Table 1.

Cohort studies of night work and breast cancer

Authors, (years) Country	Study description	Exposure assessment	Exposure categories	RR or HR
Schernhammer et al. (2001) [13] USA	Prospective cohort study of 121,701 registered nurses, with follow-up 1988–1998	Self-reported lifetime years on rotating night shifts, with rotating night shifts defined as “at least three nights per month, in addition to evenings and afternoons in that month”	Never 1–15 15–29 ≥ 30 P for trend	1.0 (ref) 1.08 (0.99–1.18) 1.08 (0.90–1.30) 1.36 (1.0–1.78) 0.02
Schernhammer et al. (2006) [14] USA	Prospective cohort study of 116,087 registered nurses, with follow-up 1989–2001	Self-reported lifetime years on rotating night shifts, with rotating night shifts defined as “at least three nights per month, in addition to evenings and afternoons in that month”	Never 1–9 10–19 20+ P for trend	1.0 0.98 (0.87–1.10) 0.91 (0.72–1.16) 1.79 (1.06–3.01) 0.65
Schwartzbaum et al. (2007) [15] Sweden	Register-based retrospective cohort study of 1,148,661 female workers, with follow-up 1971–1989	Usual occupation and work hours (three-shift schedules and others) to define occupations with a large proportion of workers with night work; from in-person interviews in annual surveys of living conditions (1977–1981)	Shift work in 1970 Shift work in both 1960 and 1970	0.94 (0.74–1.18) 0.97 (0.67–1.40)
Pronk et al. (2010) [18] China	Shanghai Women’s Health Study: a population-based prospective cohort study	Job exposure matrix of three categories with increasing scores for night-shift work: 0 = no night-shift work; 1 = incidental night-shift work; 2 = jobs likely to involve the night shift, 3 = jobs that probably involved all-night shifts	Never 1–5 years 6–17 years 18 years and more	1.0 (ref) 0.9 (0.6–1.3) 0.9 (0.6–1.4) 0.8 (0.5–1.2)
Knutsson et al. (2012) [16] Sweden	WOLF (Work, Lipids, and Fibrinogen) occupational cohort study that included subjects who were employed in different public and private companies (N = 4036), with average follow-up time of 12.4 years	Questionnaire: “Do you work shifts?” and “How many hours do you normally work per week, including overtime, and how are these hours distributed on average?”	Day Shifts without nights Shifts with nights	1.0 (ref) 1.23 (0.70–2.17) 2.02 (1.03–3.95)
Koppes et al. (2014) [19] Netherlands	14 Dutch Labor Force Surveys (1996–2009) Record linkage with national registers on hospital admission	Current exposure to night work was assessed with the question: “Do you work at nights, meaning between midnight and 6 am?”	No night work Occasional Regular	1.0 (ref) 1.04 (0.85–1.27) 0.87 (0.72–1.05)
Åkersted et al. (2015) [17] Sweden	Cohort study of 13,656 women from the Swedish Twin Registry, with 3404 exposed to night work; follow-up time of 12 years	Questionnaire: “For how many years have you had working hours that meant that you worked nights at least now and then?”	Follow-up to 60 years
			No night work	1.0 (ref)
			1–5 years	0.93 (0.66 to 1.31)
			6–10 years	0.79 (0.45 to 1.38)
			11–20 years	0.80 (0.45 to 1.42)
			21–45 years	1.77 (1.03 to 3.04)

RR Relative risk, HR Hazard ratio