The Science of Light and Health

Introduction

The Sunday Light is the closest thing to sunshine you can get from an indoor light. As such, it can provide many of the same health benefits of sunlight exposure.

As a rule of thumb, you should set your Sunday Light to be bright and cooler during the day and warmer in the evening.

This document has two aims: first, to describe the current scientific understanding of the mechanisms through which light affects our physiology, and, second, to show how you can use this knowledge to maximise the benefits of the Sunday Light.

Table of Contents

• Introduction
• Seasonal Affective Disorder
• Light and the Circadian Rhythm
  • Time of Light Exposure
  • Frequency of Light
  • Brightness and Circadian Stimulus
  • Using Light to Control Your Circadian Rhythm
    • Max Brightness at Dawn
    • After Sunset Keep Your Light on Max Warm and Below 10% Brightness
    • Try to Maximise Your Brighter, Warmer Light Exposure in the Late Afternoon
• Sunday and Time Zones
  • Travelling East to West
  • Travelling West to East
  • Making it Always Summer
  • Other Times You May Consider Shifting Your Circadian Rhythm
• The Immediate Effects of Light on Cognition
• Sharing Your Light
• UV & Health
• Conclusion
• References

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Seasonal Affective Disorder

Seasonal Affective Disorder, known as SAD, is a form of depression that comes and goes with seasonal patterns. In popular culture, SAD has become a familiar aspect of discussions on mental health. Indeed, it affects 12 million people in Northern Europe and a further 16 million in the United States (Nevarez-Flores et al., 2023) (American Psychiatric Association, 2020). Reported incidence of SAD is greater at higher and lower latitudes, suggesting a correlation between daylight exposure and mood effects.

However, the diversity of continuing research suggests that the etiology of SAD is complex and not fully understood. Popular discussions oversimplify this murky and evolving scientific consensus. Emerging research suggests that the underlying causes of SAD may vary among individuals—different factors such as genetics, hormonal changes, disruptions in circadian rhythms, cultural background, and environmental influences have all been implicated (Munir, 2024). Additionally, many people who are not diagnosed with SAD and whose symptoms may not constitute full-fledged depression will still experience moments of unhappiness, lethargy, and low mood.

What is clear, though, is that symptoms of SAD can be helped by increasing exposure to sunlight or bright, full-spectrum artificial lights (Pjrek et al., 2019). With that in mind, in this document we’ll specifically explore the ways in which bright light affects human physiology. These diverse and powerful effects can help anyone, whether or not you suffer from SAD.

Light and the Circadian Rhythm

The body has a daily internal cycle called the circadian rhythm. This cycle, characterised by daily variations in biological markers such as cortisol, glucose, body temperature, insulin, and melatonin, creates a temporal predisposition for resting and waking.

Living out of sync with our circadian rhythm has been implicated in numerous maladies, including SAD. Indeed, a leading hypothesis (called the phase shift hypothesis) states that most patients with SAD experience depression in the winter at least in part because of a phase delay in circadian rhythms relative to their pattern of waking and sleeping (Lewy et al., 2007).

Light plays a crucial role in making sure our circadian rhythm remains in sync with our sleep-wake cycles. Nina Milosavljevic writes in a review on light regulating behaviour and mood that “The most potent impact of light on physiology and behavioural state comes from its regulation of circadian rhythms” (Milosavljevic, 2019).

Acute effects of asynchrony with circadian rhythm include increased anxiety (Walker et al., 2020) and impulsivity (McGowan & Coogan, 2018) while also correlating to decreased cognitive and motor performance (Valdez et al., 2012). There is new evidence that chronic asynchrony can lead to increased risk of cancer, diabetes, and cardiovascular disease (Foster, 2021) (Davis, 2006).

Time of Light Exposure

The circadian cycle only responds to light at certain times. Exposure to light within a few hours before and after usual wake up time has the effect of advancing the cycle—you will want to go to bed earlier. Conversely, exposure to light around usual bedtime delays the cycle—you will want to go to bed later (Foster, 2021).

Between two hours after the usual wake-up time and two hours before bedtime, light exposure has minimal impact on moving the circadian rhythm. This variability in responsiveness to light is called the Phase Response Curve. The “delay zone” before night time shows the time in which you can delay the cycle; the “advance zone” immediately following night time shows the opposite. Exposure to light (or a lack of exposure) throughout the day outside has minimal effects on the circadian cycle (Lack & Wright, 2007).

Frequency of Light

There are special non-visual receptors in our eyes responsible for sensing light for our circadian rhythm called iPRGc (intrinsically photosensitive retinal ganglion cells) (Zaidi et al., 2007). These receptors are most strongly activated by bright blue light (with a peak at 480 nm), and accordingly, bluer light has by far the largest impact on setting our circadian rhythm (Blume et al., 2019).

