Associations between objectively measured sleep and cognition in older Icelandic adolescents

Abstract Background: Insufficient sleep is associated with an increased risk of a variety of negative health outcomes and has been shown to adversely affect academic and cognitive function. Despite strong evidence of the deleterious effects of short and disrupted sleep on health, studies that use objective methods to measure adolescent free-living sleep, in general, and its association with academic and cognitive performance, specifically, are scarce, particularly during the critical transition from compulsory to secondary education. Aim: The aim of this research was to use objective measures to quantify the free-living sleep of Icelandic adolescents at ages 15 and 17, as they transition from compulsory to secondary education, and to determine whether their sleep patterns are associated with academic and cognitive outcomes. Methods: The study sample came from six elementary schools in Reykjavík, Iceland. One week of free-living sleep measured with wrist actigraphy was collected at two time points. The first wave of data collection occurred in 2015, where 280 participants had valid sleep data (mean age 15.9±0.3 years). Two years later, 199 participants had valid sleep data (mean age of 17.7±0.3 years). In total, 145 participants had complete data at both data collection points. During the first wave of data collection, academic achievement was objectively quantified using the combined score from standardized national examinations administered to all 10th grade students in mathematics, English, and Icelandic. During the second wave of data collection, an n-back working memory task and Posner cue-target visual attention task were used to objectively assess cognitive function. Results: Over all measured nights at both time points, Icelandic adolescents averaged late bedtimes (00:43 at age 15, 01:12 at age 17), short total sleep time (6.6 ± 0.7 h/night at age 15 and 6.2 ± 0.7 h/night at age 17) and high variability in total sleep time (weekly standard deviations of 1.3 h and 1.4 h at ages 15 and 17, respectively). Thus, during the two year change from age 15 to age 17, students sleep duration decreased, night-to-night variability in sleep duration increased, and students went to bed 29 min/night later. Cross-sectional regression of the data collected at age 15 in those with standardized exam scores (n=253) demonstrated that both bedtime and night-to-night variability in total sleep time were negatively associated with the average score across all topics. Similarly, cross-sectional analysis of students who underwent cognitive function testing at age 17 (n=160) showed that time in bed the night before cognitive testing was negatively associated with response times during the most challenging memory task. However, sleep measures the night before did not correlate with performance on the attention task and weekly sleep parameters were not associated with either cognitive task. Conclusion: In general, Icelandic students go to bed late and have short and inconsistent sleep schedules at both age 15 and age 17. Cross-sectional results at age 15 indicate that those with earlier bedtimes and more consistent sleep schedules score higher on national exams. In addition, the cross-sectional results at age 17 demonstrated that shorter time in bed the night prior to the cognitive testing was associated with poorer performance on the most challenging short-term memory task. Despite the presence of several significant associations, the relationship between free-living sleep and academic and cognitive task performance in healthy adolescents is less clear than that identified in laboratory studies or with self-report, perhaps due to high night-to-night sleep variation. Future studies with longer observation periods and interventional components could further clarify the relationship between free-living sleep and academic and cognitive performance in adolescents.