Summary Adolescent sleep health is a global health concern. Sleep difficulties have detrimental consequences for adolescent mental health; however, prior research has typically focussed on a single dimension of sleep, used self‐reported data, and relied on cross‐sectional designs. Using 14 days of self‐reported daily diaries and actigraphy‐measured sleep data, this study addressed these gaps by examining the daily associations between sleep difficulties (e.g. longer sleep onset latency, greater sleep disturbance) and adolescent internalising symptoms, with sleep regularity as a moderator of these associations. The sample included 350 adolescents (69.1% females; M age = 14.27, SD = 0.61; 21.32% African American, 40.3% Asian American, 34.9% Latinx, and 2.5% other ethnic/racial minorities) in the northeastern United States. Adolescents provided daily actigraphy (i.e. sleep duration and wakefulness after sleep onset) and self‐reported (i.e. sleep onset latency and sleep disturbance) sleep data. Adolescent internalising symptoms (i.e. anxious mood and somatic symptoms) were also assessed via daily diary reports. The Sleep Regularity Index was a measure of sleep variability using a metric reflecting minute‐by‐minute assessment of consistency in sleep/wake patterns, averaged across 14‐day wrist actigraphy data. Multilevel modelling analysis accounted for the nested structure of the diary data. Nightly sleep difficulties were linked to more internalising symptoms (i.e. more anxious mood and somatic symptoms) the following day. Sleep regularity moderated the associations between sleep onset latency and sleep duration with adolescent anxious mood, and protected against the negative impact of sleep disturbance on adolescent somatic symptoms. Findings highlight the important role of sleep regularity in the daily links between sleep and internalising symptoms.