Aerobic exercise intervention for depression was conventionally recommended three times weekly with moderate intensity in previous studies, but little is known about the training effect of aerobic exercise at low frequency and vigorous intensity. The purpose of this study is to compare the training effect of aerobic exercise at different exercising frequencies and intensities on older adults with comorbid insomnia and depressive symptoms. In this study, the investigators will investigate two types of aerobic exercise (i.e., vigorous-intensity exercise and moderate-intensity exercise) under different exercising frequencies (i.e., regular exercising pattern and weekend warrior). Individuals with chronic insomnia and depressive symptoms will be recruited and randomly allocated into 5 groups: 1) attention control group (stretching exercise), 2) moderate intensity exercise performed thrice weekly (MIE×3/wk), 3) moderate intensity exercise performed once weekly (MIE×1/wk), 4) vigorous intensity exercise performed thrice weekly (VIE×3/wk), and 5) vigorous intensity exercise performed once weekly (VIE×1/wk). Intervention will be maintained for 12 weeks. Outcome assessments will be conducted at baseline, 6 weeks and 12 weeks after the intervention. Outcomes including depressive status, chronic insomnia, objective sleep quality and pattern (measured by Actigraphy), subjective sleep quality (measured by Epworth sleepiness scale, insomnia severity index, PSQI), anxiety status (measured by HADS and GAD-7), quality of life (measured by SF-12), attention level (measured by computer attention test), exercise enjoyment (measured by physical activity enjoyment scale), habitual physical activity level (measured by IPAQ), aerobic fitness (measured by metabolic cart), body adiposity (measured by DXA), blood chemistry, adherence, medication usage, and adverse events will be measure in this study. This proposed study will provide pilot evidence for the benefits, effectiveness, safety, adherence, and sustainability of low-frequency vigorous aerobic exercise. the investigators expect the low-frequency exercise modality will enhance the practical suitability of aerobic exercise and will provide evidence for weekend warrior aerobic training strategy as a new exercise option in the management of elderly insomnia and depression.

Inclusion Criteria:

1. borderline abnormal depression with score above 8 out of 21 on Hospital Anxiety and Depression Scale (HADS),
2. chronic insomnia according to the fifth edition of the diagnostic and statistical manual of mental disorder (DSM-5),
3. Cantonese, Mandarin or English speaking, and 4) age 50 or above. To enhance generalizability, males and females will be recruited.

Exclusion Criteria:

1. contraindications to participating in physical exercise;
2. regular exercise habit in the past three months (i.e.,>75 minutes of vigorous-intensity exercise weekly or >150 minutes of moderate-intensity exercise weekly);
3. any pre-existing medical or physical issues that affect the experimental test and exercise intervention;
4. diagnosis with psychosis, schizophrenia or bipolar disorder;
5. currently receiving non-medication treatment for depression or insomnia, such as mindfulness training or cognitive behavior therapy;
6. shift worker or other commitment that interferes with the regular sleep pattern at night; and
7. abnormal heart's electrical activity according to the exercise electrocardiography (ECG) test, such as irregular heartbeat, arrhythmia, and myocardial infarction.

Single-blind (Outcome Assessor)