Table 1. Baseline Characteristics of Participants by Bedtimea.
| Characteristic | Bedtime, No. (%) (N = 136 652) | ||||
|---|---|---|---|---|---|
| Daytime (n = 1769)b | 8 pm-10 pm (n = 31 472) | 10 pm-midnight (n = 83 751) | Midnight-2 am (n = 17 939) | 2 am-6 am (n = 1721) | |
| Age, mean (SD), y | 52.5 (10.3) | 51.7 (10.1) | 51.1 (9.8) | 49.8 (9.4) | 49.2 (9.3) |
| Sex | |||||
| Men | 727 (41.1) | 12 055 (38.3) | 33 868 (40.4) | 7601 (42.4) | 749 (43.5) |
| Women | 1042 (58.9) | 19 417 (61.7) | 49 883 (59.6) | 10 338 (57.6) | 972 (56.5) |
| Education attainment | |||||
| None or primary school | 1171 (66.5) | 17 000 (54.1) | 28 211 (33.7) | 5918 (33.0) | 536 (31.2) |
| Secondary, high, or higher secondary | 459 (26.0) | 11 372 (36.2) | 32 366 (38.7) | 6218 (34.7) | 631 (36.7) |
| Trade, college, or university | 132 (7.5) | 3029 (9.6) | 23 073 (27.6) | 5790 (32.3) | 552 (32.1) |
| Living location | |||||
| Urban | 480 (27.1) | 10 601 (33.7) | 49 655 (59.3) | 13 165 (73.4) | 1380 (80.2) |
| Rural | 1289 (72.9) | 20 871 (66.3) | 34 096 (40.7) | 4774 (26.6) | 341 (19.8) |
| Country income status | |||||
| High | 83 (4.7) | 1350 (4.3) | 12 086 (14.4) | 2443 (13.6) | 444 (25.8) |
| Middle | 1498 (84.7) | 26 137 (83.0) | 62 673 (74.8) | 14 675 (81.8) | 1168 (67.9) |
| Low | 188 (10.6) | 3985 (12.7) | 8992 (10.7) | 821 (4.6) | 109 (6.3) |
| Smoking status | |||||
| No | 1093 (62.1) | 21 954 (70.5) | 57 657 (69.1) | 10 794 (60.3) | 803 (46.7) |
| Current or former | 666 (37.9) | 9191 (29.5) | 25 737 (30.9) | 7120 (39.7) | 915 (53.3) |
| Alcohol drinking | |||||
| No | 973 (55.4) | 21 583 (69.4) | 54 413 (65.6) | 11 328 (64.3) | 984 (58.6) |
| Current or former | 783 (44.6) | 9524 (30.6) | 28 565 (34.4) | 6293 (35.7) | 694 (41.4) |
| Physical activity, mean (SD), MET-min/wk | 5886 (6338) | 4815 (5555) | 4428 (4891) | 3891 (4628) | 3568 (4738) |
| Total energy intake, mean (SD), kcal | 2320 (1122) | 2165 (975) | 2207 (957) | 2376 (1011) | 2444 (1070) |
| Nocturnal sleep duration, mean (SD), h | 9.7 (1.7) | 8.8 (1.1) | 7.6 (1.1) | 6.7 (1.4) | 6.3 (1.6) |
| Duration of daytime naps, mean (SD), h | 1.0 (0.7) | 1.0 (0.6) | 1.0 (0.6) | 1.1 (0.6) | 1.2 (0.7) |
| Wake-up time, median (IQR), h:min in 24 h | 5:30 (5:00-6:00) | 6:00 (5:00-6:00) | 6:00 (5:30-7:00) | 7:00 (6:00-8:00) | 9:00 (7:00-10:00) |
| MST, median (IQR), h:min in 24 h | NC | 1:23 (1:00-1:31) | 2:30 (2:00-2:45) | 3:30 (3:00-4:00) | 5:30 (4:45-6:15) |
| Anthropometric measurements, mean (SD) | |||||
| BMI | 25.1 (5.4) | 25.2 (5.0) | 26.2 (5.0) | 28.3 (5.7) | 28.9 (6.4) |
| Waist circumference, cm | 83.7 (13.4) | 83.3 (12.6) | 86.1 (13.2) | 91.1 (13.8) | 92.9 (14.9) |
| Hip circumference, cm | 95.0 (11.6) | 95.3 (10.7) | 98.1 (10.8) | 102.5 (11.6) | 103.8 (12.9) |
| Waist-to-hip ratio | 0.88 (0.09) | 0.87 (0.08) | 0.88 (0.08) | 0.89 (0.09) | 0.90 (0.09) |
| Waist-to-height ratio | 0.53 (0.08) | 0.52 (0.08) | 0.53 (0.08) | 0.56 (0.09) | 0.57 (0.09) |
Abbreviations: BMI, body mass index (calculated as weight in kilograms divided by height in meters squared); IQR, interquartile range; NC, not calculated; MET, metabolic equivalent of task; MST, midsleep time.
a
All variables were summarized and compared by subgroups of habitual bedtime and showed a P < .001. Continuous variables were compared by 1-way analysis of variance or Kruskal-Wallis test. Categorical variables were compared by χ2 test. All calculations were based on participants with complete data.
b
Defined as bedtime between 6 am and 8 pm. MST was not calculated for this group.