Inclusion Criteria:  (a) Hong Kong resident, (b) Aged between 18-60, (c) Diagnosed with ADHD, (d) Able to read and write Chinese, (e) Has regular access to a smart phone and internet, (f) Insomnia severity index ⩾10  Exclusion Criteria:  (a) Having comorbid bipolar disorder, psychosis, suicidality, moderately severe to severe depression, narcolepsy, and/or hypersomnolence disorder, (b) Having medical conditions that might affect sleep (e.g., serious physical concerns necessitating surgery, risk of falls, epilepsy), (c) Having night shift schedules at work, (d) Currently receiving psychological intervention for insomnia