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Abstract:
Circadian rhythms govern many biological processes and sleep regulates brain homeostasis including plasticity. Diffusion-weighted MRI is sensitive to changes in tissue microstructure and reflects learning-dependent plasticity. Here, we aimed to assess the roles of circadian rhythmicity and sleep for brain diffusivity, particularly in relation to learning.
DW-MRI was conducted at three time points (t0=0h, t1=3h and t2=13h). The wake group (n=18) was scanned twice in the morning and the following evening. The sleep group (n=17) was scanned twice in the evening, slept with a mobile EEG and returned the next morning. Participants completed a learning condition, including an object-location learning task with fMRI between t0-t1 and a control condition without learning.
Both groups showed similar circadian effects on mean diffusivity (MD) in both conditions. Total brain tissue MD was lower in the evening than in the morning accompanied by a negative correlation between MD and time spent awake. Over time, the wake group showed a linear MD decrease the sleep group a quadratic trend with t0>t1<t2. Voxels with a higher MD decrease before sleep also showed a higher increase over sleep. This relationship was stronger in the control condition. Voxels with higher task-related activation during learning showed a higher sleep-related increase than in the control.
Our analyses confirm an influence of circadian rhythm on brain diffusivity and emphasize the need for proper controls. Furthermore, our data support the idea of sleep as a homeostatic process effecting synaptic downscaling and indicate that sleep-regulated effects on brain microstructure differ after learning.
