Feeling sleepy is like feeling hungry--sometimes you do, sometimes you don't.

The reason you cannot sleep at the same time as the previous night is due to one or more of the following:

1) You ate something heavy.
2) You did something to upset your nocturnal routine.
3) You are mentally haunted by an annoying memory.
4) There is something that you want very much that you cannot have.

Suggested solutions:

1) Sip warm water.
2) Clear your mind and keep it that way.
3) Turn the TV on to cartoons, with subliminal sound, and drift off to sleep while the TV is on.
4) Change the smell of your surroundings by spraying a little cologne on your pillow.