Anna Mallach explores the phenomenon of body clocks and circadian rhythms, and asks why they arose in the first place
Whilst Nobel Prizes are intended to recognise the most important, life-changing, and exciting findings in the sciences, they have an interesting habit of raising awareness of a particular field of research, and bringing recognition to certain theories and ideas. This year, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbach, and Michael W. Young, researchers studying the circadian cycle who have now solved the mystery of how our cells keep track of time.
The finding itself is quite simple: within a cell, a protein increases in concentration until it starts to inhibit its own production. At this point, its concentration is reduced until it falls below a certain threshold. The amount of inhibition is then reduced, meaning that the concentration of the protein starts to increase again. This process is known as negative feedback, and is an important part of many natural processes.
Of course, arriving at this conclusion is a lot more complicated: how do you model day and night cycles in a cell kept alive in a tissue culture incubator? After the prize had been announced, the inevitable debate started: the implication this has for our lifestyle, the bad influence of jetlag on our health, and so forth. But what might be more interesting to consider, is why did such a system develop in the first place? Why does a cell, sitting deep within a specific organ never seeing daylight, care about what is essentially the dynamics of our solar system?
The first answer to this may be obvious: animals display different behaviours at different points during the day. For most animals, the day is spent foraging for food, nest-building, and mating, whilst during the night they undergo a restorative phase involving sleep. Whilst researchers don’t quite understand the exact role sleep plays, this much is clear: we need it so much that we cannot survive without dedicating over a quarter of our lives to it. Many cells have an internal circadian cycle, but the brain synchronises them to one “standard time”. With the help of the retina at the back of the eye, one brain region can keep track of the level of light and adjust its own internal clock to the day/night rhythm outside. This information is conveyed via melatonin, which increases in concentration slowly during the day, making you feel more and more drowsy.
What happens when the internal clocks are not regulated in that manner can be seen in babies, who are so dependent on regular food intake that prolonged sleep may prove dangerous to them. As a result, they don’t produce melatonin and therefore have yet to develop a synchronised day/night rhythm. This means they don’t display the same sleep cycle their parents do.
Displaying temporally-similar behaviours in a group of animals, who are interacting, is important in order to regulate the group’s actions. It is useful if everybody wakes up at a similar time so that activities, such as migrating, hunting or mating can be done simultaneously. Tying into this, another reason to have a circadian cycle is because others have it. As a predatory animal, knowing when your prey will sleep allows for a more successful hunt.
Finally, the master regulator within the brain, which can reset circadian cycles throughout the body, can not only differentiate between day and night, but also has the ability to keep track of seasons. By noticing the shortening or lengthening of the days, the brain can, with extraordinary precision in some species, tell the time of year without the use of a calendar. Much of an animal’s life depends on its external circumstances and, therefore, being able to predict the changing of the seasons, and associated weather, is distinctly advantageous. For example, the shortening of daytime could tell migratory birds that it is once again time to pack up and head south. On the other hand, the lengthening of daytime could signal to mice that spring is near and breeding should commence to ensure the young are raised when food is plentiful.
Overall, being able to keep track of time has been of incredible importance throughout our history. Despite us now being able to keep time using clocks and calendars, regulation of our organs and behaviour still relies on our internal time keepers. Although the Nobel Prize winners this year revealed in greater detail the intricacies of circadian cycles, there is still a lot we can learn from our own cells.
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