This is not a judgement...
...and I Am Not A Health-Care Professional:

But it sounds to me like you may have some form of mental condition. Not in itself dangerous or harmful; but as you already appear to have identified, sensible to remain aware of.

Have you discussed this with a Doctor or Counsellor? I'm *not* suggesting you get all medicated, but it might be helpful to talk this through with a professional; if only for your reassurance.

edit: on a more practical note, I've found that some quiet background ambient/electronica/"chillout" music can help me get to sleep. As an example; perhaps Brian Eno - Music For Airports. It's not the easiest to find when filesharing; so if you're in the UK I might be able to send you a CD.

edit: Also, as you say, absolute dark can be undesirable. Another idea I use is to have a string of coloured fairy lights strung around the room, all year-round. If necessary, I switch on the string of lights, then turn off the main light; so I have a room dark enough to drift off in, yet not so bright as to keep me wake. The colours provide gentle visual stimulation.
(The Incredible Sulk Finally remembered his password on, Wed 24 Dec 2008, 1:37, Reply)