Why is it so hard to quit smoking?
Research has shown that nicotine is the main silent agent in cigarettes. More than-just a physical craving, it’s also a psychological dependency.
Nicotine raises dopamine and endorphin levels, which control feelings of pleasure and euphoria. As smokers increase the amount of cigarettes they smoke, more nicotine is absorbed by their lungs and into their bloodstream. They have to maintain this level of nicotine in order to keep the nervous system stable and prevent withdrawal symptoms. At night, when you’re asleep, these pathways shut down and cravings often occur immediately upon waking up.
Wiki on nicotine: http://en.wikipedia.org/wiki/Nicotine
Article source: http://awesome.good.is/transparency/web/1103/quit-smoking/flat.html