Smoking a cigarette delivers about 1 mg of nicotine. Depending on the amount of smoke inhaled, smoking one cigarette can rapidly increase plasma nicotine by 5-20 ng/ml. Since the ½ life of nicotine in plasma is about 2 hours, repeated cigarette smoking throughout the day can increase plasma nicotine to over 40 ng/ml. Smoking the first cigarette of the day produces the greatest increase of pleasure/arousal but as smoking is repeated over a 16-hour day even though plasma nicotine levels are elevated, because of tolerance, the pleasure/arousal obtained from smoking each cigarette decreases. The withdrawal symptoms that are present in the morning are relieved by smoking the first cigarette, but later in the day even though plasma nicotine levels are now elevated, repeated smoking is necessary to avoid withdrawal symptoms. During overnight abstinence, the neurobiologic systems resensitize resulting in reduced tolerance so the cycle is repeated the next day.