The goal of the proposed work is to test: 1) the effects of exercise on mood, objective measures of sleep, and cue reactivity; 2) the effects of exercise at a given intensity and duration, after a short (few hours) and more prolonged (24 to 72 hours) abstinence from smoking; and 3) the effects of exercising in the morning versus exercising in the evening on the proposed outcome domains. We plan to implement a within-subject design that will allow us to collect measures for each subject aged 18 to 45 in four different conditions: 1) ad libitum smoking; 2) evening exercise after 2 to 72 hours of abstinence; 3) morning exercise after 2 to 72 hours of abstinence; and 4) no exercise after 2 to 72 hours of abstinence. Exercise will be conducted at a fixed and monitored dose and duration. By collecting morning and evening measures of withdrawal symptoms and cue reactivity, we will also test whether the effects of exercise in reducing withdrawal symptoms and cue reactivity will show a prolonged effect beyond the minutes/hours immediately after the exercise bout. We also hypothesize that the timing of exercise may have differential effects on different aspect of withdrawal. The proposed work will allow us to examine the extent to which exercise, a promising nonpharmacologic behavioral treatment method, can attenuate key symptoms of withdrawal recognized to impede abstinence efforts. A better understanding of these effects will allow us to implement exercise at the time of day that may be more troublesome for the individual attempting to quit. The development of new behavioral methods for attenuating withdrawal symptoms will be directly transferable to smoking cessation treatments aimed at reducing withdrawal symptoms and aiding efforts to avoid relapse after quitting smoking.