During my first SP attempts I was quite excited. I was waiting for something to happen, and this obviously kept me awake longer then usual.
I lay in bed for a long time and nothing happened.
I found it much easier to get into SP when I'm very tired, however it is also quite a bit harder to hold onto your awareness if you are that tired.
Remember that one of your goals is to fall asleep, if you're too excited this will be very hard.

If you experience your first SP's even remotely similar to mine, you will definitely notice though, I wouldn't worry about that.