Apparently, you can influence your circadian rhythm by heating up or cooling down your body at certain times of day, and a well-timed and designed bath is one way to do that. You’ll need a clock, a water thermometer, an air thermometer, and an understanding of the metric system.

In the study, researchers at the University of Freiburg in Germany assigned 45 people with depression to either soak in 40C water for up to 30 minutes and then wrap themselves in blankets and hot water bottles for a further 20 minutes, or take 40 to 45 minutes of aerobic exercise twice a week. Eight weeks later, those taking regular warm baths in the afternoon scored six points lower on a commonly used depression scale, while the exercise group scored three points lower on average…

Your bathwater should be just a little hotter than body temperature, which is about 37C. Somewhere between 40C and 45C is ideal…

Consider the temperature of the room as well. A Japanese study showed that bathing in 41C water in a 25C room increased body temperature more than taking a bath in a 14C room. However, if taking a bath to promote sleep before bed, the room temperature should be cooler: 18C is ideal.