Nighttime blue light exposure can derange the circadian rhythm. It may be a culprit behind many modern chronic health issues, including obesity, metabolic syndrome, and cancer. On the other hand, blue light exposure during the day is very beneficial. Read this post to learn how to hack your blue light exposure for your benefit.
What Is Blue Light?
Light is electromagnetic radiation of a variety of wavelengths within the electromagnetic spectrum.
There is both visible and non-visible light. Non-visible light includes ultraviolet (UV) and infrared light, while visible light includes the whole spectrum of the rainbow, including blue light. Within the visible light spectrum, each wavelength is represented by a color.
Of all the colors of the visible light spectrum, blue light (wavelength 446-477 nm) has the strongest impact on physiology and the circadian rhythm because our pigments react to this wavelength [1, 2, 3].
Humans have evolved to rise with the sun and go to sleep after sunset. Before the advent of technology, humans only used sources that emitted yellow, orange, and red light, such as fire, candles, and lamps. These lights have much less impact on our circadian rhythm and sleep-wake cycles than blue light [4, 5].
Nowadays, advances in lighting and other technologies like television, computers, and digital clocks among others, have introduced more blue-white light to our environment. In addition, these lights are available at all hours, which can affect our health and wellbeing .
The higher-efficiency “green” light bulbs are not necessarily as beneficial for humans as they supposedly are for the Earth. Compact fluorescent light (CFL) bulbs contain about 25% of blue light, while this percentage reaches 35% in LEDs .
Generally, blue light is beneficial during the day, but harmful at night.
Light exposure anchors human body functions to the rise and fall of the sun. When sunlight, which contains blue light, hits the retina, the photoreceptor cells transmit nerve impulses to the hypothalamus. The hypothalamus is the command central for hunger, thirst, temperature regulation, hormone secretion, and sleep patterns, which fluctuate according to our circadian rhythm .
Photoreceptor cells in the retina of the eye (retinal cells, rod and cone cells). The arrangement of retinal cells is shown in a cross-section.
When light hits the eye, it hits the retina. This light-sensitive tissue is actually considered part of the central nervous system (CNS) and is connected to the brain via the optic nerve. There are several layers of nerve cells in the retina, but the ones that are sensitive to light are those with photoreceptor cells. Rods and cones are the light-sensing cells that allow us to see, while the retinal ganglion cells are important for circadian rhythm entrainment .
The suprachiasmatic nucleus (SCN) in the hypothalamus coordinates light exposure with bodily functions through hormonal, autonomic (involuntary nerve impulses), and feeding-related cues .
The following adverse effects are commonly associated with increased exposure to blue night during the night. Note, however, that the majority of studies covered in this section deal with associations only, which means that a cause-and-effect relationship hasn’t been established. Other environmental and genetic factors may play a more important role.
Intrinsically photosensitive retinal ganglion cells contain melanopsin (a light sensor protein), whose job is to synchronize the body’s circadian clock to light .
Exposure to blue light at night signals the body that it is daytime, consequently, messing up the circadian rhythm, which is crucial to many body processes. To entrain the circadian rhythm, it is important to both get sunlight in the morning and avoid artificial light at night .
Even dim light can interfere with a person’s circadian rhythm and melatonin secretion. A mere 8 lux (2x the brightness of a night light) can inhibit melatonin. The brighter the light and the longer the exposure time, the less melatonin the eye cells produce [14, 15].
A theoretical model of different types of light and exposures indicates the following time needed for melatonin suppression :
- Monochromatic red light at 100 lux, a reasonable living room lighting level, would take 403 hours of exposure to suppress melatonin by 50%
- Candle: 66 min suppresses melatonin by 50%
- 60-watt incandescent bulb: 39 min suppresses melatonin by 50%
- 58-watt deluxe daylight fluorescent light: 15 min suppresses melatonin by 50%
- Pure white high-output LED: 13 min suppresses melatonin by 50%
For example, after 1 hr of light at midnight, melatonin could be suppressed up to 71%, 67%, 44%, 38%, and 16% with intensities of 3,000, 1,000, 500, 350, and 200 lux, respectively .
Melatonin levels are reduced most with dilated pupils exposed to 90 min of monochromatic blue light from 2:00 to 3:30 AM at a brightness of 0.1 lux, which is equivalent to the light of a full moon .
Blue light, even at a dim, moonlight level (0.1 lux), suppressed melatonin production more than any other wavelength in a study in rats .
People with light-colored eyes (blue or green, for example) are more susceptible to melatonin suppression by blue light than those with darker eyes .
Exposure to either short-wave blue light or long-wave red light significantly increased cortisol levels at night, with little effect on cortisol levels during the day, in a small trial on 12 people .
In clinical trials, nighttime blue light exposure:
- Increased the time it takes to fall asleep 
- Reduced total and deep sleep duration and overall sleep quality 
- Reduced melatonin effects, such as increased deep sleep in elderly insomniacs and increased REM sleep in people with reduced REM 
In a questionnaire-based study on 100,000 women in the UK, obesity parameters (BMI, waist:hip ratio, waist:height ratio, and waist circumference) were significantly associated with increased exposure to light at night .
