Meijer, J. H. | Search bioRxiv

bioRxiv2021

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 LED devices allowed to directly measure the light response of the SCN. In contrast to the general expectation, we found that SCN activity was suppressed by light. The suppressions were observed not only in response to narrowband blue light ({lambda}max: 470nm) but remarkably, also in response to green ({lambda}max: 515nm) and orange ({lambda}max: 590nm), but not to violet light ({lambda}max: 405nm). 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.

neuroscience↗

bioRxiv2019

Ultradian regulation of rest and activity bouts in mice

The suprachiasmatic nucleus (SCN), which serves as the central pacemaker in mammals, regulates the 24-hour rhythm in behavioral activity. However, it is currently unclear whether and how bouts of activity and rest are regulated within the 24-hour cycle (i.e., over ultradian time scales). Therefore, we used passive infrared sensors to measure behavior in mice housed under either a light-dark (LD) cycle or continuous darkness (DD). We found that a probabilistic Markov model captures the ultradian changes in the behavioral state over a 24-hour cycle. In this model, the animals behavioral state in the next time interval is determined solely by the animals current behavioral state and by the "toss" of a proverbial "biased coin". We found that the bias of this "coin" is regulated by light input and by the phase of the clock. Moreover, the bias of this "coin" for an animal is related to the average length of rest and activity bouts in that animal. In LD conditions, the average length of rest bouts was greater during the day compared to during the night, whereas the average length of activity bouts was greater during the night compared to during the day. Importantly, we also found that day-night changes in the rest bout lengths were significantly greater than day-night changes in the activity bout lengths. Finally, in DD conditions, the activity and rest bouts also differed between subjective night and subjective day, albeit to a lesser extent compared to LD conditions. The persistent differences in bout length over the circadian cycle following loss of the external LD cycle indicate that the central pacemaker plays a role in regulating rest and activity bouts on an ultradian time scale.

physiology↗