A cool, dark room helps because sleep starts while core temperature is falling. The heat leaves through the skin, and a cool room gives that heat somewhere to go. Colder is not better. Darkness lets melatonin rise. Bright evening light, including a phone, can hold it back.
Sleep starts while you cool down
Sleep begins during that fall in core temperature. The body is letting heat go, not fighting to stay warm. A stuffy room makes the fall harder. A little cool is enough for many people, with a cover they can push off.
Heat needs somewhere to go
The heat leaves through the skin. A cool room is simply a place for it to go. The room does not need to be cold, and colder is not better. If the air outside is safe and quiet, a cracked window does the same job as a cooler setting. There is no degree to hit.
Darkness lets melatonin rise
Darkness lets melatonin rise. Bright evening light, including a phone, can hold it back. You do not need a blacked-out room on the first night. A dim lamp instead of the ceiling, the phone out of your hand, and curtains that meet are enough to stop the room from arguing with the night.
This is a description of the room, not a treatment and not a product. If sleep stays hard most nights, a clinician is the right next conversation.
Waking in the lighter half of the night is a different question. Why you wake around 3 a.m.