I had no idea that a hot bath could engender so much controversy! I received dozens of responses to my Health-e-Byte about taking a hot bath before bed to get a better night’s sleep. Thank you for taking the time to share your thoughts. I very much appreciate your feedback and engagement.
Here are your questions:
- What is the evidence for my recommendations?
- Did I mean 9 minutes or 90 minutes before bed?
- Did the temperature of the water really need to be so hot (40 to 43 degrees Celsius or 103 to 109 degrees Fahrenheit)?
Also here is a reader’s experience:
Time and time again, after bathing in Japanese hot springs that were >40 degrees Celsius, I had disturbed sleep. I struggled to fall asleep and was not able to have deep, sustained, sound sleep. I believe that high temperatures over-stimulate the body and can make people feel restless for a long time thereafter. Body and mind are placed in almost adrenaline mode. The Japanese recommend a warm bath of 38 degrees Celsius. When I have a warm bath — not a hot bath — it feels comfortable and relaxes my mind and body. I then ease into a beautiful night’s sleep.
Here are my answers to your questions and comments:
The source of my recommendations is a meta-analysis published in the journal Sleep Medicine Reviews. A meta-analysis is a compilation of results obtained from multiple independent studies on the same subject. In other words, data obtained from many — not one — team of researchers from around the world is compared and statistically pooled to look for common findings. The combined results showed that the most favourable time to take a bath was one to two hours before bed. The temperature that yielded the most soporific effect was 40 degrees. You can read the original research here:
Shahab Haghayegh, Sepideh Khoshnevis, Michael H. Smolensky, Kenneth R. Diller, Richard J. Castriotta. Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis. Sleep Medicine Reviews, 2019; 46: 124 DOI: 10.1016/j.smrv.2019.04.008
So how do I explain the Japanese experience?
First and foremost, my philosophy is that YOU are your own best guide. Your subjective experience matters more than the results of pooled research. We are all different. If you find 40 degrees too hot, by all means drop the temperature to your level of comfort. Secondly, the meta-analysis only looked at baths taken indoors, not outdoors. The water in hot springs has high levels of minerals such as calcium, magnesium, silica, lithium and in some cases even radium. Household water also contains minerals including calcium, magnesium and potassium but in much lower concentrations. Perhaps some of the mineral salts contribute to restlessness? Or maybe the hot springs were in a stimulating outdoor environment? The other difference between hot springs and household bathtubs is that water in a tub cools down while you soak. The water in hot springs stays consistently hot. Therefore, by the time you get out of your bath at home, the water would be considerably less than 40 degrees. These are simply my thoughts on the matter — I don’t profess to have a definitive explanation for your personal experience and findings. To reiterate the critical point: listen to your body and do what works best for you.
Thank you again to everyone who contributed to this conversation.
Health-e-Byte: What do a hot bath and brisk walk have in common?