People with evening chronotypes have a 1.7 fold elevated risk of needing to retire early and take a disability pension compared with morning chronotypes, suggest the findings of research published online in the journal Occupational & Environmental Medicine.
Chronotype is the body's natural schedule for sleeping and waking. Based on the timing of body functions and alertness, people can be divided into three main groups: morning types, intermediate types and evening types.
Previous studies have indicated that evening types (E-types) appear to have poorer health and experience more difficulties in working life than morning types (M-types).
However, the link between whether a person is a "night owl" or a "morning person" and getting a disability pension has not been fully established.
In a bid to strengthen the evidence base, researchers analysed whether people classed as E-types in their mid 40s were more likely to receive a disability pension in the next 12 years than people classed as M-types.
Researchers analysed data from an ongoing population study which started in 1966 and included 96% of all people born in two northern provinces of Finland.
In 2012, when the individuals were 46, they were asked a series of questions to determine their chronotype. These included how easily they woke in the morning, how tired they were during the first half hour, when they preferred to work, and whether they thought of themselves as a morning or evening person.
The baseline sample included 5679 people (2597 men and 3082 women). The researchers classified 253 men (9.7%) and 381 women (12.4%) as E-types and 1182 men (46%) and 1355 women (44%) as M-types. The remainder were classed as intermediate types.
The researchers then linked this information with data from Finland's national registers which provide information on work and pensions.
During the 12 year follow up, 135 of the men (5.2%) and 249 of the women (8.1%) were granted their first disability pension.
Being classed as an E-type at age 46 was linked with a 1.7 increased likelihood of getting a disability pension in the following 12 years compared with being an M-type, for both men and women.
This association was the same even after the researchers adjusted for socioeconomic factors such as education level and type of occupation.
A disability pension is only granted in Finland after a period of severe and long term health decline, so the researchers believe that the poorer health of E-types compared with M-types is the main contributor to the increased incidence of disability pensions in this group.
They suggest that E-types may be more likely to have worse health because of poorer health choices and the disruption of their internal body clock which can lead to sleep problems.
Chronotype is an innate biological trait with a strong genetic basis, which was also proven in this study by utilising genetic information. Previous studies have shown that chronotype is very stable in adulthood.
A strength of the study is that it is based on a large group of people from all sectors of the economy and occupations who were followed up over a long period.
This is an observational study, and as such, no firm conclusions can be drawn about cause and effect. Another limitation of the study is that there may have been changes in health and circumstances in the years after the baseline survey which were not accounted for.
Nevertheless, the researchers say the health of some E-types approaching their 50s and 60s may have already deteriorated to such an extent that it erodes their working ability and makes them more likely to need disability pensions than M-types.
"People who are E-type represent a significant proportion of the population and their health and ability to continue working until retirement age pose challenges for sustainable economies," say the researchers.
