Go through your body from the head down, tensing each muscle for a few seconds and then letting it relax, one at a time. Then, work really hard at really envisioning somewhere, seeing all the detail in your mind's eye. Focus on it. Imagine feeling the physical sensations of being there. The gentle sway of a hammock, the warmth of the sun radiating down on your face. Giving your mind something to focus on lets it stop being so active, and lets it relax. It's like having the tv on late at night putting you to sleep.
With the physical relaxation from the tensing and loosening and the mental relaxation, sleep should come much easier. It does take practice though.
It's something that you just get used to. When I had to travel long distance in a cramped bus daily I used to sleep on the way. The effect was so strong I from doing this every day that I really had to fight falling asleep in every bus I rode on after that. Now the habit is gone again.
I think a lot of people who have that skill learned it while serving in one military or another. Infantry(wo)men either learn to grab what rest they can when they can… or they don’t work out as infantry(wo)men.
Between serving in 'nam and working a labor-intensive job with as much overtime as her could get, my dad could sleep anywhere. Sometimes so hard it took a phone or alarm to wake him up, and he'd be moving and talking before his brain even shifted into gear.
The secret is to accept some level of discomfort in whatever situation while attempting to secure a subconsciously safe-feeling location, and then your tired brain will do the rest of the work getting you to sleep.
I've fallen into a deep REM sleep right next to a diesel turbine once (wearing ear protection of course).
Best I can do is sleep sitting upright. In my office chair, road trips, on the couch. You name it.
The secret, just be tired AF and sleep deprived. I don't sleep well at night. I'm exhausted after work. I usually need a nap midday, but can't take one if I'm working. By the time I get home, I'll pass out in my chair for a bit.
I would look to get that checked out. If you need to have a nap in the middle of the day and have trouble sleeping, I would look to have a sleep test to see if you have sleep apnea.
I have 0 merit in this. I just… can. I always could, apparently. My parents organized dance competitions when I was a baby; they used to make me sleep in the DJ’s booth as it was the quietest-ish place in the venue. I slept through all of those like a (literal) baby. I don’t know why or how.
I used to struggle to get to sleep and hated those who could do it anywhere. They'd say, "just clear your mind", which wasn't helpful. It could be other things, but I figured out that my mind was always busy, sometimes from stress, sometimes from excitement. For me, it's extreme focus. Often, I'll put my mind to work on a complicated problem I'm having at work or home. If I have nothing, my go to is to see how far I can get calculating the binary digits (1, 2, 4, 8, 16, 32, 64...) or the Fibonacci sequence in my head. I don't get very far and I'm out.
I also didn't know how people did it. Then I had a baby and almost fell asleep in the shower once while standing. I've never been more tired in my life but man, these few months of just being able to fall asleep at meteoric speed anywhere anytime were incredible. It felt like a superpower from one of those pill memes. Now I am just tired all the time but I can't fall asleep like that anymore.
Beats me, people saying "if you're tired enough you can sleep anywhere", making me laugh my exhausted ass off (and feel jealous af, but can't really hold it against them).
I've spent more nights than I can count laid in bed for hours, absolutely shattered, wanting nothing but sleep, and none has come. Then when I do finally fall asleep, it isn't for long or of any decent quality (as in EtraordinaryJoe's case - chronic pain will do that to you), so I'm still always tired, and yet still unable to just fall asleep.
My trick to getting to sleep at night is weed and a meal, to help induce food coma, which again, isn't quality sleep, but it's better than nothing..