Light and Health

we evolved with natural light


natural light is full spectrum and dynamic. light and health are fundamentally linked

Light is a cornerstone of our health and well-being

natural light varies with the time of day, time of year, and place on earth

daylight is bright and cool, sunset is soft and warm

We are programmed to respond to the daily and seasonal variations of natural light = light and health are fundamentally linked.  Until 100 years ago our days were filled with bright sunlight, and our evenings were lit with the soft warm glow of a flame.  We naturally feel bright and energized on a sunny day, and down on rainy days, during dark winter months or when we’ve been stuck inside for too long.  Exposure to light drives our hormone cycles and synchronizes our circadian rhythms – profoundly impacting how we feel, work, play and rest.  Unfortunately, most of us don’t get enough natural light during the day and get too much artificial light at night.  The science linking light and health is clear, and the technology to recreate natural light is now available!

the spectrum of natural light changes with the time of day

Figure 1. Light and health are fundamentally linked, the daily cycles of natural light are our master clock!  Natural light is bright and cool during the day (contains lots of blue light), warm and soft at sunset (contains less blue and more red light), and has no bio-active blue light at night.  A prism can be used to split light into its constituent colors and show how the ratio of blue to red light changes with the time of day.  Note also that natural light contains all the colors of the rainbow which is why we look and see our best with natural light!

the human eye has a 3rd photoreceptor which senses the time of day

cycles of light intensity & color regulate our circadian rhythms and hormones

our eyes measure the time of day with ganglion cells (iPRGCs)

Figure 2 (a) (above).  In 2002 scientists discovered special ganglion cells in our retina which sense the time of day by measuring the amount of blue light hitting our eye.   During the day blue light tells our body to be awake and alert, while at night the absence of blue light tells our bodies to rest and recover.  (Figure credit:

light and health are linked as light synchronizes our circadian rhythms via our SCNFigure 2  (b) (right).  Rods and cones transmit the visual information to the visual center of the brain via the optic nerve (green path).  The ganglion cells are connected with the superior cervical ganglion in the spinal cord and with the suprachaiasmatic nucleus (SCN) by the retinohypothalamatic tract (the blue path).   The SCN uses the pineal gland and hormone balance to synchronize the body with the outside world.  (Figure credit:

exposure to light correlates with activity and relaxation

Figure 2 (c) (left).  The daily and seasonal cycles of natural light drive our activity and relaxation by regulating our circadian rhythms and hormone cycles.  The bright blue light of day signals ‘awake and alert’ while the softer warmer light of sunset and candle light signals ‘relax and recover’.  (Figure credit:

we evolved with dynamic full spectrum light

the cycles of natural light tell us when to be awake and when to rest

changes in the blue content of light with the time of day drive the connection between light and health

Figure 3 (a) Sunlight is bright and cool during the day (contains lots bio-active blue light), warm and soft at sunset (contains lots of red light, and less bio-active blue light).  Light from a candle or a fire contains almost no bio-active blue light.  Light from Sunlight Inside products is full spectrum and automatically changes throughout the day, closely matching sunlight during the day and then providing candle and fire light at night.

comparing natural light to artificial light

Figure 3 (b).  The light spectrum from most artificial light sources is fixed (does not change during the day) and is therefore set to approximate EITHER daylight (cool-white) OR sunset (warm-white).  Cool white products contain more bio-active blue, while warm-white products contain more red light.  Unfortunately even the warm-white (sunset) light sources still contain meaningful amounts of bio-active blue light (as shown in the red boxes) which means they continue to stimulate our bodies when used at night.  Incandescent bulbs (original Edison style filament bulbs) offer the least amount of stimulating bio-active blue, but should still be dimmed at night to reduce their blue content further.   While incandescent and some halogen sources can be used safely at night, they cannot deliver the bright blue light we need to feel awake and alert during the day.   Furthermore incandescent and halogen light sources are very inefficient (only ~5% of incoming energy comes out as light) which is why they are being phased out.  As light from a prism shows, fluorescents do not produce full spectrum light which is why they distort the visual appearance of people and objects.  

light spectrum from screens (smartphone, monitor, TV)Figure 3 (c) (left). The light emitted by smartphones, monitors and TVs contains a lot of bio-active blue which is one reason why these devices keep you awake at night.   To mitigate the impact of blue light from screens at night we recommend using f.lux for PCs and Andriod, and Nightshift for iOS devices!   These software solutions shift the balance of screen light from blue to red at night to reduce their stimulating effect.   Studies have shown that watching Television generally delivers much less bio-active blue to your eye because of the substantially longer viewing distances (and darker images) – but this math is different if you’re a gamer sitting up close!

The light emitted from screens intentionally does not replicate daylight in order to provide the red, green, blue (RGB) control needed to form different images.  The partial spectrum of light emitted from screens means that screens are generally a poor source of illumination, light from screens distorts the appearance of people and objects. 

circadian rhythms are a cornerstone of our health and wellness

(+) natural light synchronizes our circadian rhythms so we feel, sleep, work, and play at our best!
(-) artificial light disrupts our circadian rhythms

light and health are linked as our circadian rhythm underpins our daily activities

Figure 4.  Our circadian cycles regulate our energy levels, performance, metabolism and our recovery.  Regular exposure to cycles of natural light are required to synchronize these circadian rhythms.  Unfortunately most of us don’t get enough natural light during the day and get too much artificial light at night.  Studies show that artificial light at night disrupts our circadian rhythms and thereby increases the risk of insomnia, depression, cancer, diabetes, and cardiovascular disease.  (Figure credit


Please see below for more on the link between light and health.  If you’d like to dig deeper into the science we recommend checking the research organization websites, reading the references in the linked documents and consulting with a health professional.   Unfortunately we don’t have the bandwidth to catalog the thousands of articles on the link between light and health here.  However, please do feel free to send us articles, presentations and other resources you think should be included in our high-level summary below!