Different exposure metrics of night work were computed
on the basis of work history (Table 1). If nothing else is
stated, the term ‘‘night work’’ in this study includes working
periods from rotating, as well as permanent, night schedules
and includes the work of permanent night workers. To evaluate the influence of permanent night work, we performed
separate analyses, excluding nurses who worked only at
night as permanent night workers (11%); these odds ratios
were then compared with the overall odds ratios when including all night workers, in order to evaluate the difference.
The average number of night shifts per month was imputed for jobs reported to include night shifts if the average
number was missing (10% of night-shift jobs) or for jobs
where the reported average number exceeded 16 (1% of
night-shift jobs). For imputation, we applied the average
number of night shifts per month in jobs that started during
the same 5-year period (before 1960, 1960–1964, . . .,
1995–1999, 2000 and after) and in the same kind of work
site (hospital, other institution, others). The imputed number
of night shifts per month varied from 4.5 to 7.0. The reference category, in all but one analysis, consists of nurses who
never worked at night after graduation from the nursing
school. However, all women in this study had some exposure to night work during the 3 years at a nursing school.