Sleeping in a cold room — is it good or bad?
Sleep is important to overcome the feeling of fatigue, recover the physical strength, rehabilitate nervous and immune systems and promote the growth in young individuals.1 Sleep constitutes almost one-third of a lifetime in humans.2 It has been stated that disturbed nocturnal sleep can cause impaired cognitive and physiological functions.3 Decreased quantity and quality of sleep has been associated with many adverse health effects including chronic conditions like diabetes, hypertension, obesity, reduced quality of life and increased mortality.4 In humans, normal sleep is composed of two states- Rapid Eye Movement (REM) and Non REM (NREM) that happens alternatively across a sleep episode. Sleep generally begins with first stage of NREM state and progresses into deeper stages of NREM. The first episode of REM occurs 80 to 100 minutes after the initiation of NREM. The cycle of NREM reoccurs every 90 to 120 minutes in distinct episodes.3
The quality and quantity of sleep in humans can be affected by several environmental factors such as light, noise, temperature, humidity, air circulation, settings of bedroom.4 Ambient temperature and the core temperature of body are considered as key determining factors for sleep since thermoregulation affects the mechanism regulating sleep.5 Extremely low or high ambient temperature (Ta) may affect sleep even in healthy humans without insomnia. This article mainly focuses on the relationship between the temperature of the room and the quality of sleep, ideal temperatures conditions and benefits of sleeping in low temperature environment.
Core temperature, sleep and thermoregulation
In human body, periodic circadian rhythms or variations are ubiquitous in nature. It includes physiological and behavioural variations of core body temperature, melatonin secretion, sleep propensity and subjective alertness. The circadian rhythm of core body temperature (CBT) is closely related to the sleep-wake cycle. A rapid decline in CBT increases the likelihood of sleep initiation and promotes entry into the deeper stages of sleep.6
The thermoregulatory changes in the body during sleep are controlled by the preoptic area/ anterior hypothalamus (POAH). The thermoregulatory change in the sleep varies with the stages of sleep. Studies have confirmed that hypothalamic centres of brain mainly control NREM state of sleep whereas temperature sensitivity is reduced during the REM sleep. Most of the temperature sensitive receptors are concentrated in the face, hands, feet and specially fingers and toes. Another factor that plays an important role in thermoregulation is the temperature of the skin.3
Ideal temperature for sleep
Several standards and guidelines specify the design criteria of air ventilation and ideal temperature for an undisturbed and peaceful sleep. According to WHO a minimum temperature of 18°C is recommended for bedrooms. The Charted Institute of Building Services Engineers (CIBSE) guide A recommends a temperature range of 17-19°C for winters and 23-25°C for summers. The European standard (EN15251) recommends a minimum temperature of 20°C and a maximum temperature of 26°C in living spaces.3
Benefits of cooler environments on sleep
More restful sleep/ improves quality: It has been stated that sleep is strongly associated to thermoregulation. In humans, during the 24-hour sleep-wake cycle, the core body temperature which also rhythms with the circadian system decreases during the nocturnal sleep phase and increases during the wake phase. As the body prepares to sleep, vasodilatation in the skin helps in heat dissipation, initiating an important signal for the sleep onset i.e. the decrease in core temperature of the body. This signal is further preceded by the amplification of temperature decrease at the distal to proximal sites helping to cool the core in the evening and early morning. Once the core temperature drops during sleep it remains low throughout the sleep and rises again before the awakening. Maintaining cooler and ambient temperature of the surroundings aids to facilitate the onset of sleep by lowering the core temperature and is associated with uninterrupted more restful better quality sleep.7
Fall asleep quickly: As the night time approaches, the circadian rhythm naturally thermo regulates the body by lowering the core temperature. By keeping the temperature lower, it reinforces the body’s natural instinct to sleep. If the temperature is too hot, it could potentially block signals in hypothalamus and takes longer to fall asleep.
Improves melatonin levels: Melatonin is mainly produced by a small pea shaped pineal gland in brain. It is released into bloodstream exclusively at night following the circadian rhythm. Several studies have reported the role of melatonin in improvement of nocturnal quality of sleep, increase in total sleep time, improvement in sleep efficiency and decrease in sleep onset latency.8 Literature also state that decrease in release of melatonin during heat exposure and in humid environment lead to impairment in short wave sleep patterns and REM sleep.9 Similarly Fujii H et al. in 2015 studied the effects of increase in outdoor ambient temperatures on the health, specifically fatigue and sleep quality in 602 healthy volunteers in Osaka, Japan and observed increased fatigue and poor sleep patterns in healthy volunteers in increased air temperatures depending upon the use of air conditioners, accommodation status and age.10
Anti-aging: As mentioned above, ambient room temperature of 18 to 25°C aids in improving sleep quality and encourage the circadian release of melatonin exclusively at night. In addition to promoting sleep, melatonin also acts as an anti-aging hormone and helps in healthy aging.11 Haimov I. in 2001 also reported review evidence that biological aging leads to decrease in circulating melatonin levels and impaired melatonin secretion is associated with sleep disturbances in elderly.12
Decrease risk of metabolic diseases and helps in weight loss: A recent discovery of functional brown adipose tissue (BAT) has opened new prospective to combat obesity and metabolic disorders. Brown adipose tissue is a thermogenic organ that dissipates nutrient energy as heat, preventing hypothermia and obesity. Earlier it was reported to be present only in infants but over the last decade with advanced metabolic imaging, conclusive evidence has been found for the existence of BAT in adult humans. BAT function operates at two levels of an acute course of linking the intrinsic thermoregulation and a chronic course linking the growth and proliferation of pre-existing brown adipocytes and the differentiation of specific white adipose tissue depots into brown adipocytes. Studies have reported BAT activity reduces with obesity and is stimulated by cold exposures which increases the sensitivity of BAT towards the insulin, glucocorticoids, mineralocorticoids in humans and aids in reducing the risk for metabolic disorders facilitating fat loss and obesity by converting bad fat into good brown fat. These findings strongly suggest that cooler sleeping environment not only provide better sleeping quality but also reduces the risk of metabolic diseases and aids in weight loss.13
Prevents insomnia: Difficulty in maintaining nocturnal sleep and excessive wakefulness is generally associated with elevated nocturnal core temperature. Studies have reported that sleep is typically initiated with a drop in core body temperature (CBT) and the maximum circadian sleepiness occurs at the CBT minimum (CBTmin) and insomniacs have significantly higher core temperature approximately 0.2°C throughout their circadian rhythm. Sleep difficulties or insomnia can also occur if the temperature rhythm is relatively late with respect to the attempted sleep period. The non-pharmacological means of treating insomnia includes lowering the environmental temperature to achieve the drop in the CBT and initiating the sleep cycle.6
Improves mood and reduces stress: as we have discussed above, cooler environments improve the quality and quantity of nocturnal sleep. A well-slept and rested individual feels better overall. A better sleep helps to enhance the mood and reduces the stress levels. Another factor that helps in enhancing the mood after a good night sleep is the release of serotonin that is known as a mood enhancer and also a precursor for melatonin.14
A quality sleep is very important for the well-being and proper functioning of the human body. The physical environment especially the thermoregulation is the key factor in determining the quantity and quality of sleep. To conclude, maintaining the cooler temperature of the surroundings helps in facilitating the sleep onset, maintaining the circadian rhythm and modulating the core body temperature to achieve an uninterrupted and healthy sleep at night.
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