We are studying the effect of a mind-body treatment for people with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) with or without Long COVID (LC). Our team recognizes that ME/CFS and Long COVID are serious, debilitating, biomedical conditions. ME/CFS can affect many different parts of the body, such as the nervous system, immune system, and metabolism. It is hypothesized that central sensitization (i.e., when nerves get too excited) may influence these complex conditions and may make symptoms worse. This study explores a mind-body program called the Dynamic Neural Retraining System™, or DNRS™. Mind-body interventions (MBIs) focus on how the brain, mind, body, and behaviour interact to improve health and well-being. These techniques help people become more aware of themselves, take better care of their health, and boost mood, quality of life, and coping skills. MBIs use the brain's ability to change (neuroplasticity) by reinforcing certain thoughts, feelings, or behaviours that support changes in biology and function. MBIs may influence physical health by affecting how the brain and body communicate through chemicals such as hormones and neurotransmitters. Objective fMRI evidence shows that practising MBIs can change brain structure and function. This makes MBIs a potential good fit for people with chronic illnesses like ME/CFS and LC, which involve complex interactions between the brain, immune system, and hormones. There is no rigorous peer-reviewed evidence that DNRS is effective. Our study will address this question using subjective and objective measurements in a multiple-methods wait-list randomized controlled trial. The objectives of this study are to: 1) examine the effectiveness of the DNRS program for individuals diagnosed with ME/CFS with and without Long COVID, compared to a treatment-as-usual wait-list control group on a range of patient-reported outcomes including health-related quality of life, fatigue, pain, anxiety and depression and objective measures including daily steps, heart rate variability, and sleep; 2) quantify metabolic changes through untargeted serum metabolomics profiling, 3) determine if metabolomics screening can predict treatment responsiveness to DNRS; and 4) identify the microbial signatures of large bowel microbiota in Long COVID patients pre and post MBI. A descriptive qualitative study will also be conducted in a sample of participants to learn more about their experiences during the trial.