The human eye incorporates a non-visual biological "light meter" that regulates our internal clock, alertness, and mood. Exposure to sufficient natural light is essential to break from our modern "biological darkness" and optimize our cognitive faculties.

noli's read

The fact that the eye possesses a genuine biological "light meter" is well-validated by science: it involves light-sensitive retinal cells that regulate our life rhythms without participating in image vision. A literature review published by Blume et al. in Somnologie (2019) confirms that these cells transmit light signals directly to the brain's internal clock to orchestrate sleep and mood. Furthermore, a major meta-analysis led by Mu in 2022 robustly demonstrates the immediate stimulating effect of light on alertness and wakefulness in healthy subjects. In daily life, an observational study published in Communications Psychology in 2025 reinforces this idea by associating more stable and intense light exposure with better mental performance and less sleepiness. However, the claim that light boosts memory and cognition in a linear fashion warrants nuance. Recent controlled trials, such as the one by Reitmayer et al. (2025), instead suggest a complex "inverted U" relationship, where excessive light intensity in the evening can paradoxically increase mental fatigue rather than improve memorization capacities. Finally, the expression "biological darkness" is a popularizing metaphor used to describe our modern indoor lifestyle, rather than a rigorously defined scientific concept.

the sources

1. 1Blume C, Garbazza C, Spitschan M 2019
2. 2Brown TM et al. 2022