The Sunday Light’s variable colour temperature allows you to select an aesthetically pleasing light output that still outputs a significant amount of light at this key frequency.

Brightness and Circadian Stimulus

Sunlight is very bright. Outside on a clear day, conditions can often be 1,000 times brighter than a typical room. The brighter the light, the quicker it will have an impact on the circadian rhythm. Very high lux values from Sunday allow you to have a similar impact indoors, but brightness alone doesn’t fully capture the effect of different lighting conditions on your circadian rhythm.

Circadian Stimulus (CS) quantifies the potency of light in affecting our circadian rhythm. It does this by accounting for the coolness (specifically through spectral distribution) and brightness of a light.

Full sunlight has the highest CS score of 0.7 while standing directly under the Sunday Light at maximum brightness and 4000K provides a CS of 0.69. See below for a full table of CS scores at different brightness and colour temperature.

Brightness	Circadian Score
	Min	10%	50%	Max
2700 K	0.024	0.258	0.586	0.612
3000 K	0.044	0.283	0.586	0.646
4000 K	0.203	0.283	0.662	0.684
5000 K	0.291	0.479	0.678	0.689
6000 K	0.344	0.482	0.676	0.687

Using Light to Control Your Circadian Rhythm

Max Brightness at Dawn

Most people's circadian cycles tend to run slightly longer than 24 hours (Kronauer, 1999). Without any input from light, a typical person’s cycle will gradually shift later in the day—intuitively, they are inclined to stay up a little later every night. Being exposed to as much high CS light in the early morning advance-zone time period is vital for maintaining a natural circadian rhythm.

If you aren’t planning to go outside within a couple of hours of waking, we recommend trying to spend at least 30 minutes under your Sunday Light at the highest CS score you find comfortable.

After Sunset Keep Your Light on Max Warm and Below 10% Brightness

Conversely, within two hours of your typical bedtime, exposure to bright blue light will delay your circadian rhythm, shifting it later in the day. As our circadian rhythms tend to already last longer than 24 hours, this should generally be avoided. Importantly though, there is enormous variability between how sensitive people are to the effects of evening light, with one study finding a 50-fold variance in sensitivity (Cain, 2020) (Phillips et al., 2019).

By setting your light to maximum warmth and minimum brightness, it produces a light with a circadian stimulus of 0.024, which should have no impact on circadian rhythms for the vast majority of people. It is particularly important to avoid using your Sunday Light at high brightness and cooler settings in the evening, as its power can delay your circadian rhythm more rapidly than typical residential lighting.

Try to Maximise Your Brighter, Warmer Light Exposure in the Late Afternoon

Somewhat counterintuitively, recent studies have shown that bright light exposure in the day and ‘early-evening’ can reduce some of the sleep-disruptive consequences of light exposure in the later evening (Kulve, 2019).

What qualifies as ‘early evening’ will vary between individuals, but a conservative general rule is: maximise bright, warmer light exposure up to 4 hours before your usual bedtime. We recommend warmer light, as most people find this more aesthetically pleasing later in the day. There is some evidence that light with this spectrum may also play a secondary role influencing your circadian cycle (Hayter, 2018).

Sunday and Time Zones

Travelling East to West

Let’s say you’re travelling from London to New York, a time zone difference of five hours. A few days before your departure, you can start ‘delaying’ your circadian cycle by exposing yourself to 30 minutes of high Circadian Stimulus (CS) light after sunset and shifting the time you wake up and go to bed back successively each day. It’s best to prepare over several days, as the circadian cycle can be shifted by a maximum of one to two hours each day. With this adjustment, your circadian rhythm will be closer to the local time upon arrival, allowing you to stay up in the evening more naturally.

Travelling West to East

After a week in New York, your circadian rhythm will likely be fully adjusted to the local time. Upon returning to London, your body clock will now be delayed by five hours, making it most receptive to the phase-advancing effects of light five hours later than usual. Therefore, to adjust to London time as quickly as possible, it’s recommended to expose yourself to 30 minutes of high Circadian Stimulus (CS) light from your Sunday Light immediately upon waking, continuing until two hours after the time you would have been waking up in New York.

Making it Always Summer

In winter, the sun rises later and sets several hours earlier than in summer, which naturally affects our circadian rhythm. (In London, for example, at 51 degrees above the equator, this effect is significant: the longest day of the year is nearly nine hours longer than the shortest.) This can cause people to sleep more in winter and experience seasonal variation in the amount of REM sleep they get (Roenneberg, 2006).

Your Sunday Light can be used after sunset in winter to mimic the longer daylight hours of a summer’s day. In practice, it’s advisable to experiment with this cautiously, as maintaining a consistent light rhythm is crucial to avoiding regular ‘artificial jet lag.’

