Everything. I'm a total insomniac, no matter how exhausted I am. I work 12 hour days and still can't sleep well at night. My mind just goes over every little detail of my life until I finally pass out.
Whenever I have too much thinking going around in my head, they play like a recorder at night...and it never stops. Sometimes it can keep me up for 3-4hrs, and worst possible outcome....they even transfer to my dreams.
But once things settle down, I usually get a nice relaxing night's sleep.