Subjects
Six Korean young male subjects participated in this study (mean ± SD 22.7 ± 2.0 years in age, 175.6 ± 3.5 cm in height, 73.1 ± 8.5 kg in body weight, and 23.6 ± 2.2 in body mass index). Volunteers having any symptoms of sleep disturbance, sleep disorder, or unusual (irregular) sleep patterns were excluded. We recruited subjects only who had a regular sleep schedule (6~8 h sleep per night) and daily activity routine through pre-screening. The subjects were also instructed to keep their daily activity routines before each experiment was conducted. Also, the subjects were required to abstain from strenuous exercise and alcohol 24 h before arriving, and to refrain from eating food including stimulating or caffeinated drinks 3 h before arriving, to minimize any effect of those factors on sleep quality. All the experimental procedures were described in detail to each subject who then signed to an informed consent prior to experiments. The experimental protocol was approved by the Institutional Review Board of Seoul National University (IRB No. 1801/001-004).
Experimental clothing and bedding
The conceptual and methodological frameworks are illustrated in Fig. 1. Subjects were identically dressed in long-sleeved T-shirts, long sweat pants, and undershorts (670 g in total, 0.66 clo in estimated clothing insulation). The top cuff circumference, cuff length, foot length, fabric thickness, mass per area, and air permeability of bed socks were 16.5 cm, 12.0 cm, 20.0 cm, 2.4 mm, 0.0316 g/cm2, and 20.1 cm3/cm2/s at 125 hPa, respectively. The fabric composition of the bed socks was polyester 98.6% and polyurethane 1.4%. The socks were worn in one of the two conditions and not reused. When moving into a climatic chamber from a preparation room to sleep, subjects wore in-house slippers and were required to take them off before stepping into the chamber. Each subject was assigned to single bedding, which includes a sponge mattress (polyester 100%, 5 cm thickness) and its sheets (cotton 100%, 110 cm × 185 cm), a blanket (cover: cotton 100%, 160 cm × 200 cm, padding: polyester 100%, 3.5 cm thickness), and a pillow (cover: cotton 100%, 40 cm × 60 cm, padding: polyester 100%, 15 cm thickness). In both control and feet warming condition, subjects wore the identical bedding during sleeping.
Experimental conditions and procedures
Experiments in this study consisted of two conditions: one condition in which subjects slept wearing no socks (CON) and the other condition in which subjects slept wearing bed socks to warm their feet (Feet warming condition). At least 2 days before each experiment day and between the two experiment conditions, subjects were instructed to keep their own regular sleep patterns. Subjects were required to visit the laboratory twice in total to participate in each 7-h sleep experiment. Each visit was separated by an interval of 6 days on average (at least 2 days) to help the subjects to regain their regular sleep schedules before revisiting the laboratory for the second experimental condition. The order of the conditions was randomized to avoid the effect of familiarization. In both conditions, each subject slept in a climatic chamber at an air temperature (Ta) of 23.0 ± 0.1 °C with 55.0 ± 0.7% relative humidity (RH) from 00:00 AM to 07:00 AM.
Subjects were required to arrive at the laboratory at least 2 h before each scheduled test and after changing into experimental clothing, rest sufficiently. A group of three subjects were tested together, and the subjects became closer for the 2~3 h before starting the experiments. In the Feet warming condition, bed socks were worn approximately 1 h before going to bed to sufficiently manipulate foot skin temperature prior to the start of measurements. At 11:25 PM, subjects were asked to leave the preparation room. Before moving into the bedroom in the climatic chamber at 11:30 PM, subjects were asked to use the bathroom if needed and not to bring any kind of personal electrical devices into the bedroom. In case of emergency during experiments, subjects were encouraged to go out of the bed room immediately and let the experimenters know. After entering the bedroom, subjects lay down on a mattress which was placed on the floor and covered themselves with a blanket assigned to each person, resting their heads on a pillow. Closing eyes was discouraged until 00:00 AM to prevent subjects from falling asleep before measurements start. Each subject was instructed to start sleeping in a proper position at 00:00 AM with lights off, wearing earplugs provided to exclude influence of unnecessary noise on sleep. At 07:00 AM, lights were turned on and subjects were woken up. After being directed to the preparation room and completing a sleep quality questionnaire about the previous night, all the sensors and experimental clothing were removed from subjects (Fig. 2).
