It sounds backwards: heat yourself up in order to cool down. But the mechanism is well understood and the evidence is better than for most things sold as sleep aids.
Sleep onset runs on falling temperature
Core body temperature follows a circadian rhythm. It peaks in the late afternoon or early evening, then falls through the night, bottoming out roughly two hours before your natural wake time.
That fall is a sleep signal. The rate of decline correlates with how quickly you fall asleep. Anything that steepens it helps; anything that flattens it hurts — which is why a hot bedroom is one of the more reliable ways to sleep badly. (How far to drop the room.)
Why heating your body cools it
The counterintuitive part, and it hinges on where the blood is.
A hot bath warms your skin and triggers peripheral vasodilation — the blood vessels in your hands, feet and skin surface open up and blood flows outward from the core.
Your extremities are efficient radiators. Blood arriving there sheds heat to the air. So after you get out, your core temperature drops faster and further than it would have on its own, because you have opened the radiators.
You feel warm. Your core is cooling more steeply than usual. That gap is the whole trick.
The timing that makes it work
This is where most people go wrong, and it is the difference between the intervention working and doing nothing.
A meta-analysis of water-based passive body heating found the optimal window is roughly one to two hours before bed, with about 90 minutes the most commonly cited figure. Under those conditions, participants fell asleep on average around 10 minutes faster, with improvements in sleep quality and efficiency.
Ten minutes may sound modest. Set against most consumer sleep products it is a strong result, and it is free.
Too close to bedtime and it backfires. Get out of a hot bath and straight into bed and your core is still elevated — you have raised temperature at the moment it should be falling. The cooling has to happen before you lie down.
The specifics
Temperature: roughly 40–42°C / 104–108°F. Comfortably hot, not scalding.
Duration: 10 to 20 minutes is enough. Longer adds little.
Timing: 90 minutes before bed. If your routine only allows 60, that still works.
A shower works too, though somewhat less well — less of the body is heated for less time. Focus the water on the extremities if you are showering: hands, feet, lower legs, since those are the radiators you are trying to open.
A foot bath is a real option if a full bath is impractical. Feet are disproportionately effective at heat exchange, and studies of foot bathing before bed show measurable benefit.
Who this suits
Particularly useful if you have cold hands and feet at bedtime. That is a sign of poor peripheral vasodilation, meaning your body is struggling to shed core heat — exactly the mechanism a warm bath assists.
It also stacks well with the rest of an evening routine, and it is one of the few interventions that is pleasant enough that people actually keep doing it. (The last hour before bed.)
Where it does not help
It is not a treatment for insomnia. It helps sleep onset modestly. CBT-I is first-line for chronic insomnia and nothing here substitutes.
It does not fix a mistimed clock. If you are wide awake at 1am because your circadian rhythm has drifted, temperature will not correct that. Light timing will. (The evening dimming protocol.)
It does not fix a fragmented night. If you fall asleep fine and wake exhausted, sleep onset was never your problem. Something is breaking the night into pieces, and disrupted breathing is among the commonest and most fixable. (Titan's summary of the nasal breathing research.) (The full checklist.) (What nasal breathing changes across a night.)
And the morning inversion
Worth noting because the two get conflated: the morning version of this runs the opposite direction.
Waking is accompanied by core temperature rising, so a warm shower in the morning works with that, while a cold one triggers a noradrenaline surge that produces alertness through an unrelated mechanism. (Whether cold showers actually work.)
Warm in the evening to steepen the fall. Warm in the morning to assist the rise. Cold in the morning is a stimulant, not a temperature intervention.
The bottom line
Heating your skin opens the peripheral vessels, which lets your core shed heat faster once you get out — steepening the temperature fall that sleep onset depends on.
Roughly 40–42°C for 10 to 20 minutes, about 90 minutes before bed. Around 10 minutes faster to sleep, better efficiency, and free. A shower or even a foot bath captures much of it. The one thing that ruins it is doing it immediately before getting into bed.