Table 3.

Risk of sporadic anovulation associated with sleep characteristics in the BioCycle Study (2005–2007)

	Risk of Anovulation
	RR	95% CI
Average daily sleep duration (in hours) at the LH surge a
Continuous number of hours (increasing)	0.79	0.59, 1.06
<7 hours compared to ≥7 hours	1.63	0.84, 3.16
Employment and night/shift work statusb
Employed, No shift/night work	reference
Unemployed	0.65	0.24, 1.73
Employed, Yes shift/night work	0.70	0.32, 1.56
Chronotypeb
Morning	2.50	0.93, 6.77
Intermediate	reference
Evening	1.20	0.50, 2.86

RR=risk ratio, CI=confidence interval.

These modified Poisson regression models take repeated observations into account: up to two menstrual cycles per woman.

^aAverage hours of sleep per day is taken from the total amount of sleep on the day of and day before the clinic visit around the LH surge, as reported in daily diaries. In the first model, average sleep per day is treated continuously; in the second model, those who reported <7 hours per day are compared to those reporting at least 7 hours per day (reference). Models are adjusted for: age, race, BMI, chronotype, night/shift work status, season; and at the time of the LH surge in each cycle, average daily caloric intake, average daily caffeine intake, cigarette use, and percent of energy intake attributable to alcohol, fat, protein, and carbohydrates.

^bAdjusted for age, race, body mass index (BMI), chronotype (night/shift work model), night/shift work status (chronotype model), and season.