Physical Functions and Lifestyle Behaviours in Post-COVID-19 Syndrome Based on Circadian Rhythms

Post-COVID-19 syndrome is characterized by persistent symptoms such as fatigue, reduced exercise tolerance, and impaired respiratory function following acute SARS-CoV-2 infection. Emerging evidence suggests that circadian rhythm disruption may play a role in the pathophysiology of these symptoms by affecting metabolic regulation, hormonal balance, sleep quality, and physical performance. Circadian rhythm varies between individuals and is commonly classified into chronotypes as morning, intermediate, and evening types. These chronotypes are associated with differences in daily activity patterns, nutritional behaviors, and physiological performance. The aim of this observational cross-sectional study is to compare respiratory muscle strength, physical activity levels, nutritional habits, sleep quality, fatigue perception, and gastrointestinal symptoms in individuals with post-COVID-19 syndrome according to their chronotype. Participants aged 18-65 years with a confirmed diagnosis of post-COVID-19 syndrome will be included. Chronotype will be assessed using the Munich Chronotype Questionnaire (MCTQ). Respiratory muscle strength will be evaluated using maximal inspiratory and expiratory pressures (MIP/MEP), and dynamic respiratory parameters will be assessed using a POWERbreathe device. Respiratory muscle endurance will be measured using threshold loading protocols. Physical activity levels will be assessed using the International Physical Activity Questionnaire (IPAQ), while nutritional intake will be evaluated using 24-hour dietary recall. Sleep quality, fatigue severity, and gastrointestinal symptoms will be assessed using validated questionnaires. In addition, blood samples will be collected for genetic analysis of the CLOCK gene polymorphism (rs1801260) using PCR-RFLP techniques to provide an objective assessment of circadian rhythm differences. This study is expected to provide comprehensive insight into the relationship between circadian rhythm and functional, behavioral, and physiological outcomes in post-COVID-19 syndrome, and may contribute to the development of individualized rehabilitation strategies.