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


CONCLUSIONS
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.
National Research Centre for the Working Environment , Copenhagen , Denmark.
Anne H
AH
Garde
Åse  M
Å
Hansen
Roger
R
Persson
Kai
K
Österberg
Palle
P
Ørbæk
Björn
B
Karlson
Annemarie
A
Olsen
Jesper
J
Kristiansen
D064368:Healthy Volunteers; D006801:Humans; D016272:Occupational Health; D012537:Scandinavian and Nordic Countries; D012890:Sleep
Sleep quality; circa-annual variation; human; sleep duration