Associations between Finnish Seasons and Sleep in Early Childhood

Anja Kärki

Research output: Book/ReportDoctoral thesisCollection of Articles

Abstract

Environmental light can affect sleep. Particularly in Finland and other Nordic countries, the amount of natural light varies substantially, depending on the season. Therefore, the effects of season on sleep have been studied quite a lot over the years. These studies, however, have been conducted mostly on adults, school-aged children, or adolescents. There is a scarcity of evidence on the effects of season on sleep during early childhood, and most of the previous data are not based on the gold standard of sleep evaluation, polysomnography (PSG).

The aim of this prospective cross-sectional study is to clarify the effect of Finnish seasons on the nighttime sleep of infants and small children.

The families of the CHILD-SLEEP birth cohort were recruited to the project from maternity clinics in Pirkanmaa hospital district during the year 2011 and subsequently followed in several age points. The study cohort has been divided into several subgroups. This study was conducted on a subgroup which participated in an ambulatory home PSG study at different ages (one, eight, and 24 months). In addition to the PSG data, other data collected from comprehensive parental questionnaires and growth measurements acquired from child health clinics were utilized.
In the first sub-study, the effects of seasons on sleep were assessed when the infants were approximately one month old (n=85). In the second sub-study, the effects of seasons were assessed when the infants were approximately eight months old (n=72). In the third sub-study, the effects of seasons were analyzed in relation to the computational spectral power analysis of sleep EEG when the children were approximately eight (n=56) and 24 months (n=60) old. This allowed the analysis of sleep microstructure, such as slow wave activity (SWA) and sleep spindle frequency activity (SFA).

At the age of one months, infants born during the summer had shorter nighttime sleep than infants born during the autumn. In addition, infants born during the summer had more R sleep than infants born during the winter, and less T sleep than infants born during the autumn and winter. R sleep corresponds to REM sleep, and T sleep is an immature transient sleep stage of infancy.

At the age of eight months, infants whose PSGs were recorded during the spring, had less deep sleep (N3) and more light non-REM sleep (N2) than the PSGs conducted during the autumn. In addition, during the spring, the infants had more arousals and awakenings than during the autumn. The transition to daylight saving time also reduced the sleep quality of the infants.

The seasonal effect was also seen on sleep microstructure. In the younger age group (eight months), there were local reductions in SFA during the spring compared to darker seasons, whereas during the winter there was higher local SWA compared to the spring. In the older age group (24 months), there were only minor findings, with local SWA being lower during the winter than during the autumn. Moreover, in both age groups, SFA was decreasing across night sleep.

This is the first study to show the effects of Finnish seasons on sleep during early childhood. During very early infancy, the lightest time of year shortened the night sleep of infants but also induced some beneficial effects on sleep structure.

Later, when the infants were eight months old, lightening of sleep, more fragmented sleep, and reduction in local SFA was detected during the spring and after transition to daylight saving time. Thus, the rapid increment of natural light exposure and daylight saving time during the spring may reduce the quality of sleep in this age group.

At the age of 24 months, the effect of Finnish seasons on the sleep microstructure was minor, with the sole finding being a scarce and local reduction of SWA during the winter. Therefore, the effect of seasons on sleep structure seems to diminish with age.

This is the first study to show a diminishing overnight dynamic of SFA in children aged eight and 24 months, which differs from previous findings in adults. The present finding may be related to the immature regulation of sleep spindles during early childhood or the immature thalamocortical structures responsible for sleep spindle formation.
Original languageEnglish
Place of PublicationTampere
ISBN (Electronic)978-952-03-3127-6
Publication statusPublished - 2023
Publication typeG5 Doctoral dissertation (articles)

Publication series

NameTampere University Dissertations - Tampereen yliopiston väitöskirjat
Volume890
ISSN (Print)2489-9860
ISSN (Electronic)2490-0028

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