Wanda

Attack the daytime, not the night. A west facing top floor room absorbs heat all afternoon into the walls and ceiling, and that mass radiates all night, which is why opening the window at 11pm does so little.

What measurably worked in a similar flat: blackout blind plus a cheap reflective panel in the window from about 1pm, window shut during the hottest part of the day, then windows wide open the moment outside drops below inside. I bought a cheap thermometer for outside the window purely to know when that moment was. Took about 2.5C off my overnight low.

19C is good. Check what the room is doing at 3am though, rather than at bedtime. If your heating comes on early, or summer sun starts warming the room, the temperature you fall asleep in is not the temperature you wake in. Mine was climbing four degrees overnight because of a timer I had forgotten about.

If you have been persistently exhausted for a long stretch despite reasonable habits, mention it to a doctor rather than assuming it is a discipline problem. Plenty of ordinary things cause it and none of them are fixed by a stricter alarm.

Daytime sleep environment has to be over-engineered compared with night sleep, because you are fighting light, noise and your own body clock at once. Full blackout — foil or a proper blind, not curtains — plus a fan or white noise loud enough to mask the doorbell, plus a note on the door. Half measures give you exactly the three broken hours you describe.

Take the mask more seriously before dismissing it though. A cheap flat mask is not representative — the moulded contoured ones with eye cups do not press on your eyes and survive side sleeping much better. About 20, and it travels, which the blind does not. Within your budget I would honestly buy both.