Investigating the Effects of Intermittent Hypoxia-Hyperoxia Treatment (IHHT) in People With Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) to Improve Fatigue, Pain, and Quality of Life by Targeting Mitochondrial Dysfunction and Autonomic Nervous System Impairment

ME/CFS Patients Inclusion Criteria: * Female sex * Age 20-59 years * Established diagnosis of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) according to the International Consensus Criteria (ICC) * Disease duration \>6 months * Presence of post-exertional malaise (PEM) * Mild or moderate disease severity according to NICE guidelines * Body mass index (BMI) \>18 and \<30 kg/m² * Ability and willingness to attend study visits and complete the intervention protocol * Provision of written informed consent * Willingness to comply with all study procedures and restrictions throughout the study period * Use of highly effective contraception during study participation for women of childbearing potential Exclusion Criteria: * Severe ME/CFS according to NICE guidelines * Participation in another clinical trial that may interfere with the present study * Pregnancy, planned pregnancy, or breastfeeding * Presence of clinically significant psychiatric disease not considered part of ME/CFS and likely to interfere with study participation or interpretation of results * Presence of clinically significant neurological disease, peripheral nerve disorder, or other neurological dysfunction not considered part of ME/CFS * Active internal disease or clinically significant cardiovascular, pulmonary, metabolic, renal, hepatic, endocrine, or systemic disease that may interfere with study participation or interpretation of outcomes * Obstructive lung disease * Claustrophobia * History of spontaneous pneumothorax * Presence of implanted medical devices, prostheses, or other conditions considered incompatible with IHHT * Chronic pain conditions not considered part of ME/CFS and likely to interfere with assessments * Current structured graded exercise therapy (GET) or structured exercise programme * Significant alcohol, tobacco, or cannabis use considered likely to interfere with study participation or interpretation of outcomes * Inability to comply with study procedures or communicate meaningfully with study personnel * Any condition which, in the opinion of the investigator, may compromise participant safety or study integrity Healthy Controls Inclusion Criteria: * Female sex * Age 20-59 years * Body mass index (BMI) \>18 and \<30 kg/m² * Ability and willingness to attend study visits * Provision of written informed consent * Good general health without clinically significant chronic medical, neurological, psychiatric, or pain conditions * Activity level and BMI broadly matched to ME/CFS participants * Willingness to comply with all study procedures and restrictions throughout the study period * Use of highly effective contraception during study participation for women of childbearing potential Exclusion Criteria: * Current or previous diagnosis of ME/CFS * Participation in another clinical trial that may interfere with the present study * Pregnancy, planned pregnancy, or breastfeeding * Presence of clinically significant psychiatric, neurological, cardiovascular, pulmonary, metabolic, renal, hepatic, endocrine, or systemic disease * Chronic pain conditions, migraine disorder, or recurrent headache/back pain likely to interfere with study assessments * Obstructive lung disease * Claustrophobia * History of spontaneous pneumothorax * Presence of implanted medical devices, prostheses, or other conditions considered incompatible with study procedures * Current structured exercise programme or recent major change in exercise habits * Significant alcohol, tobacco, or cannabis use considered likely to interfere with study participation or interpretation of outcomes * Inability to comply with study procedures or communicate meaningfully with study personnel * Any condition which, in the opinion of the investigator, may compromise participant safety or study integrity