How Heat and Cold Each Disrupt Sleep, Differently

Drawing together their own and others' research on thermoregulation and sleep, this review lays out what happens to sleep architecture when the bedroom is too hot or too cold. Both extremes increase wakefulness and reduce the deep, restorative stages of sleep, slow-wave sleep and REM, but the practical picture is more nuanced than 'cooler is always better'. In semi-nude laboratory conditions, cold exposure disturbs sleep more than heat does. Once ordinary bedding and clothing are added back in, that picture flips: cold exposure is largely buffered by blankets and pyjamas, while heat, particularly combined with humidity, still disrupts sleep stages and pushes the body's cooling system to work harder through the night.

As a review rather than a single trial, its value is in synthesis rather than a new headline number, and it draws heavily on its own authors' body of laboratory work, so the underlying studies share some common limitations, including the small samples typical of thermal physiology research. Still, it offers a useful, practical corrective: bedding matters as much as room temperature itself, and a humid warm room is a bigger threat to good sleep than a slightly cool one.