In the modern world, natural light is often replaced by artificial lighting such as LEDs, phones, laptops, and televisions. These light sources emit a cold blue spectrum (around 450–480 nm) which, through non-visual pathways, maintains wakefulness by reducing melatonin secretion. Besides visual processing, the human eye contains intrinsically photosensitive retinal ganglion cells (ipRGCs) responsible for non-visual effects on the body. The activation of these cells increases melanopsin production, sending signals to the suprachiasmatic nucleus (SCN) in the hypothalamus. The SCN reduces melatonin release, consequently increasing alertness.

During the day, low melatonin levels activate dopaminergic pathways especially in brain regions responsible for motivation, attention, and rewardl eading to increased energy and productivity. The problem lies in the opposite scenario: when artificial LED lighting is used in the evening, it suppresses melatonin, the sleep hormone, causing delayed sleep, restless nights, and as a result, fatigue throughout the day.

The circadian rhythm is present in almost all living organisms—from bacteria and plants to animals and humans. It plays a key role in maintaining homeostasis and optimizing the body’s energy processes in alignment with external cycles. Circadian rhythm (from Latin circa diem – “about one day”) represents an endogenous biological rhythm with a period of approximately 24 hours (25.1h), which regulates numerous physiological, metabolic, and behavioral functions in the body. This internal rhythm enables the organism to synchronize with changes in natural light and darkness. The center of the endogenous clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN contains around 20,000 neurons that generate and synchronize circadian oscillations through specific genetic and molecular mechanisms. The hypothalamus is also the center for processing all information and acts as the main regulator. Information reaches the hypothalamus through afferent fibers from within the body as well as from the outside via sensory pathways.

At the whole-organism level, endogenous oscillators represent fundamental biological units that create rhythmic changes in cellular and tissue activity. Each cell possesses its own molecular oscillator, functioning through transcription–translation feedback loops based on the expression of genes such as CLOCK, BMAL1, PER, and CRY. These oscillators form micro-circadian rhythms and represent rhythmic processes occurring within individual cells, tissues, or organs. When micro-circadian rhythms are synchronized under the control of the SCN, they form the macro-circadian rhythm. In this way, while micro-circadian rhythms regulate local processes (such as hormone secretion in endocrine cells or enzyme activity oscillations in the liver), the macro-circadian rhythm ensures their mutual alignment and harmony with external factors, especially with the cycle of light and darkness.

The main synchronizer, known as the zeitgeber, is light, which transmits signals from the retina directly to the SCN via the retinohypothalamic pathway. From there, the SCN coordinates rhythmic activity of peripheral clocks in almost all tissues and organs including the liver, heart, muscles, and endocrine glands, achieving a coherent rhythm of functional oscillations on all levels. During the day, cortisol (the wakefulness hormone) increases, providing the necessary energy for normal and synchronized daily activity. In contrast, at night, when blue light exposure decreases, signals from the SCN lead to increased secretion of melatonin (the sleep hormone), which enables proper sleep, allowing the organism to constantly adapt to the light–dark cycle.