Effects of Messaging App Reminder on the Compliance of Zero-Time Exercise (ZTEx)

Inclusion Criteria: 1. Hong Kong residents, 2. ethnic Chinese aged 18-65 years with the ability to communicate in Cantonese or Putonghua, 3. who fulfil the DSM-5 diagnostic criteria for insomnia disorder according to a validated diagnostic tool, the Brief Insomnia Questionnaire (we include primary insomnia because it is more homogenous and less complicated than comorbid insomnia; if ZTEx is found to be effective for primary insomnia, then comorbid insomnia can be targeted in future studies), 4. scored at least 10 points in the Insomnia Severity Index, 5. are ambulant and can independently perform daily activities, 6. are physically inactive, defined as not having participated in at least 150 min of moderate intensity or 75 min of vigorous intensity physical activity a week, or the equivalent combination over the previous 3 months by self-report 7. have a smartphone compatible with WhatsApp, and 8. willing to give informed consent Exclusion Criteria: 1. any current diagnosis fulfilling the DSM-IV criteria of a major depressive disorder, generalized anxiety disorders, post-traumatic stress disorder, panic disorder, substance use disorders, or schizophrenia as screened using the Chinese version of the Structured Clinical Interview for DSM-IV; 2. other sleep disorders, including circadian rhythm disorder, narcolepsy, obstructive sleep apnoea (OSA), and restless leg syndrome (RLS)/periodic leg movement disorder (PLMD) as determined based on cut-off scores (≥ 7 on narcolepsy; ≥ 8 on circadian rhythm disorder; ≥ 15 on OSA; ≥ 7 on RLS/PLMD) using SLEEP-50. If in doubt, subjects will be referred to our team psychiatrist (KF Chung) for further assessment; 3. any current medical conditions or side-effects of medication that are contributing significantly to the subject's insomnia; 4. taking medications or currently participating in other psychotherapies for insomnia or other psychiatric disorders; 5. impaired cognitive functioning (\<22 points in HK-MoCA) or with difficulty in giving consent or understanding instructions; 6. pregnant; 7. shift work; 8. whose physical condition is such that physicians recommend that they refrain from exercising; and 9. fulfilling the criterion for obesity for Asians (body mass index, of 27.5 or above)