Contrary to popular belief, an ice-cold shower may not be the most effective way to cool down and promote sleep on a hot night. Mike Tipton, a professor of human and applied physiology at the University of Portsmouth, states that a freezing cold shower is "just about the last thing you want before you go to bed."
The shock of cold water immersion prompts the body to release stress hormones, including adrenaline and cortisol, which are designed to keep individuals alert. Furthermore, very cold water causes blood vessels near the skin to constrict, reducing circulation and trapping heat within the body.
While freezing cold water can be useful for severe heatstroke where circulation has collapsed, a tepid shower is considered more effective for general cooling before sleep. This allows blood flow to the skin, facilitating a gentle release of heat. The ideal temperature for such a shower is likely in the mid to high 20C range, in contrast to a freezing cold shower which would be below 10C.