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... Show moreIn 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. Show less
Ambient light detection is important for the synchronization of the circadian clock to the external solar cycle. Light signals are sent to the suprachiasmatic nuclei (SCN), the site of the major... Show moreAmbient light detection is important for the synchronization of the circadian clock to the external solar cycle. Light signals are sent to the suprachiasmatic nuclei (SCN), the site of the major circadian pacemaker. It has been assumed that cone photoreceptors con-tribute minimally to synchronization. Here, however, we find that cone photoreceptors are sufficient for mediating entrainment and transmitting photic information to the SCN, as evaluated in mice that have only cones as functional photoreceptors. Using in vivo electrophysiological recordings in the SCN of freely moving cone-only mice, we observed light responses in SCN neuronal activity in response to 60-s pulses of both ultraviolet (UV) (lambda(max) 365 nm) and green (lambda(max) 505 nm) light. Higher irradiances of UV light led to irradiance-dependent enhancements in SCN neuronal activity, whereas higher irradiances of green light led to a reduction in the sustained response with only the transient response remain-ing. Responses in SCN neuronal activity decayed with a half-max time of similar to 9 min for UV light and less than a minute for green light, indicating differential input between short-wavelength-sensitive and mid-wavelength-sensitive cones for the SCN responsiveness. Furthermore, we show that UV light is more effective for photo-entrainment than green light. Based on the lack of a full sustained response in cone-only mice, we confirmed that rapidly alternating light levels, rather than slowly alternating light, caused substantial phase shifts. Together, our data provide strong evidence that cone types contribute to photoentrainment and differentially affect the electrical activity levels of the SCN. Show less
