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  • br Conflict of interest br Acknowledgements This


    Conflict of interest
    Acknowledgements This research was funded by the Research Council of KU Leuven (OT/12/044) and the Research Foundation Flanders (G0920.12). Maaike Vandermosten is postdoctoral fellow of the Research Foundation Flanders. Sophie Dandache is gratefully acknowledged for participant selection and behavioural data collection. We also like to thank Ron Peeters, Alexander Leemans and Flavio Del’Aqua for technical assistance of DTI-processing.
    Inadequate sleep is endemic in adolescence (). In addition to receiving less than optimal average sleep per night, variability in sleep duration and sleep–wake rhythms peak during adolescence (). Children and adolescents often need more sleep than adults, which is attributed to the putative function sleep serves in indole-3-carbinol maturation (). Indeed, researchers have proposed that the primary purpose of sleep is the promotion of brain development (). Unfortunately, adolescents may not receive the quality environmental context needed for restorative sleep and optimal neuronal development (). Experimental research utilizing animal models and human adults has shown that impaired sleep, including sleep deprivation and variability in sleep–wake cycles, alters synaptic plasticity resulting in transcriptional alterations in protein synthesis, reduced cerebral metabolism, gray matter loss in cortical and subcortical structures, as well as neurogenesis as a result of increased circulating levels of the adrenal stress hormone, corticosterone (). Moreover, there is compelling evidence from high density EEG studies in youth measuring electrical activity of the brain showing that poor sleep can influence neural function, relating to impairments in memory formation and learning, executive functions, and emotional well-being (see ). Because sleep, and REM sleep in particular, facilitates memory consolidation, learning, and emotional development (), poor sleep in adolescence may be associated with a host of negative outcomes via alterations in brain development and function (). Yet, despite the important role of sleep in promoting brain development, we know little about how variations in sleep during adolescence are associated with alterations in the developing brain. The adolescent years represent a key developmental period when the brain undergoes significant remodeling, which is accompanied by changes in the neurophysiological features of sleep, memory systems, socioemotional processing, and emotion regulation (). Myelination of frontocortical and frontostriatal white matter tracts continues through adolescence and into adulthood (). Cellular maturation of white matter facilitates faster and more efficient neural transmission and cognitive processing (). Thus, relative delayed maturation and subtle alterations in the microstructure of white matter fibers during development can affect neurocognition and behavior, including behavioral problems (), substance use (), and poor cognitive performance (). During periods of brain development, synaptic activity exhibits high levels of plasticity and connectivity and is therefore particularly influenced by environmental inputs (). Moreover, stressors can significantly modify brain development (). Sleep may therefore have an especially large impact on the developing adolescent brain. While sleep problems may not induce neural changes in white matter that are immediately evident, chronic or frequent sleep problems may have accumulating effects, impairing neuronal integrity, decreasing neurogenesis, and altering structural plasticity over time. Adolescents demonstrate considerable variability in their sleep patterns, with intra-individual differences in sleep duration often exceeding between-person differences. Such variability in sleep is detrimental for health outcomes, perhaps more so than is chronically low sleep duration (). For example, adolescents with high day-to-day variability of sleep duration assessed by daily sleep logs over two weeks report greater depression, anxiety, fatigue, and lower subjective well-being controlling for average sleep duration (). Daily diary measurements offer an ideal method for assessing sleep. Single-time surveys do not provide estimates of the daily variability in adolescents’ sleep that go beyond differences between weekdays and weekends (). Moreover, daily reporting minimizes the amount of error that occurs in retrospective reporting of events and allows for a direct estimation of the daily variability in adolescents’ sleep time.