A 42-degree shower cools you down more than a cold one

You take a cold shower at eleven at night thinking it will cool you down for sleep, and you get the opposite. Cold closes the vessels in your hands and feet to protect the core: your skin goes icy and the heat inside has nowhere to go. Water at forty-two degrees does exactly the reverse. The vessels open, hands and feet turn into two radiators, and core heat leaves through them over the following hour.

This matters because the signal to sleep is thermal: you fall asleep when your internal temperature drops by a few tenths of a degree. A review from the University of Texas at Austin published in 2019 gathered the trials on warm baths and showers before bed, and the same pattern came up again and again: ten minutes in water between forty and forty-two and a half degrees, one to two hours before getting into bed, brought sleep on about ten minutes earlier.

The timing is the part almost everyone gets wrong. Work out when you want to be asleep, subtract two hours, shower for ten minutes with the water good and hot, and then go and do something else: the drop arrives on its own.