Other Times You May Consider Shifting Your Circadian Rhythm

With an awareness of the importance of your circadian rhythm and the tools to manipulate it, you may find other circumstances where actively modulating your rhythm can be useful. For example, bright light exposure has been proven to be helpful for night shift workers transitioning between day and night shifts (Bjorvatn, 2013). The physiological changes that occur during adolescence shift the circadian rhythm on average two hours later (Hagenauer, 2009). As such, maximising early morning light exposure can help bring teenagers' circadian rhythms more in line with their schedules and has been proven to improve their sleep quality (Crowley, 2022).

The Immediate Effects of Light on Cognition

As discussed above, living out of sync with your circadian rhythm has been shown to have negative effects on mood and may have a role in the pathophysiology of SAD (Milosavljevic, 2019). However, independent of the circadian rhythm and SAD, light exposure has been shown to have a direct and immediate impact on mood and cognition (Vandewalle, 2009) (Fernandez, 2018).

Exposure to bright blue light has been shown to directly enhance alertness and to improve reaction time and various other measures of cognitive performance (Vandewalle, 2009). Bright light exposure reduces alpha, theta, and low-frequency activity, which are correlated to sleepiness (Cajochen, 2000). There is a logistic response curve to alertness, with significant alerting effects being caused by a well-lit room of 100 lux, with double the alerting effect occurring at 9,200 lux. Higher intensity of light has quicker effects on alertness.

Despite some diversity of results on the significance of daytime light exposure, a recent meta-analysis concluded that the brighter and bluer the light, the greater the effect on alertness (Siraji, 2022). The significance of these effects was far more pronounced in sleep-deprived individuals.

Warmer light has much less effect on alertness, with one study finding that lights of 2,800 K directly suppress the amygdala, suggesting an actively calming effect (Cajochen, 2000). As discussed, bright blue light should be avoided within three hours of the time you plan to go to bed due to its effect on the circadian rhythm.

Outside of alertness, warmer colour temperatures have been shown to lead to less negative bias in the emotional processing of faces (Li, 2021).

Sharing Your Light

There is a lot of interindividual variability in how light is perceived. A person’s mood affects how they respond to light (Stefan, 2021); being in a different stage of your circadian cycle will alter your lighting preference (Campbell, 2023); men tend to be more sensitive to the effects of blue light than women (Chellappa, 2017); cataracts in older people make them less sensitive to light in general, and to blue light specifically (Yan, 2016). There is also significant cultural influence on lighting preferences, with one study theorising that a preference for cooler light is found in people from places that experience more direct sunlight throughout the year (Madan, 2023).

These variations in our physiology and preferences often come to the fore when a household is considering ordering a Sunday Light—a light 100 times brighter than a typical overhead light. Often, we find that one partner wants incredibly bright blue light first thing in the morning, while the other partner does not.

It is important for the lover of cool, bright light to recognise that this same light may be objectively perceived differently by their partner, and that they can still achieve a very high Circadian Stimulus even at warmer colour temperatures.

The other partner’s scepticism is reasonable based on the fact that almost all bright indoor lights are of very low quality, causing colours to look washed out and pale. However, Sunday is fundamentally different; due to its exceptionally high Colour Rendering Index (CRI), colours will look vibrant, almost identical to being under full sunshine. In this regard, switching on Sunday feels more like opening a curtain than switching on a light.

UV & Health

It is worth noting that ultraviolet (UV) light is an important component of sunlight, and Sunday Light produces no UV light. This is a choice we made when developing light, as UV light can cause skin and eye damage.

However, UV light does serve an important role in several physiological processes. Most importantly, UV exposure on the skin is the primary way our body produces vitamin D. While certain foods can provide vitamin D, it is difficult to maintain healthy levels from food alone. In the UK, where daylight is relatively unavailable, as many as 57% of people have deficient vitamin D levels (Braithwaite, 2024). Supplements are effective at increasing vitamin D levels and have traditionally been a first-line recommendation. However, recent research suggests that for people in low sunlight countries, increased UV exposure (through sunlight) showed improved health outcomes: the risk/reward of UV exposure is much more favourable with environmentally lower levels of UV.

Conclusion

The mechanisms through which light affects our physiology are complex. Everyone is different, and everyone will respond differently to changing light conditions. Trust the signals that your body gives you, and, in general, seek out light that feels good to you.

Despite these differences, there are some general guidelines to maximize the positive impacts that light can have on your experience of the world.

Maintain a circadian rhythm in sync with your environment by exposing yourself to light when you wake up. Actively anticipate transitions to distant time zones. Choose lighting conditions that are conducive to work or relaxation. Remain aware of the total quantity of lux hours you receive, through changing seasons and patterns of work and travel.

We hope that Sunday will be a useful tool in your lifestyle and an artistic addition to your life.

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