Table 1.

Characteristics of study population

VariableDay shiftaNight shiftPb
Mean (SE)/N (%)Mean (SE)/N (%)
Age40.5 (1.02)
BMI (kg/m2)28.4 (0.82)
# Years of shift work14.0 (1.01)
Ethnicity
White113 (95.76%)
Other5 (4.24%)
Reproductive characteristics
Age at menarche12.55 (0.13)
Number of pregnancies1.58 (0.18)
Ever been pregnant
Yes69 (58.47%)
No49 (51.53%)
Menopausal status
Premenopausal89 (75.42%)
Postmenopausal29 (24.58%)
Number of days since previous period13.83 (1.73)19.58 (1.71)0.009
Sleep characteristics
Sleep duration6.91 (0.15)5.23 (0.15)<0.0001
Sleep interrupted15 (12.71%)19 (16.10%)0.45
Lights on for more than 1 hour if interrupted3 (2.54%)8 (6.78%)0.09
Experience sleep problems69 (58.47%)
Diagnosed with sleep disorder4 (3.49%)
Medication use
Antidepressants12 (10.17%)
Beta-blockers2 (1.69%)
Hormone replacement therapy7 (5.93%)
Migraine medication6 (5.08%)
Pain medication (NSAIDs)29 (24.58%)28 (23.73%)0.86
Sedatives or muscle relaxants6 (5.08%)7 (5.93%)0.74
Oral contraceptives19 (16.10%)15 (12.70%)0.25
Lifestyle characteristics
Pack-years smoking2.77 (0.57)
Smoked during 24 hours of melatonin collection10 (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)
Teen2.52 (0.33)
20s4.30 (0.38)
30s2.83 (0.34; n = 95)
40s2.71 (0.36; n = 66)
50s2.75 (0.74; n = 26)
Chronotype
Definite morning type3 (3.57%)
Moderate morning type18 (21.43%)
Neither type55 (65.48%)
Moderate evening type8 (9.52%)
Definite evening type0
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.