‘I am sure there are things that can’t be cured by a good bath, but I can’t think of one,’ wrote the poet and novelist Sylvia Plath in The Bell Jar. Plath, who struggled with depression, would surely have felt vindicated by the findings of a study reported in New Scientist recently, which concluded that taking regular afternoon baths was associated with a moderate but persistent lift in mood among people with depression. Indeed, the size of the effect was greater than that of physical exercise — a more established mood booster.

By increasing participants’ core body temperatures, the theory goes, warm baths helped to strengthen and synchronise their circadian rhythms, the daily fluctuations in behaviour and biochemistry that affect every one of our organs, including the brain. Core body temperature usually rises during the daytime and falls at night — in fact, this nightly decrease helps us to fall asleep by promoting release of the hormone melatonin, which tells the body when it is night. However, in depressed people, this temperature rhythm is often flatter, disrupted or delayed by several hours. Morning exposure to bright light, which both strengthens circadian rhythms and shifts them to earlier in the day, has also been found to alleviate depression.

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 40-45 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.

Although the study was small, and there were some methodological issues with it (many of the participants in the exercise group dropped out), it builds on existing evidence suggesting that body temperature can be used to tweak our circadian rhythms. Other studies have indicated that taking a warm bath before bed can aid sleep, by dilating blood vessels in the skin, which helps the body to dump excess heat. This only works if you keep your bedroom relatively cool though — about 18C is ideal.

Indeed, one reason why warm baths might reduce depression is simply by improving people’s sleep .

If taking a bath to promote sleep before bed, setting the room temperature at 18C is ideal
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But circadian rhythms may not be the only explanation for the curative properties of a warm bath. Depression is thought to be related to low levels of the brain-signalling molecule serotonin, and research in rats has shown that serotonin-releasing neurons connected to mood-regulating regions of the brain fire in response to increases in body temperature. Whatever the explanation, if you find taking a warm bath relaxing and it makes you feel good, there is now even more reason to take the plunge.

How to take a good bath

• Pick a time when you are unlikely to be distracted. Peace and quiet should help you to relax.

• Consider adding an essential oil, such as lavender oil, to promote relaxation.

• 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.

• Large and sudden increases in temperature put strain on the heart, so if you suffer from heart problems, avoid hot baths — particularly on cold days.

Guardian News & Media