Table 3. Light at night at age 20 and risk of breast cancer, by menopausal status and oestrogen-receptor status of breast cancer.

				Oestrogen-receptor status of breast cancer
	All breast cancers			Positive			Negative
LAN levela	No. cases	HR (95% CI)b	P-value	No. cases	HR (95% CI)b	P-value	No. cases	HR (95% CI)b	P-value
Total (Pyrs=624 049)
Low	452	1.00		269	1.00		57	1.00
Med	846	1.02 (0.90–1.16)	0.76	674	1.07 (0.93–1.24)	0.34	118	0.86 (0.63–1.18)	0.36
High	540	1.00 (0.88–1.15)	0.97	409	1.00 (0.86–1.17)	0.96	84	0.94 (0.67–1.32)	0.73
Pre-menopausal breast cancer (Pyrs=364 008)
Low	125	1.00		93	1.00		22	1.00
Med	321	0.88 (0.71–1.08)	0.21	247	0.92 (0.72–1.16)	0.48	49	0.73 (0.44–1.21)	0.22
High	238	0.91 (0.73–1.13)	0.40	185	0.97 (0.76–1.25)	0.81	31	0.64 (0.37–1.11)	0.11
Post-menopausal breast cancer (Pyrs=260 042)
Low	227	1.00		176	1.00		35	1.00
Med	525	1.11 (0.95–1.29)	0.20	427	1.17 (0.98–1.39)	0.09	69	0.93 (0.62–1.39)	0.72
High	302	1.04 (0.88–1.24)	0.63	224	1.00 (0.821.22)	0.99	53	1.17 (0.76–1.80)	0.47

Abbreviations: CI=confidence interval; HR=hazard ratio; LAN=light at night; Pyrs=person-years of follow-up.

^aLow: ‘Too dark to see your hand, or you wear a mask’ Med: ‘Light enough to see your hand in front of you, but not see across the room’ High: ‘Light enough to see across the room, but not read’+‘Light enough to read’.

^bHazard ratios estimated using Cox proportional hazards regression with attained age as time scale, adjusted for: year of birth, history of benign breast disease, breast cancer in a first-degree relative, socioeconomic score, age at menarche, age at first birth, parity, duration of breastfeeding, oral contraceptive use, hormone replacement therapy use, menopausal status and age at menopause where applicable, pre-menopausal and post-menopausal body mass index, alcohol consumption, smoking and physical activity level.