Mice exposed to dim (5 lux) light at night had altered core circadian clock rhythms in the hypothalamus, which resulted in increased body mass compared to mice that were not exposed to light, even though they ate the same amount of food .
The hypothalamus contains high concentrations of receptors for both leptin (the satiating hormone) and ghrelin (the appetite-stimulating hormone). Exposure to light has been found to alter the secretions of these hormones, which can lead to weight gain by increasing hunger and decreasing satiety .
Artificial blue light may alter blood sugar metabolism, as seen in a small trial on 19 healthy volunteers .
Light at night disrupted the circadian clock of pancreatic islet cells and β-cell function in cell studies. This impaired circadian clock may be responsible for altered islet function and survival and, consequently, an increased risk of type 2 diabetes .
In mice, dim light at night brought on metabolic syndrome symptoms, which could be reversed by returning to complete darkness at night .
Blue light disrupted the circadian rhythm in heart muscle cells, possibly putting the heart out of synch and increasing the risk of high blood pressure, insulin resistance, and cardiovascular disease .
Its inhibition of melatonin synthesis may lead to increased inflammation, increased blood pressure, high LDL and total cholesterol, decreased antioxidant capacity, and overall elevated risk of cardiovascular disease .
Blue light exposure may lead to macular degeneration. In several animal studies, this light:
- caused cell death and destruction of the fatty acid DHA in the retina [33, 34].
- reduced melanin (not to be confused with melatonin, melanin is a pigment with antioxidant properties that can prevent eye damage) in the retina .
- increased free radicals in the presence of oxygen around the retina, leading to the breakdown of the retinal lining .
In eyes that are undergoing age-related macular degeneration, the damaged cells release VEGF, which can induce new blood vessel formation. These new blood vessels are fragile and can easily burst, which leads to blood leaking out and damaging the back of the eye where light is transformed into a picture [37, 38].
Similarly, dim light at night increased depressive behaviors in an animal model of obstructive sleep apnea .
A study of 500 elderly individuals (average age 72.8) associated exposure to light at night with depression .
As having lowered melatonin levels has been associated with an increased risk of cancer and blue light prevents melatonin production, blue light exposure at night may increase the risk of cancer, and blocking blue light may be helpful in preventing it .
However, studies on the effect of shift work – with the associated nighttime exposure to light – on various cancers (breast, prostate, colorectal, and lymphoma) showed inconsistent results .
In a study of over 50,000 women, long-term shift work, which lowers melatonin levels, was significantly associated with an increased risk of endometrial cancer .
Numerous studies show that lowered melatonin due to nighttime light exposure is a risk factor for breast cancer, with blind women being half as likely to develop this type of cancer as non-blind women .
A global study on artificial light at night and cancer in 158 countries found a significant association with several forms of cancer, including lung, breast, colorectal, and prostate cancer .
During the day, exposure to blue light may have some health benefits that allow its therapeutic use. Below is a summary of the existing research.
Actinic keratoses are rough, scaly patches on the skin that may develop into skin cancers such as squamous cell carcinomas. Photodynamic therapy combining aminolevulinic acid (a photosensitizing agent) with light therapy (especially blue light) is approved for the removal of actinic keratosis. Upon activation by blue light, aminolevulinic acid damages the cells of these lesions
This photodynamic therapy was also effective for similar lesions on the lower lips (actinic cheilitis) in a clinical trial on 15 people and against a condition that increases the risk of developing basal cell cancer (Gorlin syndrome) in a small pilot study on 3 people, although more research is needed to support these uses [61, 62].
Blue light inhibits the growth of Propionibacterium acnes, a bacteria that causes acne. Daily blue light treatments for mild-to-moderate inflammatory acne significantly reduced the number of acne lesions in 6 clinical trials on over 200 people [63, 64, 65, 66, 67, 68].
The combination of blue and red light worked better than either used alone in 2 clinical trials on almost 150 people. Broad-spectrum continuous-wave visible light therapy can be used to harness the power of both blue and red light [69, 70, 71].
Similarly, blue light alone was only effective at improving inflammatory lesions but its combination with infrared light also improved non-inflammatory lesions and fat production (seborrhea) in a trial on 20 people .
A system comprised of LED blue-light phototherapy and photo-converter chromophores was safe and effective in a 12-week clinical trial on 90 people with acne and a 12-week extension of this study [73, 74].
All in all, the evidence suggests that blue light therapy may help with acne. You may discuss with your doctor if it may work as a complementary approach in your case.
In a clinical trial on 33 male students, combining a breakfast rich in tryptophan with blue light exposure during the day induced melatonin secretion and improved sleep quality at night .
In another trial on 30 people over 60 years old, bright light therapy during the day improved insomnia. Forty-five minutes of light therapy was more effective than 20 minutes .
Similarly, blue light-enriched white light during daytime work hours improved sleep quality while reducing daytime sleepiness in a clinical trial on 94 people .
