Repeated MRI scans of the human brain: diurnal oscillations in healthy adults and bipolar disorder patients

Regulation of biological processes according to a 24-hr rhythm is essential for the normal functioning of an organism. Temporal variation in brain MRI data has often been attributed to circadian or diurnal oscillations; however, it is not clear if such oscillations exist. We provide the first evidence that diurnal oscillations indeed govern multiple MRI metrics. We recorded cerebral blood flow, diffusion-tensor metrics, T1 relaxation, and cortical structural features every three hours over a 24-hr period in each of 16 male controls and eight male bipolar disorder patients. Diurnal oscillations were detected in numerous MRI metrics at the whole-brain level, and regionally. Rhythmicity parameters in the bipolar disorder patients were similar to the controls for most metrics, except for a larger phase variation in cerebral blood flow. The ubiquitous nature of diurnal oscillations has broad implications for neuroimaging studies and furthers our understanding of the dynamic nature of the human brain. ### Competing Interest Statement The authors have declared no competing interest.