Monday, November 7, 2016

Life in Circadia

[ed. Another good reason we should get rid of daylight savings time.]

To be fully integrated with an ecosystem, an organism must cling to its niches, and one of those is a carefully carved-out temporal niche. For example, the first mammals’ warm-bloodedness allowed for successful colonisation of the nighttime world, when reptilian systems slowed down. Two similar species can comfortably occupy the same space if they do so at different times of day. Our modern built environment provides food, warmth and light at all hours, as well as safeguarding us from nocturnal predators. Superficially, we’ve been liberated from our own niche in the day/night environment but, under the surface, that desynchrony is causing all manner of problems. We have not outgrown our need for an internal timing system – far from it.

With so much of the talk about bodyclocks focused on sleep, it’s easy to forget that all of our biological processes around the clock are organised by circadian rhythms. Every day of the internal schedule is full of appointments. Nitrogen-fixing bacteria glean oxygen from the atmosphere, and they also photosynthesise to store energy. But they can’t do both at once, so they alternate between nocturnal nitrogen-fixing and daytime photosynthesis. Mammals have many such processes to orchestrate, and just about everything our body does – from metabolism and DNA repair to immune responses and cognition – is under circadian control. In humans, normal organ functioning depends on a harmony in hierarchy: synchrony among molecular rhythms within each cell, among cells in each organ, and among organs in the body. Coordinated functioning ensures that the body doesn’t work against itself.

The human body is teeming with clocks, arranged in a hierarchy. At the helm, a master clock in the brain’s hypothalamus called the suprachiasmatic nucleus (SCN) sets the overall rhythm of the body. But each organ also has its own rhythm that’s generated internally. A clock, in the broadest sense, consists of any type of regular oscillation, and these clocks take the form of a transcription-translation feedback loop that circles back to the beginning in roughly 24 hours. Clock genes activate a process that results in protein synthesis, and once the concentration of those proteins in the cell reach a critical threshold, they come into the nucleus and turn off the clock gene that produced them. Once the proteins have broken down, the gene switches on and the cycle begins again.

Every day, the body corrects its clock to match its surroundings using daylight. A photoreceptor in the retina – the third photoreceptor after our black-and-white vision rods and colour vision cones – senses only overall light levels and reports directly to that master clock to reset it when it drifts off-course.

Those other clocks, some generated within the cell and others governing the workings of organs, have basically the same molecular organisation as the SCN but they are autonomous from it. They differ enormously in the extent to which their rhythms are coupled to the central clock and they can be influenced by other factors. For example, liver and pancreas clocks are easily reset by eating late at night, which overrides the SCN signals in those organs and puts them out of sync with the rest of the body. Jetlag’s groggy unpleasantness comes more from this uncoupling of clocks than from an earlier or later internal time, per se. It takes about a day per hour of time-change to reset the master clock, but it can take even longer to corral the organs into line with each other. The effects of circadian dysfunction can be disastrous in the long term – knock out the cellular clocks in just part of a mouse pancreas, for example, and diabetes quickly ensues.

by Jessa Gamble, Aeon |  Read more:
Image: Richard Wilkinson