“…In addition, there is a wide variation in assessment of the outcome measure (sickness absence), the exposure (shiftwork), length of the observation or recall period, and the reported measure of association to allow direct comparisons across studies. For example, duration of sick leave was broadly defined as ≥1 day (Ohayon et al, 2002), ≥2 days (Bockerman & Laukkanen, 2010), ≥3 days (Duijts et al, 2007;Kleiven et al, 1998), 1-5 days (Nakata et al, 2004), 1-7 days (Morikawa et al, 2001), ≥6-8 days (Bourbonnais et al, 1992;d'Errico & Costa, 2012, Niedhammer et al, 2008, and ≥2 weeks (Lund et al, 2006;Tüchsen et al, 2008aTüchsen et al, , 2008b. With respect to shiftwork, some studies classified participants using finer categories such as day-, evening-, night-, and rotating-shift workers, whereas others utilize the general shift-(i.e., outside the regular work hours) versus day workers classification system.…”