Cool, dark bedroom
Drop bedroom temperature to 16–19°C; eliminate ambient light sources.
Why
Body temperature drops naturally during sleep onset; a cool room supports this. Light, even small LED indicators, fragments REM cycles. Both are fixable with one-time changes (thermostat, blackout curtains, electrical tape over LEDs).
Slot in your day
How to do it
How
Aim for 16–19°C overnight. Use blackout curtains. Cover LED indicators on electronics. Consider an eye mask if blackout isn't possible.
Evidence
Okamoto-Mizuno & Mizuno 2012 J Physiol Anthropol review: heat exposure and elevated ambient temperature suppress slow-wave and REM sleep and increase wake time. The AASM 2021 insomnia practice guideline endorses sleep-environment optimisation (cool, dark, quiet) as part of behavioural sleep medicine; effect sizes vary but the direction is consistent.