Researchers at the University of Texas at Austin (UoT) say that a warm bath one or two hours before going to bed is just what you need for a good night’s sleep.

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A review of thousands of previous studies on the subject allowed the authors to link bathing and showering (water-based passive body heating) with improved sleep quality. Bathing 1-2 hours before bedtime in water of about 104-109°Fahrenheit (40 to 43°C) seems to yield the best results.

A nice, relaxing bath

“When we looked through all known studies, we noticed significant disparities in terms of the approaches and findings,” said Shahab Haghayegh, a Ph.D. candidate in the Department of Biomedical Engineering at UoT and 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.”

The team collaborated with members from the UoT Health Science Center at Houston and the University of Southern California to review 5,322 studies on the effects of water-based body heating on sleep. From this body of research, they extracted information published in journals meeting predefined inclusion and exclusion criteria (so as to ensure their data was reliable). The team quantified the effects of bathing by looking at several indicators:

• Sleep onset latency: how much time it takes to fall asleep
• Total sleep time
• Sleep efficiency: how much time people spend asleep compared to the amount of time they want to sleep
• Subjective sleep quality.

Overall, the team reports, the optimum bath temperature to increase sleep quality revolved around 104-109°Fahrenheit (40 to 43°C). If taken 1-2 hours before bed, such a bath also reduced the time needed to fall asleep by an average of 10 minutes.

The team writes that body temperature is, in part, governed by our bodies’ sleep/wake cycle (the ‘circadian clock’) that is handled by the hypothalamus. Our bodies are 2 to 3 degrees Fahrenheit warmer in the late afternoon/early evening than during sleep (at which point it is the lowest of the whole day). People on average experience a reduction in core body temperature of about 0.5 to 1°F an hour before their usual sleep time. It keeps dropping as we sleep, reaching the lowest point sometime between the middle and later part of our usual sleep duration. In the morning, or around the time one is used to waking up, core temperature begins to rise. Think of it as a kind of biological alarm clock.

The temperature cycle underpins the sleep cycle and is an essential factor in falling asleep rapidly, easily, and for ensuring high-efficiency sleep, the team notes. The team says that the optimal time to have a bath is 90 minutes before going to bed — this gives your body enough time to cool down sufficiently.

Warm baths and showers stimulate the body’s thermoregulatory system, they explain, which improves blood flow from the internal organs out to peripheral areas such as the limbs — where it dissipates heat. Therefore, baths taken 1-2 hours before bedtime aid the natural circadian process and increase the chances of falling asleep quickly and improve sleep quality.

The paper “Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis” has been published in the journal Sleep Medicine Reviews.