Why light is the most underrated driver of human health
We spend a great deal of time trying to optimise our health and performance. We track steps, monitor heart rate variability, experiment with supplements, and refine our sleep routines in pursuit of better energy and focus.
However, one of the most fundamental drivers of how we feel and function everyday is often overlooked: bright light.
There is increasing research showing how light is not just for the function of visibility, but it is an important and powerful signal that helps regulate our mood, productivity and sleep-wake cycles. For most of human history, this signal came naturally from natural daylight. However, modern life has resulted in most of our time being indoors, meaning that many of us rarely experience light at the intensity our bodies have evolved to expect.
At Sunday Light, we’re on a mission to bring the benefits of sunlight indoors, not as a lifestyle trend, but because bright light is one of the most important and least considered inputs into human health.
In this blog post, we explore why light matters far beyond how a space looks, and why effective solutions for indoor living are increasingly essential for well-being.
The problem with modern life indoors
As a species, we evolved outdoors. For hundreds of thousands of years, our bodies adapted to bright days, dark nights, and gradual transitions in light from sunrise to sunset. These daily light signals shaped how our brains and bodies learned when to wake, when to rest (known as our circadian rhythm), and how to regulate everything from hormones to mood.
For most of human history, we experienced bright daylight simply by living our lives outdoors.
Today, we live almost entirely indoors, under lighting that is bright enough to see by, but far too dim to properly signal daytime to the body. Typical indoor lighting may reach around 300 lux (a measure of how much light is falling on a specific area), whereas stepping outside, even on a cloudy day, can expose you to around 5000 lux.
Although our eyes adapt easily, our biology does not. The outcome of this is subtle, but widespread across our sleep, mood and cognitive performance.
Why light matters
Light is not just a visual input, it is a fundamental biological input.
In addition to rods and cones, it contains specialised photoreceptors that send signals directly to areas of the brain responsible for regulating circadian rhythms, hormone release, alertness, and sleep timing (Zaidi et al., 2007; Foster, 2014).
These systems operate largely outside of conscious awareness. You don't necessarily feel your circadian rhythm adjusting, but you feel the downstream effects when it is misaligned.
This is why light exposure has been shown to influence mood and anxiety (Walker et al., 2020), cognitive performance and alertness (Valdez, 2012), sleep timing and quality (Cain et al., 2020) and long-term metabolic and cardiovascular health (Foster, 2021).
This increasing evidence makes one thing clear - light is not simply for visual purposes, but it plays a much more important role in our health and well-being.
Research has shown that biologically meaningful bright light exposure can support health in several key ways:
Improved energy and daytime alertness
Bright light increases alertness and reduces feelings of daytime fatigue by signalling to the brain that it is daytime. Studies have consistently shown that exposure to higher light levels improves vigilance, reaction time, and overall cognitive performance (Valdez, 2012; Siraji et al., 2022).
Better sleep quality and sleep timing
Light plays a central role in regulating melatonin, the hormone responsible for signalling sleep. Regular exposure to bright light during the day helps anchor the circadian rhythm, making it easier to fall asleep at night and wake up feeling rested (Cain et al., 2020; Crowley et al., 2022).
Improved mood and mental wellbeing
Bright light exposure has long been associated with improvements in mood, particularly during the winter months. Importantly, these effects extend beyond Seasonal Affective Disorder, with evidence showing broader benefits for emotional regulation and psychological wellbeing (Walker et al., 2020; Pjrek et al., 2019).
Enhanced focus and productivity
Higher daytime light levels have been linked to improved concentration, decision-making, and workplace performance. In practical terms, people tend to feel more engaged, motivated, and mentally clear when working in brighter, higher-quality light environments (Valdez, 2012).
Long-term metabolic and cardiovascular health
Chronic disruption of circadian rhythms has been associated with increased risk of metabolic and cardiovascular disease. By supporting circadian alignment, regular bright light exposure plays a role in maintaining long-term physiological health (Foster, 2021).
Taken together, these findings suggest that bright light is not simply a comfort or lifestyle upgrade - it is a foundational environmental factor that supports both short-term wellbeing and long-term health. Discover more about this here.
Sunday Light recreates the effect of standing under a clear blue sky indoors, delivering daylight-level brightness designed to support mood, energy, and natural sleep rhythms.
How light regulates the circadian rhythm
The circadian rhythm is the body’s internal body clock. It governs daily patterns in sleep and wakefulness, hormone release, body temperature, metabolism, and cognitive performance.
While many factors influence this system, light is by far its most powerful regulator.
