Characteristics of participants by sleep duration at the reference date are shown in . Older women reported sleeping fewer hours. Any alcohol consumption was less often reported by women reporting both short (< 6 hours) and long durations (≥ 9 hours) of sleep. Higher pack-years of smoking were also associated with shorter sleep intervals. Women reporting fewer hours of sleep were more likely to be postmenopausal, not currently married, to have less than a high school education and to not participate in leisure-time physical activity. Current employment was less often reported by women reporting both shorter (< 6 hours) and longer (≥ 9 hours) durations of sleep. The mean BMI was significantly different across sleep duration categories for both recent (p <0.0001) and lifetime (p <0.0001) sleep. The median BMI for each sleep duration category (<6, 6-6.9, 7-7.9, 8-8.9 and ≥9 hours per night) was 26.5, 25.8, 24.9, 25.0, and 25.6 kg/m2, respectively, for recent sleep and 26.6, 25.8, 24.9, 25.1, and 25.0 kg/m2, respectively, for average adult lifetime sleep duration (not shown).
Characteristics of women by recent sleep duration (n = 5,549)
presents the odds of overweight (BMI = 25-29.9 kg/m2), obesity (30-39.9 kg/m2), and severe obesity (BMI ≥ 40kg/m2) when compared to a normal body weight (BMI < 25 kg/m2) according to usual sleep duration over adult life and in the recent past. ORs are shown adjusted for age and study center and for potential confounders. All associations between sleep duration and obesity were attenuated after adjustment for potential confounders including education, marital status, alcohol consumption, cigarette smoking, leisure-time physical activity, menopause status, and current employment status.
Relationship of recent and usual lifetime sleep duration with overweight, obesity and severe obesity
For overweight (BMI = 25-29.9 kg/m2), we observed an approximately 30% excess odds associated with a shorter sleep duration (<7 hours per night) over adult life which was statistically significant for 6-6.9 hours (multivariate OR: 1.30; 95% CI: 1.09-1.56) when compared to the referent (7-7.9 hours). No similar excess was observed for short sleep duration in the recent past. Odds of overweight were unrelated to long sleep duration (≥ 8 hours) in adult life or the recent past.
For obesity (BMI = 30-39.9 kg/m2), we observed increasing odds with shorter durations of sleep both in adult life and in the recent past. For recent sleep duration, the multivariate-adjusted odds ratios were 1.52 (95% CI: 1.23-1.89) for 6-6.9 hours of sleep and 1.89 (95% CI: 1.45-2.47) for less than 6 hours of sleep when compared to 7-7.9 hours (the referent). The corresponding odds ratios for sleep duration throughout adult life were 1.46 (95% CI: 1.17-1.82) and 1.54 (95% CI: 1.07-2.21), respectively. A long duration of sleep (≥ 8 hours) was unrelated to the odds of obesity.
For extreme obesity (BMI ≥ 40kg/m2), we again observed positive associations with a shorter sleep duration, with somewhat stronger and more consistently significant associations noted for sleep duration in the recent past. The odds of extreme obesity were also increased for longer sleep durations. A significant U-shaped association (p <.001) was demonstrable for sleep duration in the recent past, with significantly elevated odds observed both for short (multivariate OR: 3.12; 95% CI: 1.70-5.75 for less than 6 hours) and long (multivariate OR: 2.53; 95% CI: 1.10-5.78 for 9 or more hours) reported sleeping hours 1-2 years prior to the interview.
We further evaluated associations in a subset of women in the survey who were asked about use of antidepressant medications, which may influence sleep patterns. Restricting analysis to 1,864 women who reported never using any antidepressant medication in the past did not materially change the results (data not shown), though all results were based on sparse data.
Associations between recent sleep duration and obesity were not materially different in younger (<50) and older (≥50) women (p for interaction = 0.54) (not shown). Likewise, associations were similar regardless of levels of recent physical activity (p for interaction = 0.19), smoking status (p for interaction = 0.54) and current employment (p for interaction = 0.17) (not shown). Similarly, null results were obtained when we considered these potential interactions with usual sleep duration in adult life (not shown).
We next examined prevalence of obesity in women who reported stable or changing durations of sleep in the recent past when compared to lifetime sleep durations (). Among lifetime long sleepers (≥8 hours per night), current short sleepers (< 7 hours per night) had a significantly higher mean BMI than women who reported longer current sleep durations (≥8 hours per night) (26.4 kg/m2 and 25.0 kg/m2, respectively (p<0.001)). Odds of obesity (≥ 30kg/m2) were 75% higher in women currently sleeping fewer hours (OR: 1.75; 95% CI: 1.33-2.32) than in the past. A similar pattern was noted among lifelong short sleepers (< 7 hours per night): current short sleepers had a slightly higher mean BMI when compared to lifelong short sleepers who recently increased their sleep time to a minimum of 8 hours per night (26.2 kg/m2 and 25.7 kg/m2, respectively (p=0.24)); consistent short sleepers had approximately 60% greater odds of obesity than women currently sleeping longer hours (OR: 1.59; 95% CI: 0.93-2.78), though the difference was not significant.
Odds of obesity in relation to usual sleep duration in adult life and the recent past
We observed no association of sleep duration with low body weight (BMI <18.5 kg/m2
] (not shown). When compared to women reporting 7-7.9 hours in the recent past, the odds of being underweight (<18.5 kg/m2
) were similar for women reporting <7 hours (OR = 0.97; 95% CI: 0.59-1.59) and ≥8 hours (OR = 0.91; 95% CI: 0.58-1.44) of sleep per night. Few women in the population were underweight (110 women had a BMI <18.5 kg/m2
) and these associations were imprecise.