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, we localized the SCN by employing an areal boundary mapping technique to resting-state functional images and investigated the SCN activity using perfusion imaging. In the first experiment (n = 27, including both sexes), we scanned each participant four times a day, every six hours. Higher activity was observed at noon, while lower activity was recorded in the early morning. In the second experiment (n = 20, including both sexes), the SCN activity was measured every thirty minutes for six hours from midnight to dawn. The results showed that the SCN activity gradually decreased and was not associated with the electroencephalography. Furthermore, the SCN activity was compatible 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 nocturnal and diurnal mammals and is modulated by physical lights rather than the local time. Significance Statement The suprachiasmatic nucleus (SCN) in the hypothalamus is the central clock for circadian rhythms in mammals. However, the circadian activity of human SCN remained elusive due to the difficulty of measuring the activity of such a small nucleus. In this study, we localized the human SCN and investigated the SCN activity using an MRI technique for measuring perfusion. We observed SCN activity patterns of higher activity in daylight time and lower at night and morning. We also observed that the human SCN activity gradually decreased during the night, compatible with the rodent SCN activity after switching off the lights. These results suggest that the diurnal variation of human SCN activity is modulated by physical lights rather than the local time.