i've been there (currently here now) ... i will generally fall asleep early afternoon on a sunday because i tend to get a lot done during the day (washing cars, mowing the lawn, cleaning the house inside out). for me, its the afternoon nap on sunday that kills me.
i'll do either of the following:
1. go for a walk
2. put on a documentary - this works for me.. documentary can be on anything
3. go to the local knock shop - laugh all you want, but sometimes its that little bit of cardio that will make me tired again
4. probably the best option, coupled with option 1 - meditate. sometimes if i cant sleep, its because i have too much going on and i cant stop thinking about certain things
personally, i stay away from melatonin as i don't want to introduce a synthetic booster into a naturally produced hormone. i've heard that some people that have taken melatonin, now rely on melatonin just to fall asleep