Month-to-month variation in sleep among healthy, Scandinavian daytime workers

Abstract

Background. The overall purpose of the present study was to attain more insight in month-to-month variation of sleep duration and quality in order to improve design and interpretation of, e.g. epidemiological studies using sleep as outcome. Methods. The study design entailed monthly self-reports from 38 (26 women/12 men) daytime workers, who completed the Karolinska Sleep Diary (KSD) once a month during one year. A subgroup (n = 16) also wore actigraphs on one day every month during a year. Self-reports of bedtime, time of awakening, sleep duration, individual sleep characteristics, disturbed sleep index (DSI, 4 items) and awakening index (AWI, 3 items) were analyzed together with actigraphy-derived measures. Hours of daylight were used to test for circa-annual variation in statistical models adjusted for intake of hypnotics and alcohol, gender, age and within-person variability. Results. Hours of daylight were found to be associated to self-reported bedtime (p = 0.032) and DSI (p = 0.030), thereby indicating a circa-annual variation. Bedtime was delayed by 1.8 min (95% CI: 0.6–2.9 min) per 1 hour increase in length of daylight. Sleep was slightly more disturbed during the winter. Conclusion. Only circa-annual variation in self-reports of bedtime and DSI were observed in a healthy daytime working population, and the effects were small. Therefore potential bias due to circa-annuality in the studied parameters appears to be of limited concern in adult daytime working populations.

Key Words::

• Sleep quality
• sleep duration
• circa-annual variation
• human

Acknowledgements

Lisbeth Prahl and Inge Christiansen are acknowledged for skillful technical and administrative assistance. Birgitta Malmberg, MD, is acknowledged for her help with identifying research participants. The authors would also like to thank the participants for their efforts to complete the study.

The work for the present paper was carried out at National Research Centre for the Working Environment and Division of Occupation and Environmental Medicine, Lund.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This study was partially financed by grants from the Swedish Work Environment Authority.