Table 5. Night waking with exposure to light^a 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
Night waking	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=640 832)
No	1450	1.00		1130	1.00		220	1.00
Yes	103	0.85 (0.70–1.04)	0.12	77	0.82 (0.65–1.04)	0.10	15	0.82 (0.49–1.40)	0.47
N/k	222	0.91 (0.78–1.05)	0.20	175	0.90 (0.77–1.07)	0.23	30	0.85 (0.57–1.26)	0.41
Pre-menopausal breast cancer (Pyrs=373 323)
No	593	1.00		457	1.00		88	1.00
Yes	50	0.74 (0.55–0.99)	0.04	36	0.69 (0.49–0.97)	0.03	9	0.91 (0.45–1.82)	0.79
N/k	47	0.65 (0.48–0.87)	0.01	37	0.65 (0.46–0.91)	0.01	6	0.64 (0.28–1.49)	0.30
Post-menopausal breast cancer (Pyrs=267 509)
No	857	1.00		673	1.00		132	1.00
Yes	53	0.96 (0.73–1.27)	0.80	41	0.95 (0.69–1.30)	0.74	6	0.72 (0.32–1.63)	0.43
N/k	175	1.02 (0.86–1.21)	0.80	138	1.02 (0.84–1.23)	0.85	24	0.93 (0.60–1.46)	0.76

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

^aWake and put the lights on or go into a bright room.

^bHazard ratios estimated using Cox proportional hazards regression with attained age as time scale, adjusted for: light at night at age 20, light at night in the year before recruitment, 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.