Staying up late at night has little impact on how long “night owls” live, but data based on nearly 23,000 twins shows that evening types have a slightly increased risk of dying than morning types—a risk largely linked to smoking and drinking.

The study, published in Chronobiology International, tracked people over the course of more than 37 years in Finland. It suggests that lifestyle should be considered when analyzing the impact on health of chronotype, the body’s natural inclination to sleep at a certain time.

“Our findings suggest that there is little or no independent contribution of chronotype to mortality,” says author Christer Hublin, MD, PhD, from the Finnish Institute of Occupational Health in Helsinki, in a release. “In addition, the increased risk of mortality associated with being a clearly ‘evening’ person appears to be mainly accounted for by a larger consumption of tobacco and alcohol. This is compared to those who are clearly ‘morning’ persons.”

There is increasing evidence that sleep duration and quality, as well as night shift work, affect health. Earlier studies have linked night owls with a higher risk of disease, especially heart problems.

Data published in 2018 from the UK Biobank, looking at people over the course of 6.5 years, found evening types have a small increased risk of death from any cause, including disease and from heart conditions.

It was this previous research which inspired this study, as the authors wanted to analyze some factors which were not measured—alcohol consumption and the amount people smoked, rather than just status.

This new research, which was co-led by Jaakko Kaprio, MD, PhD, from the Finnish Twin Cohort study at the University of Helsinki, followed 22,976 men and women aged 24 years from 1981 to 2018. At the start of the study, the twins were asked to pick from four possible responses: “I am clearly a morning person,” “I am to some extent a morning person,” “I am clearly an evening person,” and “I am to some extent an evening person.”

The researchers followed up with the participants in 2018 to establish if any had died. They based this on data provided by nationwide registers. The authors took into account education, daily alcohol consumption, smoking status and quantity, body mass index, and sleep duration.

Results showed that 7,591 of the twins identified as “to some extent” and 2,262 as “definite” evening types. The figures for morning types were 6,354 and 6,769, respectively. Compared to morning types, night owls were younger and drank/smoked more. Definite evening types were also less likely to report getting eight hours of sleep.

Of the total participants, 8,728 had died by 2018, and the chance of dying from any cause was 9% higher among definite night owls compared to early birds. However, the study found that smoking and alcohol largely caused these deaths, not chronotype. This finding was highlighted by the fact non-smokers were at no increased risk of dying.

The causes of death from alcohol included related diseases as well as accidental alcohol poisoning.

Kaprio notes in a release that they were more able to relate their findings to society as a whole. Their participants’ health was no different than the general population, whereas the UK Biobank’s were healthier than average. They highlight the access to comprehensive data on lifestyle factors as a strength of their research. However, the findings were based on self-reported data from asking one question.

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