Warming up for 10 minutes cools you down, and that is why you sleep

Your core temperature drops between half a degree and a full degree in the half hour before you fall asleep, and that fall is the signal the brain is waiting for. A hot bath looks like it should work against that, but it does the opposite: heat dilates the vessels in your hands and feet, blood rises to the skin and the body dumps heat through it far faster. You come out of the water hotter on the outside and you cool down inside sooner.

It has been measured. In 2019 a team at the University of Texas at Austin led by Shahab Haghayegh screened 5,322 studies and kept seventeen; the result, published in Sleep Medicine Reviews, was that the time it takes to fall asleep drops by roughly a third, around ten minutes on average.

Instructions, using the study's own numbers:

Get into water at 40 to 42.5 degrees. Ten minutes is enough; a shower works, a bath is better because it covers more skin.

Do it between one and two hours before bed, never right before: the drop needs that margin.

Get out and do not warm up again. No electric blanket, no bedroom at 24 degrees; the range the sleep literature works with is 18 or 19.

If you are going to bed at midnight tonight, the shower is at half past ten.