Taking a warm shower or bath before bedtime is a known way to improve sleep, but when's the perfect time to do it? A new study has the answer.

For those who struggle to get a decent amount of sleep each night, there are a number of tips to try to improve the chance of falling asleep and the quality of our rest. For example, we know that exercising regularly, keeping the bedroom cool, and avoiding alcohol before going to sleep are some great ways to ensure we get a good night's rest. Taking a bath or shower before bedtime is also a well-known sleep remedy.

This is partly because a warm bath can help us relax, but also because our body temperature tends to drop after a warm bath, which can induce better sleep. However, does it make any difference to sleep quality exactly when we bathe? Researchers led by Shahab Haghayegh, a doctoral researcher in the Department of Biomedical Engineering at the University of Texas at Austin, set out to investigate.

Analyzing bathing time and sleep quality

To find out, Haghayegh and colleagues searched through a total of 5,322 studies from databases such as PubMed, CINAHL, Cochran, Medline, PsycInfo, and Web of Science.

They have published the results of their meta-analysis in the journal Sleep Medicine Reviews.

The researchers analyzed the effects of "water-based passive body heating" on several indicators of sleep quality: "sleep onset latency (SOL), wake after sleep onset, total sleep time, sleep efficiency, slow-wave sleep, and subjective sleep quality."

A longer SOL—that is, how long it takes to fall asleep—is a standard measurement of sleep satisfaction. Sleep efficiency is also a conventional way of measuring sleep that divides the time a person is asleep by the total time they spend in bed minus the SOL. Finally, slow-wave sleep describes the deepest phase of sleep, which many believe to be critical for learning and memory consolidation.

Bathe 1–2 hours before bed for best sleep

The results of the analysis revealed that the best time for taking a shower or a bath is 1–2 hours before going to bed. The duration of the shower or bath does not need to be longer than 10 minutes for a person to reap the benefits. This cools the body down by improving the blood circulation from the core of the body to its periphery—that is, to the hands and feet. Taking a warm shower or bath at this time improves the "temperature circadian rhythm," helping people fall asleep more quickly and improving sleep quality, explain the study authors.

"When we looked through all studies, we noticed significant disparities in terms of the approaches and findings," explains Haghayegh. "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 [them] through a new lens."

"Yes, the data [prove] that a warm shower or bath before bed does make a huge difference in your overall sleep quality," says Haghayegh, adding, "I shower every night before bed now."