The Journal of neuroscience : the official journal of the Society for NeuroscienceJan 18, 2024

Daily Changes in Activity of the Brain’s Internal Clock Center

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗OA ↗

Updated Mar 28, 2026

Abstract

Higher activity in the human suprachiasmatic nucleus (SCN) was observed at noon compared to the early morning.

Daily rhythms in neuronal activity have been identified in the SCN of humans.
In the first experiment with 27 participants, SCN activity was scanned every 6 hours.
In the second experiment with 20 participants, SCN activity was measured every 30 minutes from midnight to dawn.
The SCN activity exhibited a gradual decrease during the night and was not linked to brain wave activity.
The observed patterns in human SCN activity align with those seen in nocturnal and diurnal mammals, indicating light modulation rather than reliance on local time.

Simplified

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 ( = 27, including both sexes), we scanned each participant four times a day, every 6 h. Higher activity was observed at noon, while lower activity was recorded in the early morning. In the second experiment ( = 20, including both sexes), the SCN activity was measured every 30 min for 6 h 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. n n

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Daily Changes in Activity of the Brain’s Internal Clock Center

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