This is a proof-of-concept pilot nested in the RCT titled, 'Zero-time Exercise on Inactive Adults With Insomnia Disorder'. It aims to evaluate the effects of a simple WhatsApp intervention on exercise adherence and on sleep and related outcomes.

This is the second phase of another study - 'Zero-time Exercise on Inactive Adults With Insomnia Disorder'.

In the first phase (baseline to week 8), 136 subjects will be randomly allocated to the ZTEx intervention or the sleep hygiene education (SHE) comparison group. They will receive two 2-hour ZTEx training or SHE lessons and practice ZTEx or follow the sleep hygiene instruction for 24 weeks.

The second phase of the study (weeks 9-24)will commence after completion of the assessment of the RCT at week 8. Subjects in the Zero-time Exercise (ZTEx) group will be randomly divided into either an ZTEx with WhatsApp reminder (ZTEx+R) group or an ZTEx without WhatsApp reminder (ZTEx alone) group to evaluate the effects of WhatsApp reminders on exercise adherence and sleep and related outcomes.

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)