It’s another sticky summer evening. You’re too hot to sleep. Surely an ice-cold shower is the best way to cool off and drift away quicker? Not really, says Mike Tipton, a professor of human and applied physiology at the University of Portsmouth.

“If you’re trying to go to sleep, the last thing you want to do is go into a freezing cold shower,” he says. The shock of cold water immersion causes the body to produce adrenaline and cortisol, stress hormones designed to keep us alert. “That’s just about the last thing you want before you go to bed.”

Cold water immersion also affects blood circulation, one of the body’s main ways of regulating temperature. When we get too hot, blood vessels near the skin dilate, warm blood moves towards the surface and heat is released into the air.

Very cold water triggers the opposite response: the body defends its temperature by decreasing circulation and shutting down blood flow to the skin. “What you do then is you trap the heat inside the body.” Eventually it will come out, Tipton says, but transfer from the body’s core to the skin and then into the air takes longer.

Freezing cold water is useful during severe heatstroke, when circulation has collapsed. “The best way to get heat out of the body then is by immersing somebody in very cold water,” he says. But for someone simply trying to cool down before bed, a tepid shower is more effective because it allows blood flow to the skin, where heat can be gently removed.

The ideal temperature varies between people but is likely in the mid to high 20C range, whereas a freezing cold shower would be below 10C.