Life on Earth has evolved for more than 3.5 billion years under an environmental cycle that has remained essentially unchanged: the alternation of light and darkness every 24 hours. The extraordinary stability of this cycle has made it the main environmental synchronizer of circadian rhythms in all living organisms. However, for little more than a century, life has been facing — for the first time in its evolutionary history — nights illuminated artificially.

In every organism — and humans are no exception — the way these biological rhythms (cortisol, melatonin, blood pressure, sleep…) organize themselves internally is a fundamental condition for maintaining health, performance and wellbeing.

We now know that the retina is not responsible only for vision and image formation through cones and rods. In addition to these photoreceptors, it contains a specialized group of neurons known as intrinsically photosensitive retinal ganglion cells (ipRGCs), which express a photopigment called melanopsin and respond especially to blue light, with wavelengths around 460–480 nm. These cells project directly to the suprachiasmatic nucleus, informing the brain about the presence, intensity and spectral composition of ambient light. This non-visual system, known as circadian photoreception, regulates essential functions such as hormone secretion, body temperature, alertness and — most importantly — the sleep–wake cycle.

Lighting isn’t just a functional or aesthetic element — it’s a key determinant of human health and ecosystem balance.

One of the most studied effects of light is its capacity to suppress melatonin secretion, the hormone released by the pineal gland during biological night. Melatonin doesn’t directly induce sleep, but it acts as an internal temporal signal that prepares the organism for sleep and for the nocturnal fasting period. Exposure to artificial light at night — especially if rich in blue wavelengths — can delay its secretion, disrupt the circadian clock, and harm both sleep onset and quality. By contrast, warm light — orange or reddish — of low intensity and with minimal blue content has a much smaller impact on melatonin and circadian rhythms.

However, the problem isn’t only the presence of light at night, but also the lack of appropriate light during the day. Numerous studies have shown that insufficient exposure to natural light — common in indoor spaces — reduces the amplitude of circadian rhythms, weakens the contrast signal between day and night, and promotes daytime sleepiness, night-time insomnia and sleep fragmentation.

The contrast between bright days and truly dark nights is essential for correct circadian synchronization. When this contrast is disrupted — as happens in shift work, jet lag, or chronic exposure to inadequate artificial lighting — the organism enters a state of circadian misalignment, or chronodisruption. This condition has been associated, in both epidemiological and experimental studies, with an increased risk of metabolic and cardiovascular disorders, memory impairment, mood alterations and even certain types of cancer. This evidence led the International Agency for Research on Cancer (IARC) to classify night shift work as probably carcinogenic, Group 2A.

The disruption of the natural light–dark cycle doesn’t affect only humans. Light pollution has become a major factor of ecological stress, altering activity, reproduction, migration and predation patterns across many species, from insects to birds and mammals. The loss of natural darkness is transforming entire ecosystems, compromising their balance and, ultimately, biodiversity.

All of this should lead us to recognize that lighting, beyond being a purely functional or aesthetic element, is a key determinant of both human and ecosystem health. For this reason, the design of luminous environments should consider not only the amount of light (illuminance), but also its temporal distribution, its spectral composition and its variability throughout the day.

Concepts such as human-centric lighting seek to align technology with biology: intense and blue-enriched light during the day to support alertness and circadian synchronization, and dim, warm light at dusk to ease the transition toward rest — which should ideally happen in darkness.