The goal of this interventional study is to test the heart-rate variability biofeedback intervention (HRV BI) in 2 cohorts of patients. Cohort A will evaluate the effect of the addition of HRV BI in patients with breast cancer treated in the neoadjuvant setting (vs. standard of care alone, SOC) followed by local therapy (surgery +/-radiotherapy). Cohort B will evaluate the effect of the addition of HRV BI in patients with colon cancer after surgery in the adjuvant setting (vs. standard of care alone, SOC).