I can half drift off anywhere - but that isn’t sleep, it isn’t relaxing, and I come out of it (it isn’t really waking up - I was sort of there all along, just not really fully there) just as tired as (if not more tired than) I was before.
But to actually sleep it must be as dark as possible. (curtains closed, all lights in the near vicinity off, if I can’t turn them off then something along the crack of the door so that the light doesn’t come through) & all doors must be closed. (Cannot sleep with the closet doors open. Or the down the hall bathroom door open.)
As quiet as possible - eventually I’ll get used to the white noise in the place, but no music, no television, no fans, no humming electric things that aren’t essential (it took me a month to get used to the refrigerator - I decided that I really did need to keep it on at night) etc.
Under a huge number of blankets (right now I’ve got the down comforter, the other comforter, the two blankets, and the flannel sheets.) Blankets must be tucked in on the bottom. If they’re not - my feet will be cold. There are generally anywhere from 2-6 pillows at the foot of my bed stacked up to keep my feet warm. I also cannot sleep without socks on. (It can be so hot that even I end up sleeping with no covers - but still, must have socks on.)
After that, the mattress has to be ridiculously soft, I don’t sleep with a pillow (under my head - people have mistaken which side is the head and foot of my bed), and I sleep on my stomach with the blankets tucked in around me. (I can sleep on my side, it just isn’t very comfortable…but I have never been able to sleep on my back)
If for some reason anything changes (there was a noise where there wasn’t before, a light goes on, a light goes off, movement, one sock falls off, a pillow falls off) I’ll wake up.