<qa>  What is the effect of abstinence or smoking denicotinized research cigarettes onTo compare the effects of complete abstinence or smoking denicotinized research cigarettes to smoking ad libitum nicotinized research cigarettes on the following measures: withdrawal symptoms and quality of sleep; MAO-A and MAO-B activities; hormonal responses HPA, HPT; mood, cognitive and psychometric performance; functional brain activity, and physiological responses. 