But scientists have now pinned that early-morning buzz down to the drug's particular effects on nerve receptors in the brain which respond to dopamine, a chemical which regulates mood and behaviour.
And as behaviour is partly shaped by the reinforcement of rewarding events, we keep trying to repeat that first "hit".
This reinforcement process goes on in a part of the brain called the ventral tegmental area (VTA) where neurons respond to the chemical messenger dopamine. Addictive drugs act on the VTA and exploit the reinforcement- reward cycle.
John Dani and colleagues at the Baylor College of Medicine in Houston, Texas, explain today in the journal Nature how they examined rat brains which contained groups of dopamine receptors, and looked at what happened when they added nicotine in the concentrations found in smokers' blood.
At first, the neurons fired more powerfully but then the responses moved down - meaning that it would take a greater input to excite the same response.
Also, that first cigarette will tend to last a long time: nicotine in the blood has a half-life of about two hours. But once they have been exposed to nicotine for more than five minutes, the dopamine receptors' sensitivity begins falling. The researchers found in three tests that the nicotine concentrations found in a smoker's blood during the day do not let the dopamine receptors recover their sensitivity. To provide enough nicotine to excite the receptors to their first-of-the-day level you would probably need so much that you would vomit.
Only when nicotine intake stops during sleep do the receptors recover and make the first cigarette seem so delightful.