Adolescents frequently experience insufficient sleep, which can negatively impact physical and mental health. We sought to examine differences in symptoms of insufficient sleep and response to a sleep extension manipulation by chronotype in habitually short-sleeping adolescents. Twenty-six adolescents with insufficient sleep (≤ 7 h on school nights) were randomised to 1 week of typical sleep (TS; usual school schedule) and sleep extension (EXT; ≥ 1-h additional time in bed) in counterbalanced order. Questionnaires (Morningness/Eveningness Scale for Children, PROMIS Paediatric Anxiety and Depression Scales, Chronic Sleep Reduction Questionnaire) were completed at screening, and actigraphy-estimated sleep and dim-light salivary melatonin onset and offset (DLMOn/Off) were assessed during both conditions. A slight majority of participants identified as evening types (57.7%). Evening types had significantly greater depression and anxiety symptoms, more chronic sleep restriction, and more daytime sequelae of insufficient sleep (irritability, loss of energy, and sleepiness) compared to morning types. Both morning/intermediate and evening types significantly advanced sleep onset time and increased sleep duration from TS to EXT (all p < 0.001). However, only evening types significantly delayed sleep offset from TS to EXT (p = 0.049) while morning/intermediate types did not change the timing of sleep offset between conditions (p > 0.05). The phase angle between DLMOn and sleep onset narrowed significantly only for morning/intermediate types (p = 0.02), while the phase angle between DLMOn and sleep offset widened significantly only for evening types (p = 0.021). To improve health, individualised approaches to improving sleep health should be considered for habitually short-sleeping adolescents based on chronotype.
