Background
Sleep problems show associations with negative outcomes in both physical and mental health in adolescents, but the associations may be reciprocal. We aimed to assess bidirectional associations between sleep problems and mental health symptoms including behavioural difficulties (internalising and externalising difficulties) and low health-related quality of life (HRQoL).

Methods
A total of 6616 adolescents (52.4% females) across Greater London completed baseline assessments when they were aged 11–12 years, and 3803 of them (57.2% females) completed follow-up assessments at aged 13–15 years. Weekday and weekend sleep duration were derived from self-reported bedtime, sleep onset latency and wake time. Sleep disturbance was assessed using a standardized sleep disturbance scale. Internalising and externalising difficulties were assessed using subscales of the Strength and Difficulties Questionnaire. HRQoL was assessed using the KIDSCREEN-10 questionnaire. Cross-lagged structural equation modelling was used with multiple imputation to examine bidirectional associations between sleep problems and mental health symptoms.

Results
Females had greater internalising difficulties, worse HRQoL and more sleep disturbance than males. Persistent insufficient weekday and weekend sleep, and sleep disturbance (i.e., at both baseline and follow-up) were associated with internalising and externalising difficulties and low HRQoL at follow-up (ORs ranged from 1.53 to 3.63). Persistent externalising difficulties and low HRQoL were also associated with insufficient weekend sleep and sleep disturbance at follow-up (ORs ranged from 1.68 to 4.25). Using continuous variables, we found bidirectional associations between weekday sleep duration and HRQoL, weekend sleep duration and externalising score, sleep quality and internalising score, and sleep quality and HRQoL. The association magnitudes were mostly similar in the two directions.

Conclusions
Our study showed bidirectional associations between sleep problems and mental health symptoms during adolescence, indicating that early intervention and treatment on the first-occurring symptom may prevent the development of subsequent problems.