This study will examine the effect of Heart Rate Variability (HRV) biofeedback as adjunctive therapy to self-help Cognitive Behavioral Therapy on insomnia (CBT-I) in Chinese adult population. Heart Rate Variability Biofeedback is a therapy training aiming at increasing heart rate oscillations through real-time feedback and slow breathing training. Several literature has found that HRV biofeedback training is effective in improving HRV and improving sleep quality and sleep efficiency (Gevirtz, 2013; Lin et al., 2019). On the other hand, self-help CBT-I is well-studied in efficacy of improving cognition and behaviors and relieving insomnia (Ho et al., 2014). Rare research study HRV biofeedback as adjunctive therapy to self-help CBT-I (Lehrer, 2017). In the light of this, this study will compare the efficacy of combination of both treatments with that of CBT-I alone, examining whether HRV biofeedback's focus on psychophysiological domain can help self-help CBT-I in more significantly improving HRV and sleep quality.

This study will be a pilot randomized controlled trial on the efficacy of HRV biofeedback as adjunctive therapy to self-help CBT-I on insomnia. Potential participants will be recruited from the community through posters, social media sites and/or emails. Prior to all study procedures, the research purpose and data collection procedure will be given to the participants and an informed consent will be obtained from potential participants. Around 44 eligible participants will be randomly assigned to either self-help CBT-I with HRV biofeedback (combined group) or self-help CBT-I only group in a ratio of 1:1. The randomization will be performed by an independent assessor using a computer-generated list of numbers. No deception is necessary. Participants in combined group will receive a booklet with content related to both self-help CBT-I and HRV biofeedback based on guidelines of self-help CBT-I and protocol for HRV biofeedback training per week for 6 consecutive weeks. Participants are required to use around 1 hour to read through the content. The combined group is requested to perform the HRV biofeedback practice with a Bluetooth sensor connected with Inner Balance mobile application for around 20 minutes a day, 5-7 days a week and record their practice. Self-help CBT-I only group will receive a booklet with content related to self-help CBT-I only per week for 6 consecutive weeks. Participants are required to use 30 to 40 minutes to read through the content. The combined group and self-help CBT-I only group will complete a set of questionnaires and HRV measurement before the treatment commences, immediately after treatment, and 4 weeks after the treatment sessions are completed.

Inclusion Criteria:

1. Hong Kong residents aged 18-59 years
2. Cantonese language fluency
3. A global Insomnia Severity Index (ISI) score of 10 or higher [A cutoff score of 10 was optimal (86.1% sensitivity and 87.7% specificity) for detecting insomnia cases (Morin, Belleville, Bélanger & Ivers, 2011)]
4. A willingness to provide informed consent and comply with the trial protocol

Exclusion Criteria:

1. have major psychiatric, medical or neurocognitive disorders that make participation infeasible or interfere with the adherence to intervention
2. previous suicide attempt, severe active suicidal ideation with a specific plan, severe self-harm, active substance abuse, or a history of psychosis.
3. having cardiovascular diseases that affect the measure of heart rate variability
4. having severe lung infections, central respiratory failure, electrolyte imbalance, fever and other diseases affecting the heart activity
5. having with other organic diseases, previous history of arrhythmias (atrial fibrillation and frequent premature beats), hyperthyroidism, history of syncope and autonomic nervous system dysfunction
6. have insomnia due to specific medical conditions, side effects of medication intake or other sleep disorders
7. change in medication or its dosage 2 weeks before the baseline measurement