What is the ingredient in a cigarette that allows me to get back to sleep when?

I am the same way,I have to smoke a ciggy before I go to bed.Otherwise I am waking up in the middle of the night.It's the addiction and nicotine...I'm sure it's the same for any addiction ,even food,I've known people who had to eat something before they go to bed or they wake up in the middle of the night starving...addiction is just that we must feed it! :-(