Table 1.
Baseline characteristics by reported night shift work in three UK prospective studies and results of follow-up for breast cancer
| Million Women Study* |
EPIC-Oxford |
UK Biobank |
||||
|---|---|---|---|---|---|---|
| Never night shifts | Ever night shifts | Never night shifts | Ever night shifts | Not current night shifts | Current night shifts | |
| Baseline characteristics and breast cancer follow-up | (n = 450 232) | (n = 72 014) | (n = 19 289) | (n = 3270) | (n = 241 972) | (n = 9073) |
| Baseline characteristics | ||||||
| Mean age at baseline (SD), years | 68.8 (4.7) | 68.5 (4.6) | 58.0 (12.2) | 56.6 (11.7) | 56.3 (8.0) | 51.0 (6.6) |
| Socioeconomic status, No. (% in lower third) | 145 392 (32.5) | 26 129 (36.6) | 5628 (33.0) | 1009 (35.1) | 79 450 (32.9) | 4109 (45.4) |
| Not married nor living with a partner, No. (%) | 66 529 (17.8) | 14 006 (23.7) | 5693 (29.6) | 1093 (33.4) | 72 399 (30.0) | 3676 (40.7) |
| Nulliparous, No. (%) | 49 313 (11.0) | 7798 (10.8) | 7216 (37.7) | 1247 (38.5) | 45 000 (18.7) | 1922 (21.3) |
| Mean No. of children (parous women) (SD) | 2.3 (0.9) | 2.5 (1.0) | 2.2 (0.9) | 2.2 (1.0) | 2.2 (0.9) | 2.3 (1.0) |
| Mean age at first birth (parous women) (SD), years | 24.3 (4.2) | 24.0 (4.4) | 26.1 (4.7) | 26.0 (4.7) | 26.0 (5.1) | 25.3 (5.4) |
| Obese, No. (%) | 63 554 (14.7) | 13 307 (19.2) | 1053 (5.6) | 246 (7.7) | 55 962 (23.6) | 2670 (30.0) |
| Strenuous physical activity >2 h/wk, No. (%) | 103 199 (23.4) | 17 636 (25.1) | 5512 (29.0) | 1122 (34.6) | 37 364 (17.5) | 1717 (21.9) |
| Mean alcohol consumption (SD), g/d | 6.4 (7.6) | 6.1 (7.6) | 8.1 (9.8) | 7.7 (9.1) | 8.9 (10.9) | 8.5 (11.7) |
| Current smoker, No. (%) | 57 198 (13.3) | 13 122 (19.1) | 1638 (8.5) | 372 (11.4) | 20 865 (8.7) | 1390 (15.4) |
| First-degree relative with breast cancer, No. (%) | 41 339 (9.7) | 6783 (10.1) | N/A | N/A | 15 522 (6.8) | 471 (5.5) |
| Ever oral contraceptive user, No. (%) | 281 855 (63.0) | 48 044 (67.1) | 14 798 (77.2) | 2615 (80.4) | 195 681 (81.1) | 7671 (84.9) |
| Ever menopausal hormone therapy user, No. (%)† | 226 674 (54.2) | 41 679 (62.1) | 1420 (40.2) | 219 (45.9) | 70 624 (51.4) | 1361 (49.6) |
| Mean amount of sleep (SD), hours | 6.8 (1.3) | 6.7 (1.8) | 6.9 (1.1) | 6.8 (1.2) | 7.2 (1.1) | 7.0 (1.2) |
| Take medication to sleep on most days, No. (%) | 22 613 (5.3) | 4865 (7.0) | 369 (1.9) | 87 (2.7) | N/A | N/A |
| More evening than morning type, No. (%) | 117 848 (28.8) | 21 526 (33.1) | 5252 (29.5) | 1008 (33.8) | 80 068 (36.4) | 3683 (45.2) |
| Follow-up for breast cancer | ||||||
| Mean person-years of follow-up per woman | 2.6 | 2.6 | 3.1 | 3.1 | 3.8 | 3.9 |
| Total No. of incident breast cancers | 4136 | 673 | 153 | 28 | 2653 | 67 |
*For the Million Women Study, the tabulation is of strenuous physical activity more than once per week.
†Restricted to women aged 55 years or older.