 4 h, and slow wave sleep (SWS) decreased in the first 4 h under 32-26 compared to 26. Mean Tsk was significantly higher during the first 4:15 h. Tcm decreased in 32-26 compared to 26 just after the 30-min transition due to cooling effects. Tre was higher during the first 5 h under 32-26 compared to 26. These results suggest that humid heat exposure during the initial segment of sleep may be more disruptive to sleep stage distribution, Tre decline, and maintenance of Tcm than the same exposure during the later sleep segments.

Effects of an electric blanket on sleep stages and body temperature in young men

The aim of this study was to investigate any effects of electric blanket on sleep stages and body temperature. Nine male subjects slept under two conditions: using the electric blanket (HB); and not using the electric blanket (C). The ambient condition was controlled at 3°C relative humidity 50 – 80%. Electroencephalography, electrooculography (EOG) and electromyography, rectal temperature, skin temperature and microclimate temperature and humidity were recorded continuously through the night. Body weight was measured before and after sleep. The amount of stage 1 and number of stage 1 and rapid eye movement sleep decreased in HB compared to C. No significant difference was observed in other sleep stages. Rectal temperature was higher in HB compared to C. The thigh, leg and foot skin temperature was higher in HB than C. The microclimate temperature of the foot area was higher in HB compared to C. No significant difference was observed in whole body sweat loss between the conditions. These results suggest that use of an electric blanket under low ambient temperature may decrease cold stress to support sleep stability and thermoregulation during sleep.

Effects of long-term microgravity exposure in space on circadian rhythms of heart rate variability

We evaluated their circadian rhythms using data from electrocardiographic records and examined the change in circadian period related to normal RR intervals for astronauts who completed a long-term (≥6-month) mission in space. The examinees were seven astronauts, five men and two women, from 2009 to 2010. Their mean ± SD age was 52.0 ± 4.2 years (47–59 yr). Each stayed in space for more than 160 days; their average length of stay was 172.6 ± 14.6 days (163–199 days). We conducted a 24-h Holter electrocardiography before launch (Pre), at one month after launch (DF1), at two months after launch (DF2), at two weeks before return (DF3), and at three months after landing (Post), comparing each index of frequency-domain analysis and 24-h biological rhythms of the NN intervals (normal RR intervals). Results show that the mean period of Normal Sinus (NN) intervals was within 24 ± 4 h at each examination. Inter-individual variability differed among the stages, being significantly smaller at DF3 (Pre versus DF1 versus DF3 versus Post = 22.36 ± 2.50 versus 25.46 ± 4.37 versus 22.46 ± 1.75 versus 26.16 ± 7.18 h, p < 0.0001). The HF component increased in 2 of 7 astronauts, whereas it decreased in 3 of 7 astronauts and 1 was remained almost unchanged at DF1. During DF3, about 6 months after their stay in space, the HF component of 5 of 7 astronauts recovered from the decrease after launch, with prominent improvement to over 20% in 3 astronauts. Although autonomic nervous functions and circadian rhythms were disturbed until one month had passed in space, well-scheduled sleep and wake rhythms and meal times served as synchronizers.

Sleep and Skin Temperature in Preschool Children and Their Mothers

The purpose of this study was to investigate and compare sleep and skin temperature (Tsk) of preschool children with those of their mothers. The subjects included 18 pairs of preschool children and their mothers. The actigraphic measurement of sleep, Tsk, heart rate, bedroom climate, and the microclimate temperature and humidity (bed climate) were measured. Proximal and distal Tsk, the temperature gradient of distal and proximal Tsk (DPG), and bed climate temperature were significantly lower in the children. Approximately 70% of the children slept without bed covering. Heat dissipation during sleep in preschool children may primarily rely on the proximal Tsk. The lower Tsk than adults, and behavioral thermoregulation, may be important for sleep in preschoolers.

Effect of cardboard under a sleeping bag on sleep stages during daytime nap.

Fourteen healthy male subjects slept from 13:30 to 15:30 under ambient temperature and relative humidity maintained at 15 °C and 60%, respectively. They slept under two conditions: in a sleeping bag on wooden flooring (Wood) and in a sleeping bag with corrugated cardboard between the bag and the flooring (CC). Polysomnography, skin temperature (Tsk), microclimate, bed climate, and subjective sensations were obtained. The number of awakenings in the CC had significantly decreased compared to that in the Wood. The mean, back, and thigh Tsk, and bed climate temperature were significantly higher in the CC than that in the Wood. Subjective thermal sensations were warmer in the CC than in the Wood. These results suggest that using corrugated cardboard under a sleeping bag may reduce cold stress, thereby decreasing the number of awakenings and increasing subjective warmth; the mean, back, and thigh Tsk; and bed climate temperature.

Effects of voluntary abdominal breathing on sleepiness and heart rate variability after modest sleep restriction

examined to establish how strain pressure affects heart rate, heart rate variability, blood pressure, blood pressure variability, and intrarectal pressure, in the supine and sitting positions, during a Valsalvas maneuver (VM) performed with a mouthpiece connected to a manometer. Bioelectrical impedance variables indicating intrathoracic blood volume, strain pressure, intrarectal pressure, heart rate, and blood pressure were measured simultaneously. The variables were recorded before, during, and after a 15-s Valsalvas maneuver at 10, 20 and 30 mm Hg of strain pressure. The increase in intrarectal pressure during the Valsalvas maneuver was greater in the sitting than in the supine position, and the effects of the Valsalvas maneuver were more marked in the subjects in sitting position. We conclude that it was easier to raise intrarectal pressure in the sitting position than in the supine position, because in the sitting position gravity made the effects of Valsalvas maneuver on hemodynamics more marked. However, no significant difference in hemodynamics was seen during strain pressure at 30 mm Hg, because other factors, e.g., muscle activity, exceeded the influence of gravity observed with a lesser strain pressure. Defecation may be safer when an increase in abdominal pressure is obtained with less strain pressure. The heart and peripheral vessels thus apparently play different roles in regulating cardiac output in higher position-related strain pressure. (The Autonomic Nervous System, 48: 48–55, 2011)