Population-Based Study Subjects

Carried out from 2000 to 2001 by the National Public Health Institute of Finland, the Health 2000 survey is an epidemiological cohort representative of the Finnish population over the age of 30 (n = 8028).¹⁹ Subjects in our studies comprised individuals under 65 years old who perform three-shift work, two-shift work, or regular night work (Table 1). These subjects were defined based on the following questions asked of those indicating that they had been working in the last 12 months: “What sort of hours do/did you work in your main occupation: regular day-job (between 6 am and 6 pm), regular evening job, regular night job, two-shift work, three-shift work, periodical work, only weekend work, other sort of working time?” We included those reporting two-shift work, three-shift work, or regular night work. In Finland, three-shift work usually refers to 8-hour shift work schedules with morning/day, evening, and night shifts; while, two-shift work refers to shift work with only two different shifts which are normally the morning/day and the evening shift.

Study Characteristics.

GWAS = genome-wide association study; MBI-GS = Maslach Burnout Inventory-General Survey.

^aGenmets subcohort of the Health 2000 Survey designed originally for case-control study of metabolic syndrome.

^bRespondents of the Health 2000 survey who participate in the genetic study and were not part of Genmets.

^cSubset of nurses from the Finnish Public Sector Study.²⁵

^dEmployees of the airline company in the follow-up phase of the study for diabetes screening and prevention implementation program in the occupational health care of the Finnish airline.²⁴

Comprised of equal numbers of cases and controls for metabolic syndrome, the GenMets subcohort from the Health 2000 survey was selected for genome-wide genotyping (n = 2130).²⁰ Our initial GWAS analyses comprised GenMets shift workers (Health 2000 GWAS study, n = 176 with successful genotyping and answered to MBI-GS) and the analyses were adjusted for the case-control status of GenMets (73 cases and 103 controls). A description of the selection of cases and controls is provided in the Supplementary Methods and basic characteristics of the cases and controls are provided in Supplementary Table S1. The two variants with the strongest association in GWAS were genotyped in the rest of the Health 2000 cohort, which was not part of GenMets, and those shift workers were used in our first replication analysis (Health 2000 replication study, n = 241 with successful genotyping and answered to MBI-GS) (Table 1). The interaction analysis of the shift-work status included the shift working subjects in both the Health 2000 GWAS and Health 2000 replication studies and, in addition, non-shift-working subjects of the Health 2000 (n = 2484 with successful genotyping and answered to MBI-GS).

The genetic fine mapping investigation was supplemented with two additional Finnish population cohorts, Vantaa 85+ (n = 532)²¹ and Kuopio 75+ (n = 601),²² which contained no information on shift work or job-related exhaustion. For a total of 14 subjects from Health 2000 and Vantaa 85+, we performed capillary sequencing, and thereafter Vantaa 85+ Kuopio 75+ were used in the fragment analysis of the microsatellite marker found by sequencing (Supplementary Methods).

Used as additional replication material and in methylation analyses, occupational subjects comprised employees from two sectors. The baseline phase of the airline study for prevention of type 2 diabetes was collected during the volunteer employee health checks of a Finnish airline operating primarily along Asia–Europe traffic routes.²³ Of the 2312 study subjects, those with full baseline data and no diabetes (n = 40 with diabetes based on their baseline glucose levels) were invited to the follow-up study 2.5 years later, and 1347 participated.²⁴ The participants of the follow-up study who had information from MBI-GS and from work schedules, gave permission for DNA sampling, and had successful genotyping, numbered 821. The shift-working subjects were defined by the questions “How many night shifts do you have per month (at least 3 hours between 23.00–06.00)?” and “How often in your shift work schedule you have following working times: Early morning shifts starting before 6 am?” The answers were further verified from the working schedule system. As shift workers, we classified subjects with night shifts ≥3/mo, and/or early morning shifts ≥1/wk. Of the included study subjects, 606 were classified as shift workers and comprised our airline study, which included in-flight workers (n = 263), who served as pilots or flight attendants, and non-flight workers (n = 343), who held various occupations at the airport. The non-flight workers constituted a separate group, analyzed post hoc, because they had much higher exhaustion scores compared to the other occupational and population-based groups (Table 1). In addition to shift-working non-flight workers, non-flight workers who did not perform shift work (n = 193 with successful genotyping and answered to MBI-GS) were included in the interaction testing of shift work.

The second occupational group included nurses from the Finnish Public Sector Study (n = 73 with successful genotyping and answered to MBI-GS)²⁵ (Table 1). All the nurses had a shift work schedule with three different shifts: morning, evening, and night.

For all the contributing studies, the local institutional review boards on human research approved the study protocols, and the participants gave their informed consent.