Have you noticed how time seems to be passing more quickly? How many times have we scheduled various activities for a day and at the end of it, we hope, at any cost, that it will be extended, that it will have 6 more hours....and in the midst of your rush, someone says, “In the past, time passed much more slowly...”.
The exchange of day for night and the changes of the seasons are caused by the movements of the Earth around its own axis (rotation) and around the Sun (translation) respectively. This variation of light and dark is responsible, from a physical point of view, for the alternation of time. Time that is increasingly being filled with activities and utilized by humans at every moment.
However, it is not just time that passes, the night that falls, and the day that dawns. All living beings accompany this dynamic process and also exhibit seasonal behavioral patterns. The dynamics of biological processes can be exemplified by hibernation, mating, sexual maturation, aging, as well as flowering and egg-laying at certain times of the year. But how is there synchronization between environmental rhythm and biological rhythm? How do all the cells that form a multicellular organism manage to act in an integrated and synchronized manner?
The synchronization of the activities of living beings with temporal variations is governed by an internal control, a central biological clock, which has humoral and neural sensors that inform the entire organism about the state of environmental light. This time marker is involved with genes that regulate cyclic physiological events, and therefore, is related to the passage of hours in a given day. In addition to this central clock, peripheral secondary clocks are present in almost all cells; they seem to be self-sustaining, but certainly depend on the oscillation coordinated by the central clock.
Have you ever stopped to think, physiologically, why do we sleep, preferably at night and not during the day? Why do we feel hungry at the same time? Why are certain medications prescribed to be taken in the morning and not before bed? The correct answer is: because of chronobiology.
Chronobiology is the study of the interaction of these periodic phenomena with environmental rhythm; and the main synchronizer for many living beings is light.
The cellular and molecular pathways that are initially activated by light for the synchronization of biological rhythm are different from those responsible for color vision. Light excites photoreceptors located in ganglion cells of the retina, and the light information is transmitted to the cells that will have their “clock genes” modulated. It is fascinating to think that only 40,000 cells, clustered in a space of 0.1mm3, known as the suprachiasmatic nuclei, are the central pacemakers for another 70 trillion cells!
The functioning of this circadian control system is basically regulated by transcription factors and proteins. They are expressed approximately every 12 hours and are capable of inhibiting their own synthesis while simultaneously stimulating the next molecule, in an alternation of phases, constituting a positive and negative feedback loop. An example of the cycle's functioning in our body is given by melatonin, a hormone secreted by the pineal gland in the absence of light, affecting sleep and innate immune response. A little before waking, a hormone called cortisol, which prepares the body for activity, is then released.
Chronobiology is very interesting, yet complex. With each reading, more questions arise than answers. Molecularly, how is all this controlled? How does regulation occur in the hypothalamus? And how is the cyclical oscillation transmitted to the organism?
And remembering that there are also active peripheral biological clocks in various cell types, what would be their role in skin cells? Do physiological responses among different cell types get regulated in the same way?
Stay tuned as these and other questions will be discussed in the next post!
Consulted and suggested Bibliographic Reference:
Regina Pekelmann Markus, Eduardo José Mortani Barbosa Junior, Zulma Silva Ferreira. Biological rhythms: understanding hours, days, and seasons. Einstein, 2003, 1:143
Stehle JH, Saade A, Rawashdeh O, Ackermann K, Jilg A, Sebestény T, Maronde E. A survey of molecular details in the human pineal gland in the light of phylogeny, structure, function and chronobiological diseases. J Pineal Res. 2011 Aug;51(1):17-43