Specialised light-sensitive cells in the eye send signals directly to the brain’s master clock, located in a region known as the suprachiasmatic nucleus. These signals help the body determine whether it is daytime or nighttime and adjust internal processes accordingly (Sack, 1992; Foster, 2014).
When light exposure is well timed, the circadian rhythm stays aligned with the external day. Morning light helps signal wakefulness and anchors the start of the biological day. Bright daytime light reinforces alertness and cognitive function. As light levels fall in the evening, the body begins preparing for rest.
Problems arise when this signalling becomes weak or inconsistent.
Spending most of the day in dim indoor environments can blur the distinction between day and night from the body’s perspective. Without a strong daytime light signal, the circadian rhythm can drift later, making it harder to feel alert during the day and harder to fall asleep at night.
This mismatch, often described as living “out of sync", is increasingly common in modern indoor life. Importantly, it does not require extreme behaviour or night-shift work to occur. Even small, repeated disruptions in daily light exposure can accumulate over time.
Understanding the relationship between light and the circadian rhythm helps explain why bright light exposure during the day is so important, and why indoor lighting, despite appearing sufficient, often fails to meet the body’s biological needs.
Why indoor lighting isn’t enough (even if it seems bright)
A normally sighted human can function comfortably across an enormous range of light levels, from a dim living room (~100 lux) to full daylight (~100,000 lux).
However, this stark difference, tends to go unnoticed because our eyes are extremely good at adapting to different lighting conditions. So although we can see perfectly well indoors, this doesn’t mean our bodies are getting the same quality of light signals as we do if we were to go outside.
As an example, modern indoor environments are typically lit at 300–500 lux. Even well-lit offices rarely exceed 1,000 lux. By contrast, a cloudy outdoor day can provide 5,000 lux, and direct sunlight up to 100,000 lux.
Over time, this can show up subtly as:
- feeling flat or low-energy during the day or as afternoon approaches
- difficulty concentrating
- sleep that feels unrefreshing
- a sense of being “out of sync” with your schedule
Indoor lighting is often a forgotten-about factor in these feelings, often because it is passive, and our visual system adapts so well that we forget how different real daylight truly is.
A new way to think about indoor lighting
At Sunday Light, we approach lighting differently.
Sunday was created around a simple idea: to make biologically meaningful bright light a part of everyday life, without an extra step in your routine or conscious effort.
Sunday Light enables this by creating an effortless environment which replicates the look and feel of being outdoors in sunlight, so that you can fully experience the benefits of bright light in a way that feels natural, and whilst remaining indoors.
“I was incredibly impressed by the Sunday Light. It produces a level and quality of light that approximates sunlight, yet its design allows prolonged use that is both comfortable and convenient. Critically, you do not have to sit next to the device to get the levels of light that will deliver physiological benefits.”
Dr Russell Foster, Head of the Sleep and Circadian Neuroscience Institute at the University of Oxford
Bringing bright light indoors
At Sunday Light we are on a mission to bring the benefits of bright light indoors into everyday spaces, so that we can benefit from the powerful signals this brings within an indoor environment.
The most common areas we see application of this are:
Bright light in homes:
Lighting shapes how we feel throughout the day: from morning energy to evening calm. Spaces that support healthy bright light exposure, particular in the morning, can feel more restorative, more uplifting, and easier to live in.
"I can't believe the difference the light has made. It's like a beam of sunshine above our kitchen table. Every morning, I notice the immediate effect it has on me and my family's mood." Matt Cummings, Sunday Light Customer
Bright light for office productivity:
Light influences focus, alertness, and cognitive performance. Environments that support circadian alignment can help people feel more awake and engaged during working hours, without relying solely on caffeine or willpower.
Bright light in hotels and wellness spaces
Guests may not consciously analyse lighting, but they feel its effects. Bright, naturalistic daytime light and warm, calming evening light contribute to experiences that feel more restorative and memorable. This is especially useful where there is a darker space you would like to illuminate, or give a more ‘outdoor’ feeling to.
"It felt like being bathed in sunshine." Women's Health
In all cases, light quietly sets the tone for how a space is experienced.
Summary
We need bright light indoors, integrated into our environment, to help regulate our sleep, mood and energy.
Light, and in particular bright light, is a foundational biological signal that shapes how we feel, think, and sleep. In the modern world, where we spend the majority of our lives indoors, as humans we are experiencing significant light disruption, and we need a solution which can address this in an indoor environment.
Sunday Light is a first-of-its-kind lighting system, designed to address this problem, creating ‘indoor sunshine’ that is the closest replica of real sunlight in an indoor environment.