. Author manuscript; available in PMC: 2018 Sep 1.

Published in final edited form as: Am J Epidemiol. 2017 Sep 1;186(5):532–540. doi: 10.1093/aje/kwx140

Table 3.

Associations of duration of rotating night shift work and invasive breast cancer, stratified by follow-up period, during 24 years of follow-up (NHS: 1988–2012; NHS II: 1989–2013)

	Follow-up <=10 years				Follow-up >10 years				MV p_interaction

	No. of cases	Person-years	Age-adjusted HR (95%CI)	Multivariable-adjusted HR (95% CI)^b	No. of cases	Person-years	Age-adjusted HR (95%CI)	Multivariable-adjusted HR (95% CI)^b
NHS rotating night shift work history
Never	977	298,701	Ref	Ref	1405	336,729	Ref	Ref
1–14 yrs	1415	383,622	1.11 (1.02–1.21)	1.09 (1.00–1.18)	1747	427,392	0.97 (0.91–1.05)	0.96 (0.89–1.03)
15–29 yrs	146	40,739	1.03 (0.86–1.23)	1.07 (0.90–1.28)	185	43,381	1.01 (0.87–1.18)	1.05 (0.90–1.23)
30+ yrs	60	12,537	1.23 (0.95–1.60)	1.26 (0.97–1.64)	36	12,418	0.65 (0.47–0.91)	0.68 (0.49–0.95)

	2598	735,599	p_trend = 0.08	p_trend = 0.04	3373	819,920	p_trend = 0.15	p_trend = 0.25	p_int = 0.03
NHS II 1989 baseline rotating night shift work history (early career)
Never	416	412,724	Ref	Ref	902	553,730	Ref	Ref
1–9 yrs	637	622,782	1.03 (0.91–1.17)	1.02 (0.90–1.15)	1434	833,620	1.07 (0.98–1.16)	1.07 (0.98–1.16)
10–19 yrs	57	47,867	0.94 (0.71–1.24)	0.97 (0.73–1.28)	111	63,327	0.94 (0.77–1.14)	1.01 (0.83–1.23)
20+ yrs	6	1,491	2.13 (0.95–4.80)	2.35 (1.04–5.31)	7	1,801	1.63 (0.77–3.45)	1.97 (0.93–4.20)

	1116	1,084,864	p_trend = 0.76	p_trend = 0.70	2454	1,452,478	p_trend = 0.65	p_trend = 0.25	p_int = 0.86
NHS II cumulative rotating night shift work (updated)^a
Never	341	321,600	Ref	Ref	609	346,804	Ref	Ref
1–9 yrs	621	602,095	0.98 (0.86–1.12)	0.97 (0.85–1.11)	1381	767,303	1.06 (0.96–1.16)	1.07 (0.97–1.18)
10–19 yrs	60	50,481	0.92 (0.70–1.21)	0.94 (0.71–1.24)	141	88,801	0.90 (0.74–1.07)	0.95 (0.79–1.14)
20+ yrs	12	2,956	1.99 (1.11–3.56)	2.12 (1.18–3.80)	23	10,637	1.10 (0.72–1.66)	1.20 (0.79–1.82)

	1034	977,132	p_trend = 0.83	p_trend = 0.74	2154	1,213,546	p_trend = 0.58	p_trend = 0.88	p_int = 0.73

For NHS II, analyses using updated duration of shift work excluded participants during the cycles in which they were missing shift work exposure information, resulting in fewer cases and person-years, compared to analyses using history of shift work reported at baseline in 1989.

Multivariable-adjusted models. See Table 2 footnotes for list of covariates.