That's a tricky one. Its hard to change one's sleeping habbits without some external instrument (like medicine, a particular food type, etc).
But certainly, stress, mental occuption, negative thoughts can keep you from sleeping restfully. You lie in your bed tossing and turning without really able to catch sleep, and when you do you awake again an hour later, to repeat the entire process.
Additionally, being slightly ill or constantly chancing your sleeping times can lead to lighter sleep, as your biorhythme gets temporarily messed up or the body fighting off the illness is having too much internal activity.
Normally though, I wouldn't worry about it, unless it really persists for a long time. The body tends to have internal balance systems that usually kick in after a while. Sleeping lighter leads to more exhaustion leads to sleeping deeper again when the reserves run out, etc etc. So depending on 'how recently' recently is, I wouldn't let it keep me awake.
Additionally, when I'm having difficulty sleeping due to stress, I like to just grab me a good book, the more fantastic the better, and read it just before going to sleep. Or alternatively watch a good movie, again something that has nothing to do with your regular waking life. I find that this takes my attention far away from all the waking things that keep occupying my consciousness, and makes it far easier to fall asleep into a more restful sleep afterwards.
It might be a good idea to cut out on the lucidity exercises for a little bit too, and just focus solely on sleeping.
Just my 2 cents, I don't really know about medicines you can take or food you can eat, so i'll leave that to others.