Cold, dark, and boring

Once light is handled, the room itself is the next lever. The good news is that the useful changes are cheap and the expensive ones are mostly optional.

Temperature

Your core temperature has to fall for sleep to begin, and a warm room makes that harder. Somewhere between 16 and 19 degrees suits most people, which is cooler than most bedrooms.

A warm shower an hour before bed helps, counterintuitively: it moves blood to the skin, and the subsequent drop is steeper than if you’d done nothing.

If your feet are cold you’ll struggle to fall asleep regardless of room temperature. Socks are a legitimate intervention and nobody will know.

Darkness

Properly dark, not curtain-dark. Even modest light through the eyelids affects sleep depth. Blackout curtains are the good version; a cheap eye mask is the ninety-percent-of-the-way version and travels well.

Deal with the small lights too - standby LEDs, chargers, a bright clock. A strip of tape solves most of them.

Sound

Consistency matters more than silence. A fan or a white-noise source that runs all night is usually better than a quiet room punctuated by a door or a car, because it’s the change in sound that wakes you rather than the level.

What to spend money on

In rough order of value: blackout for the windows, then something to fix the temperature, then the mattress. Mattresses matter far less than the industry suggests, provided yours isn’t actively uncomfortable and isn’t fifteen years old.

What not to bother with

Air purifiers marketed for sleep, most “sleep-optimised” bedding, and anything with an app. None of them are harmful; none of them beat a dark, cool room.