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“When we looked through all known studies, we noticed significant disparities in terms of the approaches and findings,” said Shahab Haghayegh, lead author on the paper.
Researchers analysed around 5,322 studies and extracted pertinent information from publications meeting predefined inclusion and exclusion criteria to explore the effects of water-based passive body heating on a number of sleep-related conditions: sleep onset latency – the length of time it takes to accomplish the transition from full wakefulness to sleep; total sleep time; sleep efficiency — the amount of time spent asleep relative to the total amount of time spent in bed intended for sleep; and subjective sleep quality.
The findings showed that an optimum temperature of between 104 and 109 degrees Fahrenheit improved overall sleep quality. If people scheduled their bath 1-2 hours before bedtime, it can also increase the speed of falling asleep by an average of 10 minutes.
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The average person’s circadian cycle is characterized by a reduction in core body temperature of about 0.5 to 1 F around an hour before usual sleep time, dropping to its lowest level between the middle and later span of nighttime sleep. It then begins to rise, acting as a kind of a biological alarm clock wake-up signal. The temperature cycle leads to the sleep cycle and is an essential factor in achieving rapid sleep onset and high-efficiency sleep.
The researchers found the optimal timing of bathing for cooling down of core body temperature in order to improve sleep quality is about 90 minutes before going to bed.
Warm baths and showers stimulate the body’s thermoregulatory system, and show a marked increase in the circulation of blood from the internal core of the body to the peripheral sites of the hands and feet, resulting in efficient removal of body heat and decline in body temperature.