Some people swear by a cool flannel. Others pop their pillow cases in the freezer. And more still are spending hundreds of pounds on portable air conditioning units. But there’s one method to help you cool down and sleep during this relentless summer of heatwaves that is so counterintuitive you’ve likely not tried it yet. How about a hot bath before bed?

Yes, it may sound mad. Surely an icy shower would be a better way of cooling off before hitting the sack? The answer to that question is a resounding “no”, according to science. Even though it might feel nice at the time, “if the water is too cold, the skin blood vessels constrict, preventing dissipation of heat from the skin and delaying cooling,” says GP Dr Deborah Lee.

Essentially, a cold shower is tricking your body into thinking it doesn’t need to use the mechanisms it has at its disposal to cool your core temperature – it thinks it has to conserve heat instead.

By contrast, though having a warm bath will slightly raise your body temperature, it causes the blood vessels to dilate so that they’re closer to the surface of the skin, better able to radiate out and get rid of excess heat.

In its advice on how to sleep well when it’s hot, Bupa recommends having “a hot or lukewarm bath (not cold). Your body temperature will go down when you get out of the bath; this will help you to sleep.”

This cooling down process is not just welcome to make us more comfortable when the mercury rises – it also promotes sleepiness, making it easier to drop off.

As your body cools, that drop in temperature helps stimulate the release of melatonin from the brain

“Immersing yourself in a warm bath temporarily raises your core body temperature,” says Dr Tom Cullen, a senior lecturer in high-performance physiology at the University of Hull. “The important part happens when you get out. As your body cools, that drop in temperature helps stimulate the release of melatonin from the brain. Melatonin is the hormone that promotes sleepiness, and under normal circumstances it's released in the evening as body temperature naturally declines.”

Having a bath mimics this natural temperature reduction and aids the temperature regulation process, leading to improved sleep. Researchers have dubbed the phenomenon, for obvious reasons, the “warm bath effect”, according to the Sleep Foundation.

In a recent survey of 2,000 UK adults conducted on behalf of Drench, over half (52 per cent) of respondents had experienced this effect first-hand, reporting that having a warm bath before bed improved their sleep quality.

One separate meta-analysis of 17 scientific studies discovered that taking a warm evening bath or shower improved self-reported sleep quality and sleep efficiency – the percentage of time you actually spend asleep while in bed. It also found that immersing oneself before bed resulted in a “significant shortening” of sleep onset latency – the exact amount of time it takes to transition from full wakefulness to the first stage of sleep. In other words, participants fell asleep much faster after taking a bath. The researchers hypothesised that warm water stimulates blood flow to the hands and feet, which allows body heat to escape more quickly.

Timing and temperature are crucial in all this, though. Just as freezing water is not conducive to sleep in a heatwave, so a scalding tub could do more harm than good. The research has flagged that between 38C and 40C is a good temperature – warm enough to feel comfortable, but not so hot that your heart rate is significantly raised or you start to sweat heavily.

The optimal time to hop in the tub, meanwhile, is one to two hours before bed: “You need to leave enough time to allow the body to cool afterwards, reinforcing the signals that tell the brain it's time to prepare for sleep,” says Dr Cullen.

And, though wiling away several hours in the tub with a book and scented candles might be presented as the epitome of “wellness”, shorter is better for the purposes of sleep; 15 to 20 minutes is ideal.

Finally, throw some good sleep hygiene into the mix. Swerve that bathtime glass of wine – it will likely hinder, rather than help, sleep – and no doomscrolling during or after if you want to avoid an overstimulated brain.