Taking a hot bath at least 90 minutes before bedtime is your ticket to sound sleep. Just remember that the water temperature should be around 40-42ºC, else you may not get better shuteye.
Biomedical engineers at University of Texas-Austin reached this conclusion after analyzing thousands of studies linking water-based passive body heating, or bathing and showering with warm/hot water, with improved sleep quality.
"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.
"The only way to make an accurate determination of whether sleep can, in fact, be improved was to combine all the past data and look at it through a new lens."
In collaboration with the UT Health Science Center at Houston and the University of Southern California, the researchers reviewed 5 322 studies.
Meta-analytical tools were used to assess the consistency between relevant studies and showed that an optimum temperature of between 40-42ºC improved overall sleep quality.
When scheduled one-two hours before bedtime, it can also hasten the speed of falling asleep by an average of 10 minutes.
It is understood that both sleep and our body's core temperature are regulated by a circadian clock located within the brain's hypothalamus that drives the 24-hour patterns of many biological processes, including sleep and wakefulness.
The average person's circadian cycle is characterised by a reduction in core body temperature around an hour before usual sleep time - dropping to its lowest level between the middle and later span of night-time sleep.
It then begins to rise, acting as a kind of a biological alarm clock wake-up signal.
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.
"If baths are taken at the right biological time - 1-2 hours before bedtime - they will aid the natural circadian process and increase one's chances of not only falling asleep quickly but also of experiencing better quality sleep," showed the findings, which appeared in the journal Sleep Medicine Reviews.