Night owls' extra disability-pension risk fades once a Finnish study adjusts for health

Seventy percent is a relative figure. Over the follow-up, 8.3% of evening-type men drew a first disability pension against 5% of morning-type men, and among women the split was 12.1% to 7.2% ^[10]. The raw gap is 3.3 percentage points for men and 4.9 for women ^[24][25]. That works out to roughly one extra pension for every 30 evening-type men, and for every 20 evening-type women, across 12 years ^[27]. The crude ratios sit near 1.7 in both sexes ^[26]. The published estimate also accounted for when pensions were awarded ^[1].

The outcome is the strongest part of the design. Participants come from the Northern Finland Birth Cohort 1966, people born in two northern provinces ^[4], who answered questions about morning tiredness and preferred working times at 46, in 2012 ^[5]. Anyone already retired or pensioned was left out ^[6], and pensions were read from national records through 2023 ^[7]. In all, 384 people received a first disability pension, under 7% of the cohort ^[9][29].

The outcome is also a medical judgement. Finland grants a disability pension when health problems substantially reduce long-term working ability despite treatment and rehabilitation ^[8]. Evening types entered the study with poorer self-rated health, more sleep difficulties and less physical activity ^[12]. Education and income did not explain the association ^[3]. Adjusting for self-rated health, body mass index, smoking, alcohol and physical activity weakened it until it was no longer statistically significant in either sex ^[13]. Working hours and schedules, entered separately, weakened it too. The model with everything included left a small, non-significant difference ^[14].

"We think that the generally poorer health of evening chronotypes (compared to morning chronotypes) was the most likely explanation of the heightened risk found in our study," Tapio Räihä, the first author and a life course health researcher at the University of Oulu, told ScienceAlert ^[2][23].

The thing this doesn't tell you is which way the health runs. Finnish working days commonly start around 8 am ^[17]. If years of that schedule against a late body clock wear people down, then health sits on the path from chronotype to disability, and adjusting for it strips out part of the effect being studied. The authors raise that caution themselves ^[15]. Chronotype has a strong genetic component ^[22], so the preference may well come first, but the study measured chronotype and health together, at 46 ^[5][12]. "Our study was observational, so we cannot prove cause-and-effect relationship," Räihä said ^[16].

The male estimate rests on few cases: 21 pensions among roughly 260 evening-type men ^[11][28]. The uncertainty around its size is considerable ^[11].

In my view the evidence supports a modest claim. In this cohort, an evening chronotype marks a group that is in poorer health and draws more disability pensions ^[12][10]. The data cannot show that chronotype causes either ^[16]. More than 70% of evening types who reported their schedules worked daytime jobs ^[18]. Räihä suggested morning outdoor light, dimmer lighting before bed and fewer late-night screens, none of which the study tested ^[19]. "If you can choose a job that allows flexibility in schedules, that could also help," he said ^[20]. Nobody yet knows whether changing schedules lowers the risk of a disability pension ^[21].