The photobiology of the human circadian clock

In modern society, the widespread use of artificial light at night disrupts the suprachiasmatic nucleus (SCN), which serves as our central circadian clock. Existing models describe excitatory responses of the SCN to primarily blue light, but direct measures in humans are absent. The combination of state-of-the-art neuroimaging techniques and custom-made MRI compatible light-emitting diode devices allowed to directly measure the light response of the SCN. In contrast to the general expectation, we found that blood oxygen level-dependent (BOLD) functional MRI signals in the SCN were suppressed by light. The suppressions were observed not only in response to narrowband blue light (lambda max: 470 nm) but remarkably, also in response to green (lambda max: 515 nm) and orange (lambda max: 590 nm), but not to violet light (lambda max: 405 nm). The broadband sensitivity of the SCN implies that strategies on light exposure should be revised: enhancement of light levels during daytime is possible with wavelengths other than blue, while during nighttime, all colors are potentially disruptive.

Automated summary

In modern society, the widespread use of artificial light at night disrupts our central circadian clock, the suprachiasmatic nucleus (SCN), by suppressing blood oxygen level-dependent (BOLD) functional MRI signals in the SCN. Contrary to expectations, not only blue light, but also green and orange light, can cause this suppression, while violet light does not. This suggests that strategies for light exposure need revision, with the possibility of using wavelengths other than blue to enhance light levels during the daytime and potential disruption from all colors during the nighttime.