There is an increasing awareness of time-of-day (TOD) effects in MRI. The underlying causes of these fluctuations may be related to diurnal physiological variations, reflecting the processes that govern transition between sleep and wake. We probed TOD effects in a prospective manner, scanning 47 healthy individuals in the morning and the same evening using a five-shell diffusion-weighted imaging protocol at 3T, where a number of important Zeitgeber signals were rigorously controlled. We found significant changes in an number of derived DWI parameters (FA, MD, AWF, MK), and speculate that these changes are pointing towards underlying mechanisms of sleep-wake homeostasis.
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