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By Sabrina Tullo
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Public debate over modern light exposure has long been dominated by screens. As highlighted in the World Screen Health Report, concerns about digital eye strain and the effects of nighttime blue light on sleep have grown, and reducing evening screen exposure has become familiar advice. Far less recognized is the daytime counterpart. As Discover magazine highlighted in its coverage of circadian rhythms, the human body depends on abundant natural daylight to regulate its internal clock.
While modern indoor environments can appear comfortably bright for visual tasks, they may fail to provide the melanopic, blue sky light signal associated with natural daylight that the brain expects. The issue is that most indoor lighting was designed for vision rather than to reproduce the biological information carried by a blue sky.
Research highlights just how critical daylight patterns are for human physiology. In a 2026 study published in npj Biological Timing and Sleep, researchers tracked real-world light exposure and sleep metrics. They found that earlier, more consistent sleep and wake timing are directly associated with longer, higher-intensity daytime light exposure. Furthermore, regular and less fragmented daytime light exposure was linked to enhanced deep sleep intensity during the initial portion of the night. By capturing melanopic irradiance alongside objective sleep metrics, the findings demonstrate that brighter days and stable light exposure patterns actively strengthen sleep architecture and support overall circadian entrainment.
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This emerging scientific consensus underscores that screens represent only a fraction of a person’s daily light environment. Most waking hours unfold beneath electric lighting in offices, classrooms, hospitals, homes and public buildings. Indoor illumination is typically an order of magnitude dimmer than natural sunlight. More importantly, much of it provides significantly less of the blue sky light signal that drives the body’s non-visual circadian system. While conventional LED lighting systems were designed primarily for energy efficiency and visual clarity, they frequently fall short of the biological signals the human body requires. Combined with excessive evening light, this chronic deficiency in daytime light exposure represents an understated public health concern that impacts daily alertness, long-term health and overall sleep quality.
Robert Soler, founder and chief scientist of BIOS Lighting, believes this broader perspective deserves greater public consideration. “We’ve become highly aware of the screens we hold in our hands,” Soler says. “We spend much of our lives biologically in the dark because the spaces where we learn, work and heal often provide visual illumination without delivering the blue sky light signals associated with the daylight environment our biology evolved to expect.”
His perspective builds on decades of research into the human circadian system. The eye contains specialized photoreceptors known as intrinsically photosensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, which exist to support vision, ipRGCs are specialized receptors whose primary role is not seeing at all. Instead, they are specifically tuned to detect the blue-enriched sky light signal carried by natural daylight.
These melanopsin-containing cells continually monitor whether the brain is receiving the daytime light cue it evolved to expect, transmitting that information directly to the body's internal clock to regulate alertness, sleep-wake cycles, hormone production and countless other biological processes. According to a study published in the Journal of Cognitive Neuroscience, blue light research has shown that a subset of totally blind individuals retain this non-visual light-sensing pathway, proving that the blue sky light signal can continue influencing circadian biology and alertness even in the absence of conscious vision.
To evaluate whether indoor environments adequately stimulate this pathway, researchers and lighting experts rely on melanopic equivalent daylight illuminance, commonly known as m-EDI. Unlike traditional lux, which measures light purely for human visual perception, m-EDI provides an actual, concrete metric to evaluate whether people are receiving sufficient biologically active light to stimulate the melanopsin system. In practical terms, it measures how effectively a lighting environment reproduces the blue sky light signal that ipRGCs are designed to detect throughout the day. Scientific consensus increasingly points to a target threshold of at least 250 melanopic lux at eye level during daytime hours. Soler notes that by utilizing m-EDI as a measurable standard, designers and health researchers gain a precise tool to assess biological light deficits and establish lighting conditions that properly support human health.
Soler’s own work on these questions began in an environment where circadian biology became impossible to overlook. Early in his career, he contributed to NASA projects supporting astronauts aboard the International Space Station, where a sunrise appears roughly every 90 minutes. Traditional schedules alone could not fully address the biological demands created by the station’s unique orbital cycle. “Space simply magnified a challenge that already existed,” Soler says. “Once we understood how light communicates with human biology, it became clear that the same principles extend well beyond astronauts.”
That realization eventually expanded into everyday buildings. Offices, classrooms, hospitals and care facilities all depend heavily on electric lighting, yet many people spend most of their day indoors with relatively limited daylight exposure. Soler argues that this creates a constant biological light deficit, not because indoor spaces are visually dim, but because they often fail to deliver enough of the blue sky light signal that ipRGCs evolved to detect, even in spaces that appear visually bright. Many people recognize that a clear blue sky day often leaves them feeling more alert and engaged than an overcast afternoon. In his view, indoor lighting design has an opportunity to draw inspiration from those familiar experiences.
The implications extend across daily life. According to Soler, limited daytime biological light may contribute to reduced alertness, focus and mood during waking hours, while also influencing sleep later that evening. Students, shift workers, hospital patients and people whose routines unfold primarily indoors may experience these environments differently because of the amount of time spent under electric lighting.
Increasing brightness alone, however, does not necessarily resolve the issue. Additional light can introduce glare, visual discomfort, practical design constraints and higher energy consumption. Soler notes that circadian lighting research increasingly examines spectrum, direction and delivery to the eye alongside overall brightness. The SkyView lighting developed by his team was engineered within this broader scientific framework. “Our goal is to deliver sky-inspired biological signals while preserving visual comfort in architectural spaces,” Soler states.
Public awareness of sleep science, environmental health and the relationship between daily habits and long-term well-being continues to grow. Within that expanding conversation, Soler believes indoor lighting deserves greater attention alongside nutrition, exercise and digital wellness. “The built environment participates in our biology every day,” he says. “If we understand that relationship more completely, we gain another opportunity to design spaces that work in greater harmony with how people naturally function.”
Many elements influence human health, and lighting represents only one piece of that larger picture. Even so, growing scientific interest suggests that indoor illumination may become an increasingly important consideration for architects, employers, educators, healthcare leaders and policymakers seeking healthier environments.
As the conversation evolves, the question becomes how indoor spaces can better reflect the biological cues people have long experienced under natural daylight. That includes not only providing enough light to see comfortably, but also delivering the blue sky light signals and melanopic cues that specialized circadian receptors evolved to expect each day. That perspective offers an opportunity to view lighting as more than a tool for visibility and instead as an environmental factor worthy of continued research, thoughtful design and informed discussion.
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