The method could work because raising body temperature in the afternoon helps restore the normal circadian rhythm of temperature, which is often disturbed in people with depression. The baths also improved people's sleep patterns.
Depression is one of the most common mental-health issues and is usually treated with antidepressants and talking therapy. However, the medication may cause side effects, and a course of talking therapy can be expensive, sometimes with long waiting lists in the UK National Health Service, for instance.
The root cause of depression is unclear. The dominant view is that there is not enough of a brain-signalling molecule called serotonin, because antidepressants seem to boost levels of this compound.
Another suspect is disturbed circadian rhythm, the physical and biochemical changes that happen to our bodies over the day. Your body temperature would normally rise in the morning, peak in the afternoon, then dip back down when you sleep, following a wave-like curve with a difference of about a 1 degree C between day and night.
But if you have depression, the cycle may be flatter, erratic or delayed by a few hours causing the peak to occur later in the day and improve mood.
His team looked at 45 people with depression, about half of whom were taking antidepressant medicines, which they continued with. People were randomly allocated to either twice-weekly exercise sessions or thermal therapy.
The bathing involved going to a spa to soak in a pool at 40 degrees C for up to 30 minutes, then getting out and wrapping up with blankets and hot water bottles for another 20 minutes. After two weeks, some people chose to carry on with the routine at home, while the rest continued with it at the spa. The baths raised body temperature by nearly 2 degrees C.
After eight weeks, the hot-bath treatment reduced symptoms by about 6 points on a commonly used depression scale, down from an average starting place of 21, while the exercise programme lowered it by about 3 points. The baths also started working within two weeks, unlike exercise, say the authors in their paper.
Nick Stafford, a psychiatrist at the Black Country Partnership NHS Foundation Trust, UK, says the approach makes sense given what happens to circadian rhythms in depression. "I'm not surprised they found a benefit, I'm just surprised no one has tried doing this before."