The association between sleep and fecundability: a Danish preconception cohort study

We estimated the effects of sleep duration, trouble sleeping, and job time (day, evening, shift, and night work) on fecundability, the per-cycle probability of conception. We analysed data from a preconception cohort study of 10 475 Danish females aged 18-45 years attempting to conceive. On baseline questionnaires, females reported on sleep duration (hours/day), trouble sleeping (never, sometimes, approx. 50% of the time, most of the time), and job time. Pregnancy status was ascertained from follow-up questionnaires completed every 8 weeks for up to 12 months. We used proportional probabilities regression models to estimate fecundability ratios (FRs), adjusted for potential confounders. Compared with 8 h/day of sleep, FRs were 0.94 for ≤6 h/day (95% CI, 0.83-1.08), 0.97 for 7 h/day (95% CI, 0.89-1.06), and 0.96 for ≥9 h/day (95% CI, 0.81-1.13). Compared with no trouble sleeping, FRs were 1.00 (95% CI, 0.94-1.06) for trouble sleeping sometimes, 0.94 (95% CI, 0.86-1.03) for trouble sleeping approx. 50% of the time, and 0.95 (95% CI, 0.82-1.10) for trouble sleeping most of the time. Relative to day work, FRs were 1.12 for shift work (95% CI, 1.04-1.20), 0.77 for night work (95% CI, 0.49-1.22) and 1.10 for evening work (95% CI, 0.91-1.33). Self-reported sleep duration and trouble sleeping were not appreciably associated with fecundability. Shift work was associated with a slightly higher fecundability, while night work was associated with slightly lower fecundability, although associations were imprecise. Potential limitations include misclassification of sleep and residual or unmeasured confounding.