Light levels play a significant role in influencing the lives of animals and humans. Scientists have conducted experiments to investigate how animals respond when deprived of light. They were particularly interested in determining whether animals could perceive the changing seasons without exposure to light. These studies revealed that animals, and by extension humans, possess an internal biological clock capable of keeping approximate time even in the absence of external stimuli such as sunlight.
What's intriguing is that this biological clock is, at best, an approximation and not meant to keep precise time over extended periods. For instance, it senses the day as being longer than 24 hours, with the cycle ranging from 24 to 48 hours when light is absent. We rely on natural daylight to "fine-tune" or correct our body clock so that it synchronizes with the external environment. The consequence of prolonged desynchronization of our body clock with natural light is the emergence of health issues such as hormonal imbalances, sleep disorders, and mood disturbances.
Similar to how animals depend on signals from the sun to maintain the accuracy of their biological clocks and coordinate their activities, humans also require adequate daylight to synchronize their circadian and circannual rhythms. Some illnesses linked to hormonal imbalances - including sleep, appetite, mood, and reproductive disorders - could result from disturbances in circadian rhythms, often arising from insufficient sunlight. Seasonal Affective Disorder (SAD) is a prime example of disrupted sleep patterns, appetite and weight issues, and depression, all exhibiting yearly and daily cycles with symptoms peaking in the depths of winter and worsening in the evening. Treating SAD patients with artificial daylight (light therapy) has proven effective in rectifying these disorders, suggesting a direct association between SAD and light deficiency.
Other diseases are also influenced by disturbances in circadian rhythms. For instance, classical depression follows a daily rhythm, worsening in the morning and improving in the evening. This rhythmic pattern leads to mood swings, accompanied by insomnia or disrupted sleep. Presently, classical depression is often treated with medications that impact circadian rhythms. Research is exploring the efficacy of treating it with light therapy. Additionally, menstrual irregularities in women, such as irregular cycles or premenstrual syndrome, represent further instances of biological clock disturbances in the body. Research is ongoing to investigate the potential benefits of therapy in treating these conditions.