Contents lists available at ScienceDirect Journal of Adolescence journal homepage: www.elsevier.com/locate/adolescence Cognitive performance in adolescents with Delayed Sleep-Wake Phase Disorder: Treatment effects and a comparison with good sleepers C. Richardson, G. Micic, N. Cain, K. Bartel, B. Maddock, M. Gradisar * Flinders University, School of Psychology, Australia ARTICLE INFO Keywords: Cognition Working memory Processing speed Adolescence Sleep Intervention ABSTRACT The present study aimed to investigate whether Australian adolescents with Delayed Sleep-Wake Phase Disorder have impaired cognitive performance and whether chronobiological treatment for Delayed Sleep-Wake Phase Disorder improves adolescents' sleep, daytime functioning and cog- nitive performance. Adolescents with Delayed Sleep-Wake Phase Disorder (mean = 15.68 ± 2.1 y, 62% f) reported significantly later sleep timing (d = 1.03–1.45), less total sleep time (d = 0.82) and greater daytime sleepiness (d = 2.66), fatigue (d = 0.63) and impairment (d = 2.41), compared to good sleeping adolescents (mean = 15.9 ± 2.4 y, 75% f). However, there were no significant between-group differences (all p > 0.05) in performance on the Operation Span (ηp 2 = 0.043), Digit Span (forwards: ηp 2 = 0.002, backwards: ηp 2 = 0.003), Letter Number Sequencing (ηp 2 < 0.001) (working memory) and Digit-Symbol Substitution Tasks (ηp 2 = 0.010) (processing speed). Adolescents with Delayed Sleep-Wake Phase Disorder went on to receive 3 weeks of light therapy. At 3 months post-treatment, adolescents with Delayed Sleep-Wake Phase Disorder reported significantly advanced sleep timing (d = 0.56–0.65), greater total sleep time (d = 0.52) and improved daytime sleepiness (d = 1.33), fatigue (d = 0.84) and impairment (d = 0.78). Performance on the Operation Span (d = 0.46), Letter Number Sequencing (d = 0.45) and Digit-Symbol Substitution tasks (d = 0.57) also sig- nificantly improved. 1. Introduction Adolescence is the developmental period when sleep timing tends to become later (Crowley, Acebo, & Carskadon, 2007; Gradisar, Gardner, & Dohnt, 2011; Roenneberg et al., 2004). Adolescents are uniquely at risk of delayed sleep timing due to physiological (i.e., the delay and/or lengthening of the circadian rhythm, slower build-up of sleepiness) (Gradisar & Crowley, 2013; Micic et al., 2015, 2016) and psychosocial influences (i.e., increased number of school, work, extra-curricular, and recreational activities to complete before bed) (Carskadon, 2011). Additionally, adolescents typically strive for independence, with reduced parental influence over bedtimes contributing to later bed- and sleep-times, and impaired sleep quality and daytime functioning (Gangwisch et al., 2010; Short et al., 2011). It is unsurprising then, that some teenagers (∼1–16%) develop a more severe form of this sleep problem, Delayed Sleep-Wake Phase Disorder (American Academy of Sleep Medicine, 2014; Lovato, Gradisar, Short, Dohnt, & Micic, 2013; Saxvig, Pallesen, Wilhelmsen-Langeland, Molde, & Bjorvatn, 2012). Delayed Sleep-Wake Phase Disorder is a circadian rhythm disorder, whereby the https://doi.org/10.1016/j.adolescence.2018.03.002 Received 24 September 2017; Received in revised form 12 February 2018; Accepted 5 March 2018 * Corresponding author. Flinders University, School of Psychology, GPO Box 2100, Adelaide, S.A., 5001, Australia. E-mail address: michael.gradisar@flinders.edu.au (M. Gradisar). Journal of Adolescence 65 (2018) 72–84 0140-1971/ © 2018 The Foundation for Professionals in Services for Adolescents. Published by Elsevier Ltd. All rights reserved. T