Differences in sleep duration in a territory with the same time zone according to the geographic longitude: the Spanish case.

Objective/background: In some territories, such as Spain, daytime starts with around 1 h of difference between the easternmost and westernmost areas, but the time zone is the same in these areas. This difference might have an effect on children's sleep. The aim of this study is to assess if there are differences in the prevalence of short sleep duration between children under 15 years from easternmost (Catalonia) and westernmost (Galicia) continental territory areas in Spain.

Methods: Cross-sectional study using data from the 2017 Spanish National Health Survey (n = 6106). The final sample includes 1004 children under 15 years living in Catalonia and Galicia. We categorized sleep duration according to the National Sleep Foundation recommendations with respect to age in (1) not short sleep duration and (2) short sleep duration. We calculated overall percentages of short sleep duration, and unadjusted and adjusted Prevalence Ratios (PR) with 95% Confidence Intervals (95% CI), according to area and stratified by confounders and covariates. Unadjusted and adjusted PR were obtained through generalized linear models with Poisson family and robust variance. We adjusted the associations for confounders.

Results: Children from Galicia were less likely to have short sleep duration than children from Catalonia, aPR = 0.61 (95% CI: 0.45,0.83).

Conclusions: Children living in the same country, potentially with a similar timetable, could be having shorter sleep durations according to their longitude position. Further studies are needed in order to consider promoting policies to apply timetables based on the sun position instead of on the national time zone.