Evening types are more often current smokers and nicotine-dependent-a study of Finnish adult twins.

Aims: To examine the association between diurnal type and smoking status and nicotine dependence (ND).

Design: A cohort study using random-effects model regressions for repeated longitudinal panel data was used to analyse smoking status by diurnal type. Regression analyses examined the association between diurnal type and ND.

Participants: A total of 23, 289 same-sex adult twin individuals from Finnish Twin Cohort. Nicotine dependence was studied in a subsample of 676 twin individuals.

Measurements: Subjects were classified by self-report into four categories: morning type, somewhat morning type, somewhat evening type, evening type (in 1981). Smoking status was defined as current and ever smoking (in 1975, 1981 and 1990). ND was measured by DSM-IV and Fagerström Test for Nicotine Dependence (FTND) (during 2001-05). Findings  Evening types of both genders were much more likely to be current (OR = 2.91, 95% CI 2.50, 3.38) and life-time smokers (OR = 2.67, 95% CI 2.96, 4.07) compared to morning types. Evening types were less likely to stop smoking. The risk of nicotine dependence assessed by DSM-IV criteria was higher among evening types (OR = 2.78, 95% CI 1.64, 4.72). Evening types scored 0.59 (95% CI 0.01, 1.17) points higher than morning types on the FTND. Adjustment for potential confounders did not change these associations.

Conclusions: Being an evening type is associated independently with a higher risk of being a current smoker, being more highly dependent upon cigarettes and a lower likelihood of stopping smoking. Understanding the cause of these associations could elucidate the causes of tobacco addiction.