Diurnal variation of brain activity in the human suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the central clock for circadian rhythms. Animal studies have revealed daily rhythms in the neuronal activity in the SCN. However, the circadian activity of the human SCN has remained elusive. In this study, to reveal the diurnal variation of the SCN activity in humans, the SCN was localized, and its activity was investigated using perfusion imaging. We scanned each participant four times a day, every six hours, and higher activity was observed at noon while lower activity was recorded in the early morning. The SCN activity was then measured every thirty minutes for six hours from midnight to dawn and showed a decreasing trend and was comparable with the rodent SCN activity after switching off the lights. These results suggest that the diurnal variation of the human SCN follows the zeitgeber cycles of mammals and is modulated by physical lights rather than the local time., In this study, two experiments were conducted, wherein the first investigated the whole cycle of the diurnal activity of the SCN in humans. Participants were scanned four times within 24 hours (18:00, 24:00, 6:00, and 12:00 local time). In the second experiment, we investigated the human SCN activity in more detail during the night. Participants stayed in the scanner throughout the night, except for brief unavoidable interruptions, and were scanned every 30 min from 24:00 to 6:00. Perfusion images of pseudo-continuous arterial spin labeling (pCASL) in each participant were acquired. Twenty-seven and twenty right-handed participants without neurological/psychiatric illness or sleep disorders participated in the first and second experiments.
Whole brain perfusion images were acquired using pCASL imaging with multiband-EPI (number of measurements = 90, repetition time = 4.0 s, echo time = 25.2 ms, partial Fourier = 6/8, flip angle = 90°, labeling duration = 1.5 s, post labeling delay = 1.6..., , # Diurnal variation of brain activity in the human suprachiasmatic nucleus

**Oka et al. (2024) Journal of Neuroscience**

This study investigated the diurnal variation of brain activity in the suprachiasmatic nucleus (SCN) in humans. In the first experiment, the SCN activity was measured four times a day, every six hours, using perfusion imaging. In the second experiment, the SCN activity was measured every thirty minutes for six hours from midnight to dawn, from 24:00 to 6:00. The results showed that the diurnal variation of the human SCN activity follows the zeitgeber cycles of mammals.

Nifti files showing the regions of interest (ROIs) of the hypothalamus and hypothalamic nucleus and the boundary probability map in the hypothalamus are provided. CSV files showing the data of brain activity in the SCN, MPO, SO, and PVH, which were used to make the line and bar graphs, are provided.

## The ROIs of the hypothalamus and hypothalamic nuclei and the boundary probability map

The areal b...