Falling asleep requires core body temperature to drop; a hot shower or bath raises core temperature, so its placement relative to bedtime determines whether it helps or hinders sleep onset.
Protocol: take a hot shower/bath roughly 1.5 hours before bedtime, not immediately before. This gives the body time to complete the post-heat rebound — core temperature rises from the heat, then drops below its starting point as the body dissipates heat — so the natural pre-sleep temperature drop is already underway at lights-out, instead of being fought by a still-elevated core temperature from a shower taken right before bed.
Small studies cited link this earlier timing to faster sleep onset and more deep sleep, compared with showering immediately before bed.
One of the specific, low-cost timing changes emphasized as higher-impact than people expect — moving when an already-existing habit happens, without adding anything new to the routine.
Dr. Mah frames the timing rule with a driving analogy: "It's easier to slowly pump your brakes and then try to sleep versus if you were going 60 mph on a freeway and then suddenly slamming on the brakes and just jumping into bed." A hot shower right before lying down is the equivalent of slamming the brakes — bathing raises core body temperature at the exact moment sleep onset needs it to fall, so shifting the shower/bath to roughly 90 minutes before bed lets temperature drop gradually instead of being forced down all at once.