Characteristics of study population
| Variable | Day shifta | Night shift | Pb |
|---|---|---|---|
| Mean (SE)/N (%) | Mean (SE)/N (%) | ||
| Age | 40.5 (1.02) | — | — |
| BMI (kg/m2) | 28.4 (0.82) | — | — |
| # Years of shift work | 14.0 (1.01) | — | — |
| Ethnicity | |||
| White | 113 (95.76%) | — | — |
| Other | 5 (4.24%) | — | — |
| Reproductive characteristics | |||
| Age at menarche | 12.55 (0.13) | — | — |
| Number of pregnancies | 1.58 (0.18) | — | — |
| Ever been pregnant | |||
| Yes | 69 (58.47%) | — | — |
| No | 49 (51.53%) | — | — |
| Menopausal status | |||
| Premenopausal | 89 (75.42%) | — | — |
| Postmenopausal | 29 (24.58%) | — | — |
| Number of days since previous period | 13.83 (1.73) | 19.58 (1.71) | 0.009 |
| Sleep characteristics | |||
| Sleep duration | 6.91 (0.15) | 5.23 (0.15) | <0.0001 |
| Sleep interrupted | 15 (12.71%) | 19 (16.10%) | 0.45 |
| Lights on for more than 1 hour if interrupted | 3 (2.54%) | 8 (6.78%) | 0.09 |
| Experience sleep problems | 69 (58.47%) | — | — |
| Diagnosed with sleep disorder | 4 (3.49%) | — | — |
| Medication use | |||
| Antidepressants | 12 (10.17%) | — | — |
| Beta-blockers | 2 (1.69%) | — | — |
| Hormone replacement therapy | 7 (5.93%) | — | — |
| Migraine medication | 6 (5.08%) | — | — |
| Pain medication (NSAIDs) | 29 (24.58%) | 28 (23.73%) | 0.86 |
| Sedatives or muscle relaxants | 6 (5.08%) | 7 (5.93%) | 0.74 |
| Oral contraceptives | 19 (16.10%) | 15 (12.70%) | 0.25 |
| Lifestyle characteristics | |||
| Pack-years smoking | 2.77 (0.57) | — | — |
| Smoked during 24 hours of melatonin collection | 10 (8.47%) | 11 (9.32%) | 0.56 |
| Caffeine consumption (# drinks during 24 hours melatonin collection) | 2.61 (0.22) | 2.97 (0.22) | 0.10 |
| Alcohol consumption (# drinks during 24 hours melatonin collection) | 0.33 (0.07) | 0.06 (0.07) | 0.008 |
| Lifetime alcohol consumption (# drinks/wk) | |||
| Teen | 2.52 (0.33) | — | — |
| 20s | 4.30 (0.38) | — | — |
| 30s | 2.83 (0.34; n = 95) | — | — |
| 40s | 2.71 (0.36; n = 66) | — | — |
| 50s | 2.75 (0.74; n = 26) | — | — |
| Chronotype | |||
| Definite morning type | 3 (3.57%) | — | — |
| Moderate morning type | 18 (21.43%) | — | — |
| Neither type | 55 (65.48%) | — | — |
| Moderate evening type | 8 (9.52%) | — | — |
| Definite evening type | 0 | — | — |
| Light exposure | |||
| Log-transformed mean light intensity (log lumens/m2) | −2.14 (0.06) | −0.06 (0.06) | <0.0001 |
| Urinary 6-sulfatoxymealtoninc | |||
| Morning 6-sulfatoxymelatonin (ng/mg creatinine) | 27.25 (1.11) | 25.49 (1.11) | 0.65 |
| Change in 6-sulfatoxymealtonin (ng/mg creatinine) | 23.48 (1.14) | 24.53 (1.14) | 0.80 |
↵aCharacteristics assessed in the study questionnaire (administered once) are shown in the “Day shift” column.
↵bDifferences between day and night shifts are compared using difference of least squares means estimates in a mixed model with a random subject effect for continuous variables and using McNemar's test for categorical variables.
↵cGeometric means (calculated by back-transforming log-transformed variables) are presented here.