Table 2. Associations between habitual sleep duration and risk of colorectal cancer in the WHI-OS (1994–2010).

Sleep duration (hours per night)	No. of cases	Age-adjusted incidence rate/100,000 person–years)	HR a	95% CI	HR b	95% CI	HR c	95% CI
Colorectal cancer d
Total cases	851
⩽5	82	131.4	1.46	1.14, 1.86	1.42	1.11, 1.81	1.36	1.06, 1.74
6	227	100.8	1.11	0.93, 1.32	1.09	0.92, 1.30	1.08	0.91, 1.28
7	303	91.4	1.00		1.00		1.00
8	185	93.8	1.03	0.86, 1.23	1.00	0.84, 1.20	1.01	0.84, 1.21
⩾9	54	142.4	1.57	1.17, 2.09	1.49	1.12, 2.00	1.47	1.10, 1.96
Colon cancer
Total cases	700
⩽5	62	98.4	1.32	1.01, 1.73	1.29	0.98, 1.68	1.20	0.91, 1.58
6	192	85.4	1.12	0.94,1.33	1.11	0.94, 1.32	1.08	0.90, 1.28
7–8	404	76.4	1.00		1.00		1.00
⩾9	42	106.6	1.46	1.06, 2.00	1.40	1.02, 1.92	1.36	0.99, 1.87
Rectal cancer
Total cases	111
⩽5	12	19.0	1.58	0.85, 2.92	1.56	0.84, 2.89	1.53	0.81, 2.88
6	26	11.6	0.94	0.60, 1.49	0.94	0.60, 1.48	0.94	0.59, 1.48
7–8	65	12.2	1.00		1.00		1.00
⩾9	8	20.5	1.72	0.82, 3.58	1.68	0.81, 3.50	1.68	0.80, 3.50
Proximal cancers
Total cases	461
⩽5	41	64.7	1.32	0.95,1.83	1.28	0.92, 1.77	1.20	0.86, 1.68
6	122	53.9	1.07	0.86,1.33	1.06	0.86, 1.32	1.04	0.84, 1.29
7–8	269	50.5	1.00		1.00		1.00
⩾9	29	73.3	1.51	1.03, 2.22	1.44	0.98, 2.12	1.39	0.95, 2.04
Distal cancers
Total cases	288
⩽5	29	46.6	1.42	0.96, 2.10	1.40	0.95, 2.08	1.32	0.89, 1.98
6	69	30.7	0.93	0.71, 1.23	0.93	0.70, 1.23	0.92	0.69 1.21
7–8	174	32.8	1.00		1.00		1.00
⩾9	16	41.0	1.29	0.77, 2.15	1.26	0.75, 2.10	1.21	0.72, 2.02

Abbreviations: CI=confidence interval; HR=hazard ratio; WHI-OS=Women’s Health Initiative–Observational Study.

^a Crude HR.

^b HR was adjusted for age.

^c HR was adjusted for age, ethnicity, fatigue, hormone replacement therapy, waist to hip ratio, and physical activity.

^d Includes cancers of colon, rectum, and rectosigmoid.