I know what you mean, although right now i sleep like a baby.
My problem was i used to work outside by myself all day, so i'd spend the whole day thinking about things and getting worked up over minor stuff that normally would just get shunted from your mind, i walked 16 miles in an 8 hour shift, so when i got home i was knackered, but couldn't sleep because my mind was buzzing with all sorts of shit, because there was no-one around to complain about stuff to.
Also i spent all day around cars, i think it was breathing in all the exhaust, at the end of every shift i felt absolutely shite, like i was hungover.
Glad i left.