Time is a funny thing, everyone experiences it, but no one has it. Everyone knows what it is but it's hard to define it and to explain what it is. Often is defined from the perspective of a reference point, for example, our current date system. Currently, it's the year 2024, but it's the year 2024 since an accepted reference point. This simple convention allows us to schedule our daily activities. In turn, this organization allowed the synchronization of our social and economic activities around specific dates and times of the day. It's not a perfect system but allows us to have a somewhat natural flow and common understanding.
Different and more accurate measures of time can also be possible. Time can be measured by small increments to specify specific periods on a given day. Or by large increments to measure and understand the development of society and to describe historic events.
Time measurement is not exclusive to human activity. Organisms also measure time as this capacity allows them to incorporate environmental cues to initialize physiological processes. One of the processes that rely the most on time is the circadian rhythm. This rhythm is defined as the set of physiological changes inside an organism during a period of 24 hours. There are different environmental changes across a 24-hour period, however, perhaps the most evident is the day/night cycle.
The synchronization of the circadian rhythm allows an organism to align its wake/sleep cycle to the natural sleep/wake cycle. Physiologically the synchronization of the circadian rhythm relies on a series of oscillators to track time, just like old pendulum clocks. These oscillators can be divided into two categories. The central oscillators are regulated by the central nervous system. And a series of local oscillators inside each cell and tissue that synchronize themselves to the central oscillator.
The central oscillator is synchronized to local time by sensing the luminous stimulus from the environment. Rods and cones inside the eye sense the amount of light or solar irradiation. This information is then passed to the central oscillator by the action of adenosine. Exposure to light/solar irradiation can alter the circadian rhythm by changing the local time that is being measured. Exposure to light/solar radiation in the morning advances the circadian rhythm whereas exposure in the afternoon delays the circadian rhythm.
Among other stimuli used to synchronize the circadian rhythm are the temperature, earth's magnetic field, and social activity. Although there are more components and mechanisms on which the circadian rhythm relies to synchronize itself it's safe to assume that the light/solar radiation stimulus is a major driver.
Why is this important?
The synchronization of the circadian rhythm allows a series of physiological processes to be initialized. Perhaps the easiest to identify is the sleep/wake cycle. But most of our physiology relies on the circadian rhythm. Secretion of hormones, digestion, respiration, the immune system, and more rely on the circadian rhythm. Understanding the influence of the environment on the circadian rhythm and our physiology will help us to understand how the environment could impact our function.
Furthermore, it can help us to understand possible behavioral changes driven by physiological problems and their possible effects on society. As the circadian rhythm heavily relies on solar activity changes in solar activity could have unexpected effects on our physiology. Although the effects could be small as almost every organism on earth is susceptible the effect could be quite noticeable.