Measurements and calculations
Over a period of 7 h, rectal temperature (Tre) was measured every 30 s by a thermistor probe inserted 16 cm beyond the anal sphincter. Skin temperatures on the ten sites (the forehead, chest, abdomen, forearm, hand, thigh, calf, foot, ankle, and sole) were measured every 30 s by thermistor probes. All the skin temperature probes were attached on the left side of the body except for the forehead, which was measured in the center. Ankle temperature was measured on the back. Tre and skin temperatures were recorded automatically using a data logger (LT-8A; Gram Corporation, Japan). Mean skin temperature (\( {\overline{T}}_{sk} \)) was calculated using a Hardy and Dubois’ equation [17]. Distal-proximal skin temperature gradient (DPG) was calculated by subtracting the proximal temperature (mean of the chest, thigh, abdomen, and forehead skin temperature) from the distal temperature (mean of the hand and foot skin temperature) [16]. Negative value in DPG hence indicates that proximal skin temperatures were higher than distal and vice versa for positive value in DPG. Heart rate was monitored every 1 s (RC3 GPS, Polar Electro, Finland), and the data were sorted out at the interval of 30 s. Total sweat rate (TSR) was estimated using a change in total body mass before and after the experiment (ID2, Mettler-Toledo, Germany; resolution of 1 g). The wrist-worn Actigraphy (wGT3X-BT, Actigraph, FL, USA) monitor, a tri-axial accelerometer, was worn on the non-dominant side of each subject to obtain objective sleep variable data. The primary sleep variables measured were sleep-onset latency (SOL—the amount of time in minutes taken to be scored as “asleep” by an algorithm), total counts (TC—the sum of total counts of actigraphy during the whole period of sleep; activity was “counted” by wrist-worn actigraphy when it caused the acceleration signal to exceed the threshold, and the number of activity “counts” would be calculated at the end of the measurement period; larger total counts indicate that there were more activities counted [18]), sleep efficiency (SE—the number of “asleep” minutes divided by the number of minutes of the whole sleep period), total sleep time (TST—the number of minutes in total scored as “asleep” during the sleep period), wake after sleep onset (WASO—the number of minutes in total when the subject was “awake” after sleep onset), number of awakenings (NOA—the number of awakenings per sleeping period scored by the algorithm; or frequency of awakening), average awakening length (AAL: the average length of “awaken” period in minutes). There were three indexes with regard to sleep fragmentation which represent the degree of being non-relaxed during the sleep period: movement index (MI) and fragmentation index (FI), and sleep fragmentation index (SFI). Greater values in MI, FI, and SFI indicate that sleep was more disrupted.
Sleep quality questionnaire
Subjective sleep quality was measured using a developed sleep quality questionnaire. Each questionnaire filled in by each subject was completed without being interrupted. The ten-question questionnaire consisted of two parts: Part 1 (questions 1 to 7) asked the degree of fragmentation and depth of the previous sleep, and part 2 (questions 8 to 10) obtained a subjective evaluation of the bedroom environment in terms of temperature, humidity, and thermal comfort.
Seven questions asking about fragmentation and depth of sleep were developed from Verran and Snyder-Halpern (VSH) Sleep Scale factors [19]. Five out of seven questions used a 5-point scale, even though the VSH Sleep Scale followed a visual analog format, to help subjects respond with ease [1. “Estimate of the amount of movement during sleep”: 0 Tossed all night, 1 Tossed frequently, 2 Neutral, 3 Hardly tossed, 4 Did not toss at all; 2. “Estimate of depth”: 0 Slept lightly, 1 Slept somewhat lightly, 2 Neutral, 3 Slept somewhat deeply, 4 Slept deeply; 3. “Estimate of how rested you are upon awakening”: 0 Awoke exhausted, 1 Awoke somewhat exhausted, 2 Neutral, 3 Awoke somewhat refreshed, 4 Awoke refreshed; 4. “Spontaneity with which you awoke in the morning”: 0 Awoke abruptly, 1 Awoke somewhat abruptly, 2 Neutral, 3 Awoke somewhat spontaneously, 4 Awoke spontaneously; and 5. “Estimate of sleep along dimensions of satisfaction, quality, and disturbance”: 0 Bad night, 1 Somewhat bad night, 2 Neutral, 3 Somewhat good night, 4 Good night. Such modification between categorical scales and visual analog scales was validated by Lee et al. [20]. Similar to the VSH Sleep Scale, the total scores of five questions were calculated. The higher the total score was, the better the quality of sleep. Two out of seven questions asked subjects to directly write down “6. Estimate of number of awakenings during the sleep period” and “7. Estimate of amount of time from settling down to sleep until falling asleep.”
One question inquiring about temperature of the bedroom was provided using a 9-point scale [8. How hot was the room you slept in: − 4 Very cold, − 3 Cold, − 2 Cool, − 1 Slightly cool, 0 Neutral, 1 Slightly warm, 2 Warm, 3 Hot, 4 Very hot]. Two questions on humidity and thermal comfort of the bedroom were given with a 7-point scale [9. How humid was the room you slept in: − 3 Very dry, − 2 Dry, − 1 A little dry, 0 Neutral, 1 A little wet, 2 Wet, 3 Very wet; 10. How thermally comfortable was the room you slept in: − 3 Very uncomfortable − 2 uncomfortable − 1 A little uncomfortable, 0 Neutral, 1 A little comfortable, 2 Comfortable, 3 Very comfortable].
Data analyses
Sleep variable analyses with Actigraphy data were performed with Actilife 6 using the Sadeh algorithm [21]. All temperature and heart rate data were averaged into 30 min for analytical and graphical purpose. The normality of the data was tested using the Shapiro-Wilks test. To examine differences in physiological and questionnaire responses between the two conditions, the paired t test and the Wilcoxon signed-rank test were used, with parametric data and with non-parametric data, respectively. All the statistical analyses were undertaken using SPSS Statistics 24.0. Significance level was set at 0.05.