Table 1.

Distribution of indoor and outdoor light-at-night estimates by selected characteristics of study population (n=106,731).

Characteristics	Used a Bright Light While Sleeping		Outdoor Light-at-Night Quintiles					All
	No	Yes	Lowest	2nd	3rd	4th	Highest
	(n=101,779)	(n=4,952)	(n=21,350)	(n=21,350)	(n=21,336)	(n=21,346)	(n=21,349)	(n=106,731)
	%	%	%	%	%	%	%	%	N
Age at baseline (years)
20–39	19	22	15	18	20	20	21	19	19,910
40–49	27	28	30	29	27	26	24	27	29,223
50–59	25	24	25	25	24	25	24	25	26,325
60–69	16	14	16	15	16	16	17	16	16,968
≥70	13	12	14	13	13	13	14	13	14,305
Race/Birthplace
White	87	74	92	89	88	86	77	87	92,339
Non-white US-born	10	21	6	8	9	10	17	10	10,871
Non-white foreign-born	2	4	1	2	2	3	5	2	2,684
Other/Unknown	1	1	1	1	1	1	1	1	837
Family history of breast cancer
No	84	85	85	85	85	84	85	85	90,369
Yes	12	11	12	11	11	12	11	12	12,461
Unknown	4	4	3	4	4	4	4	3	3,901
Age at menarche (years)
≤11	22	26	22	22	22	22	23	23	23,761
12–13	56	53	56	57	57	56	56	56	59,893
≥14	20	19	21	20	20	20	20	20	21,535
Unknown/Never	2	2	1	1	1	2	1	1	1,542
Age at first full-term pregnancy (years)
Nulliparous or Nulligravid	26	32	21	24	25	29	34	27	28,261
≤24	25	26	29	26	26	24	22	25	27,117
25–29	30	25	31	31	30	28	25	29	31,223
≥30	17	15	17	17	17	17	17	17	18,144
Unknown	2	2	2	2	2	2	2	2	1,986
Breastfeeding history (months)
Nulliparous	20	24	16	19	20	22	26	21	21,912
Nulligravid	6	8	5	5	6	6	8	6	6,223
Never breastfed	16	14	15	16	16	16	16	16	16,851
<6	17	16	18	18	17	18	16	17	18,598
6–11	14	12	14	14	14	13	12	13	14,425
≥12	25	23	30	26	25	22	20	25	26,245
Unknown	2	3	2	2	2	3	2	2	2,477
Strenuous Physical Activity (hours per week)
0–0.50	29	29	28	29	29	30	32	29	31,570
0.51–2.00	32	32	32	33	32	31	31	32	34,167
2.01–3.50	18	17	18	18	17	18	17	18	18,768
3.51–5.00	10	9	10	10	10	9	9	10	10,235
>5.00	10	12	11	10	11	11	10	10	11,252
Unknown	1	1	1	<1	1	1	1	1	739
Body mass index (kg/m2)
16.0–24.9	59	49	59	60	59	59	57	59	62,765
25.0–29.9	24	26	24	23	24	23	24	24	25,273
30.0–54.8	13	21	13	13	13	14	14	13	14,395
Unknown	4	4	4	4	4	4	5	4	4,298
Alcohol Consumption (g/day)
None	32	38	30	31	31	32	36	32	34,113
<20	55	50	56	56	56	56	52	55	58,808
≥20	8	6	9	8	8	7	6	8	8,251
Unknown	5	6	5	5	5	5	6	5	5,559
Menopausal Status/Hormone Therapy Use
Premenopausal	41	44	40	42	42	42	42	41	44,264
Peri/post-menopausal
never used hormone therapy	12	13	11	11	11	11	14	12	12,293
former hormone therapy use	7	7	7	6	7	6	7	7	7,038
current estrogen use	13	11	14	13	13	13	12	13	13,842
current estrogen/progestin use	14	11	14	15	14	15	13	14	15,144
Other/unknown	13	14	14	13	13	13	12	13	14,150
Smoking Status
Never	66	62	66	66	66	66	66	66	70,758
Former	28	30	29	28	28	28	27	28	29,922
Current	5	7	4	5	5	5	6	5	5,390
Unknown	1	1	1	1	1	1	1	1	661
Smoking pack-years
Never smoker	67	62	66	66	66	66	66	66	70,758
≤10	16	16	17	16	16	16	16	16	17,378
11–20	6	6	6	6	6	6	6	6	6,036
21–30	3	4	3	3	3	3	4	3	3,573
≥31	5	8	5	5	5	5	5	5	5,244
Unknown	3	4	3	4	4	4	3	4	3,742
Neighborhood Socioeconomic Status
Lowest quartile	25	29	33	19	21	23	29	25	26,684
2nd quartile	25	24	27	24	24	23	26	25	26,688
3rd quartile	25	25	21	25	26	27	26	25	26,677
Highest quartile	25	22	19	32	29	27	19	25	26,682
Urban/Rural (2000 block group baseline residence)
Rural	8	5	36	1	<1	<1	<1	7	7,904
Town	4	3	14	5	1	<1	<1	4	4,346
Non-metropolitan City	29	24	33	48	36	23	3	28	30,331
Suburban	54	59	17	45	60	71	79	55	58,272
Urban	5	9	0	1	3	6	18	6	5,878
Residentially stable (non-movers)	53	49	54	53	52	52	53	53	56,112