Nighttime is always the worst. For me it's because of the feeling of loneliness and isolation. My fiance goes to bed before I do and is a heavy sleeper. If something were to happen to me, he wouldn't find me until morning. I used to think I needed one of those life alert alarms, but they would just take it away from me for too many false alarms LOL
Nighttime can definitely be the worst when I am in full blown hypochondria mode. One thing that helps me is downloading books to my mp3 player and listening to them all night. Even if I don't sleep, it helps me to focus on something besides my fears and sometimes it even helps me sleep a little. But I too hate the loneliness and isolation.