Having a fixed schedule is great for a lot of things. It allows us to have an established and recurrent pattern of work and services. If public transportation follows a fixed schedule and route you will easily know how to use it. This simple system will also help you and others commute to work. Arriving on time will prevent delays that could impact other people's jobs and businesses. A fixed schedule system will allow for things to work and function without delay. For this system to function it requires at least two things, the ability to measure time and the ability to adapt to changes. The ability to measure time will allow us to follow a fixed schedule. Adaptation will ensure that a given task is done, even if there are some delays or troubles along the way.
Time measurement and adaptation can also be found in biological systems. Time measurement is done primarily by different receptors found inside the eye. This information is then passed to the central nervous system to synchronize the circadian rhythm. This allows the organism to adapt to current environmental conditions. Although the model of time measurement and adaptation allows our physiology to respond to environmental changes it might have a big problem.
Let's look at the environmental variable used to track time: light/solar radiation. This variable changes daily creating the the day-night cycle. Daily changes in light/solar radiation are gradual and follow a sinusoidal curve. Those changes also create a series of visual phenomena at specific periods of time. Being the most noticeable ones the sunrise, the sunset, and mid-day. At those particular periods of time, the change in light/solar radiation is near zero. At those particular periods of time, the time measurement will be particularly challenging as the measurement cannot discriminate between three similar periods of time.
Although the organism likely relies on other environmental cues to synchronize the circadian rhythm. But if this problem is real it might become noticeable at different time scales. A characteristic of light/solar radiation is its variability, a consequence of both solar phenomena and planetary movement.
The day/night cycle is a consequence of the earth's rotation movement. And the revolution movement changes the day duration. Each day will have the same 24 hrs duration but the amount of sunshine will change both latitude-wise and time-wise. Shortening and lengthening of days will result in periods of time both yearly and daily where the time measurements could be indistinguishable from one another.
Furthermore, the sun has also periods of high and low activity. Those changes in activity generate a sinusoidal wave with many minima and maxima. The most studied cycles of solar activity are the Schwabe cycle and the Gleissberg cycle. Those periods of high and low activity could also increase the odds of encountering periods of time where the synchronization of the circadian rhythm couldn’t be made or at least delayed due to environmental reasons. This lack of synchronization could lead to a series of metabolic, physiological, or immunological problems. A delayed immune response could ease the propagation of pathogens across different organisms.
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