Honestly, I think it's an individual thing.
After having slept in some of the finest places imaginable (cold mud, humid jungle air, ten million bugs, snakes, rats etc. etc), and with luck a fire (which means one half of your body stays somewhat warm while the other half freezes) or... without luck, we all just pile into a huddle (On the outside you freeze on one side while staying warm on the other... or you get to be in the middle where you are warm all over but you've got everyone else sticking their feet, elbows and body odor onto you).
I seem to have developed a knack for sleeping anyplace, anytime.
Things I've found that helped me sleep during the day::
- Covering the windows. I usually fold bedsheets over.
- Going to bed at a constant time every morning
- Reading a light book (if I'm desperate, I'd go over my Statistics notes. Knockout guarantee. I also had saved one of my professor's lectures on tape... just that droning voice is enough! :chuckle)
- Everyone from my grandmother to my buddies know that to waken me during my sleep is fraught with grave danger. They are good people about it anyways and don't bother me unless it is an emergency - they know that I value few things in life like I value my sleep
Now this may sound strange, but mostly I just 'will' myself to sleep. I lie down, close my eyes and 'command' my brain to go to sleep and in about 15 minutes I'm out. I'm also able to program a 'mental alarm' into my brain. Like tonight, I'll go to bed in about 1/2 hour. I'll tell myself 'ok, need to wake up in about 6 hours' and 98% of the time I wake up a few minutes before my alarm clock at the bedstand is set to go off.
I know it sounds wishy-washy but give it a shot. You never know
PS: I also find sitting in a bus is an almost instant ticket to sleep. While it is ofcourse not practical to take your naps on the enarest city bus you need to recreate that environment. Something that hums - like the sound of a very muffled engine