Nicotine, one of the substances found in cigarettes, is the ingredient which stimulates dopamine secretion, a chemical responsible for feelings of happiness and satisfaction. Thus, whenever a regular smoker is between cigarettes; for instance, when travelling long-haul on an airplane, the level of dopamine in their brain drops, making them crave and feel a need for a cigarette to restore the falling dopamine level. In several occasions, smoking is just part of a routine with apparent link to a trigger. For example, many people light up after seeing someone else smoke, during coffee sipping or even before going to bed each night.