Night owls may be more likely than early birds to smoke, and less likely to kick the habit over time, a new study suggests.
Researchers found that among more than 23,000 twin pairs followed for up to 30 years, those who described themselves as "evening types" -- usually alert at night and bleary-eyed in the morning -- were more likely to be current smokers and less likely to quit over time compared with morning people.
In line with that lower quit rate, night owls who smoked were also more likely to fit the diagnostic criteria for nicotine dependence than their early-bird counterparts.
The findings, reported in the journal Addiction, do not necessarily mean that there is something about being a night person that raises smoking risk.
One possibility is that nicotine, which is a stimulant, tends to keep smokers up at night, according to lead researcher Dr. Ulla Broms of the University of Helsinki in Finland.
However, she told Reuters Health in an email, "We primarily believe that it is more likely that evening persons are more prone to initiate smoking and have greater difficulty quitting smoking."
Whether that is in fact the case, and the underlying reasons for it, are not yet known, Broms said.
One potential explanation, she noted, is that evening types are more likely to stay out late at bars and restaurants, where smoking is common (or was during the study period). Such surroundings can reinforce current smoking and thwart quit efforts.
In addition, Broms and her colleagues write in their report, evening types could be more prone to addiction and pleasure-seeking behavior than morning people. Aspects of the brain's dopamine and opioid systems, which are involved in reward-seeking and addiction, might be related to both a person's tendency to be a night owl and to smoke.
The findings are based on data from a long-running study of Finnish twins born before 1958. In 1975, 1981 and 1990, participants were asked about their current smoking habits; during the 1981 survey, they were also asked about their tendency to be evening or morning types.
Overall, 30 percent said they were "clearly" morning types -- being "morning bright and evening sleepy." Ten percent said they were clearly evening types ("morning sleepy and evening bright").
The rest of study participants said they were to "some extent" either a morning or evening person.
Broms' team found that evening types had the highest rates of smoking at all three surveys. In the 1975 survey, 43 percent were current smokers, versus 27 percent of morning types. In 1991, those figures were 35 percent and 21 percent, respectively.
Smoking rates among "somewhat" morning types were slightly higher than those of full-fledged morning types, but lower than those of "somewhat" evening types.
When the researchers accounted for several other factors -- including age, self-reported drinking habits and participants' scores on a measure of depression symptoms -- evening types showed a three-fold greater chance of being a current smoker at any point compared with morning types.
In addition, among participants who were smokers in 1981, evening types were 27 percent less likely than morning types to have quit by 1990.
Night owls also showed higher rates of nicotine dependence, as measured by a standard questionnaire given to a sub-group of heavy smokers between 2001 and 2005: 73 percent of current evening-type smokers were nicotine-dependent, versus 48 percent of morning-type smokers.
One of the limitations of the study is that people's tendency to be evening or morning types was measured only once. Broms said that future studies should look at whether changes in those patterns over time are related to smoking habits and quit rates.
If the tendency toward night owlish-ness does in fact have some effect on quitting, there could be practical implications, according to Broms. Evening types who are aiming to stop smoking could try changing their sleep-wake habits to see if that aids their quit attempt.