That's me pretty much every night. Some nights it's better than others, but the majority of the time I can't fall asleep in a reasonable amount of time.
It's kind of paradoxical for lack of a better term, because I can't fall asleep very easily if there is light/noise - the more the worse - but at the same time, sometimes when it's completely dark and silent, I can't just lay there and fall asleep without my mind wandering off into who knows where.and not being able to stop my thoughts.
I could be dog tired too the whole day, yawning every 60 seconds, the whole time feeling like I could lay down and sleep for 10 hours, but then when I do lay down it takes a while before I finally fall asleep.
Even when I was in the hospital this last time, they would give me melatonin and trazodone, the melatonin would definitely make me drowsy, but even with them both helping I would still have trouble falling asleep sometimes.
I think personally for me maybe it's just a matter of not being able to sleep if I'm just not 'ready' to, and being tired doesn't necessarily always mean I'm ready.