It can be from stress, or working too hard during the day, sounds weird, but its true.

Here are some things that I do:

1. eat something light, like a piece of bread or something, that you like

2. heat up a glass of milk, and drink it, its gross, but that sometimes sooths you.

3. when you are laying in bed, start at your feet, and SLOWLY relax you whole body

4. my sister does this: we have heating blankets, and she turns her's on before she goes to bed, and puts her pillow under it, and when she goes to bed, she layes her head on the warm pillow that was under the blanket, and she says that helps.

I'll post more, as I think of more ideas. Good luck! I hope you can fall asleep soon!