To help us fall and stay asleep, experts have long recommended that we take a warm shower or bath before going to bed.
What hasn’t been clear, however, is when we should immerse ourselves in water. A half hour before bedtime? An hour? Longer?
A new study, published in the August issue of Sleep Medicine Review, offers an answer. A team of researchers from the University of Texas at Austin analyzed data from the 17 best studies (narrowed down from more than 5,000) that had previously investigated whether warm water — in the form of either full-body showers or baths or just footbaths — affected the onset and quality of sleep.
The studies involved diverse groups of people, including young, healthy soccer players, middle-aged patients who had experienced traumatic brain injuries and older people with sleep apnea. Other groups included people living with cancer or heart disease, as well as people being cared for in hospital intensive care units.
Thirteen of the studies contained enough comparable data to pool for a meta-analysis.
All in the timing
That analysis found that the optimum time to take a shower, bath or footbath was one to two hours before going to bed. It also found that the best water temperature to bath in — the temperature most conducive to improving sleep — was 104-109 degrees Fahrenheit.
Those factors had the biggest impact on something sleep researchers call “sleep onset latency” — the time it takes to fall asleep. Showering or soaking in the tub one to two hours before bedtime moved up the onset of sleep by an average of 8.6 minutes, or almost 40 percent.
But the activity improved other aspects of sleep as well, including the total time spent asleep and the time spent in slow wave sleep, the “deep sleep” believed necessary for memory consolidation, energy restoration, cell regeneration and other important functions.
As little as 10 minutes in a bath or shower was all that was necessary to trigger these sleep improvements.
Why the findings make sense
Why does bathing help us fall — and stay — asleep? It’s mostly because of its effect on body temperature.
Our core temperature declines naturally toward the end of each 24-hour cycle — usually about an hour or two before our usual time of sleep. When we get out of a warm bath or shower, that process gets accelerated, causing us to feel sleepier more quickly.
In other words, it’s the cooling of our body after the bath, not the warmth of the bath itself, that helps us fall asleep.
But don’t wear socks to bed.
The authors of the current study omitted several footbath studies from their meta-analysis because those studies included a treatment in addition to soaking in warm water: wearing socks to bed. None of those studies showed any improvement in their participants’ sleep — a failure that is likely due to the socks, say the authors of the current study.
As they explain, the body cools itself by sending blood from its internal core to the hands and feet, where it can be quickly and efficiently dissipated.
Socks may slow down that process.
For more information: You’ll find an abstract of the study on Sleep Medicine Review’s website, but the full study is behind a paywall. For more tips on getting a good night’s sleep, go to the National Sleep Foundation’s website.