Can I Be Deficient in Sleep Chemicals?

Please excuse such a technical question; but I am wondering if my now chronic insomnia can be caused by a lack of those brain chemicals which induce sleep. I am often on the verge of sleep for hours and can't cross over into sleepland. And I also find it odd that I am mentally exhausted, but not drowsy when awake. I am trying to figure why some people fall asleep easily and others don't or why sleep becomes a problem after years of decent sleep. Is there a neurochemical reason and if so, how can a person replenish the necessary chemicals? If anyone knows of a site which explains the chemistry of sleep or insomnia or where I could have such a question answered, that would be helpful.