Although limited, the evidence suggests that daytime blue light may improve sleep quality. You may use blue light therapy for this purpose if your doctor determines it may be helpful.
Narrow bandwidth blue light out-performed dimmer red light in reversing the symptoms of seasonal affective disorder (SAD) in 5 clinical trials on 176 people suffering from this condition [79, 80, 81, 82].
Blue light was effective at lower intensity than traditional fluorescent sources in a small trial on 18 depressed people .
Again, limited evidence suggests that blue light therapy may help with seasonal affective disorder. Discuss this approach with your doctor and never use it in place of what your doctor recommends or prescribes.
The following purported benefits are only supported by limited, often low-quality clinical studies. There is insufficient evidence to support the use of blue light for any of the below-listed uses. Remember to speak with a doctor before using blue light for any of these purposes. Blue light should never be used as a replacement for approved medical therapies.
In a clinical trial on 35 healthy people, exposure to blue light during daytime improved activation of a brain region (prefrontal cortex) during a working memory task .
In another trial on 17 people, morning blue light improved cognitive performance, mood, and general well-being .
Similarly, exposure to blue light during memory consolidation improved long-delay verbal memory performance in a trial on 30 people .
In a clinical trial on 20 preschool children, exposure to blue light-enriched light improved task-switching performance .
Blue light improved cognitive performance better than red light and placebo in a clinical trial on 18 elderly people in long-term care .
In another trial on 29 healthy elderly people, blue light from 8 to 9 AM improved cognitive function but only in women .
Exposure to bright light after lunch had the same benefits on cognitive flexibility as a short nap in a clinical trial on 25 people .
However, blue light didn’t improve reaction time in a clinical trial on 72 male athletes .
Although the results are overall promising, all the trials were very small and include a study with negative results. Larger, more robust clinical trials are needed to validate this potential use of blue light therapy.
Bright light combined with melatonin improved mood and attenuated cognitive decline in those with dementia in a clinical trial on 189 elderly people .
In a clinical trial on 20 people with traumatic brain injury, blue light therapy alleviated fatigue and daytime sleepiness .
Two clinical trials cannot be considered sufficient evidence to claim that blue light therapy may help with cognitive impairment. Further clinical research is needed.
Similarly, full-body irradiation with blue light reduced itching, recurrence, and need for corticosteroids, as well as improving sleep and quality of life, in a clinical trial on 36 people with severe eczema .
Again, the results are promising but the evidence comes from only 3 small clinical trials. More clinical studies on larger populations are needed to verify these preliminary results.
A single clinical trial is clearly insufficient to back this potential health benefit until more research is conducted.
No clinical evidence supports the use of blue light for any of the conditions listed in this section. Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies should not be interpreted as supportive of any health benefit.
In animals, daytime blue light exposure was helpful for raising melatonin at night, which significantly slowed down prostate cancer growth .
Note, however, that these effects may not be the same in humans. Never use blue light as a replacement for proven anticancer therapies.
The following strategies may help you exploit the full potential of daytime exposure to blue light while avoiding the negative effects associated with this light in the evening. Discuss with your doctor if any of them may be helpful in your case.
Getting more bright light during the day may lessen your sensitivity to light in the evening as well as melatonin suppression by nighttime light exposure .
If you work in an office and can’t get it naturally, bright light devices are available.
Red or amber colored glasses block blue light from entering your eyes. UVEX glasses will only block out blue light. Users and manufacturers recommend wearing these glasses 4 hours before going to sleep.
Switching from regular light bulbs to blue blocking or red light bulbs can reduce blue light exposure.
Computers, iPads, smartphones and other digital devices emit blue light. You can reduce their blue light by installing F.lux and using a blue light filter app on your smartphone. You can also use blue-blocking red sheets to cover your iPad or other screens.
In addition to cutting blue light out at night, dimming all your light-emitting devices to the lowest setting may help.
Sleeping in total darkness will reduce your exposure to blue light. You can buy blackout curtains for your windows and black tape for blue-emitting electronics.
Glasses and contact lenses filter out UV lights. UV light should directly hit your eyes to have an effect on your circadian rhythm.
You can take supplements with lutein and zeaxanthin, which may reduce the oxidative effects of blue light .
In a study on almost 1,200 Norwegian shift-working women, TT carriers for CLOCK allele rs3749474 had a lower incidence of breast cancer compared to the other shift workers, indicating that they were less negatively impacted by nightlight. In the same study, GG carriers for CLOCK rs11133373 had an increased incidence of breast cancer .
Again, in the study of Norwegian shift-working women, TT carriers for BMAL1/ARNTL rs2278749 had a reduced incidence of breast cancer compared to the other shift workers, indicating they were less negatively impacted by nightlight .
Women with the highest number of successive night shifts and carrying at least one variant allele of SNPs in the two core circadian genes BMAL1/ARNTL (rs2290035, rs969485) and ROR-b (rs3903529, rs3750420) had an increased incidence of breast cancer .
Regarding AANAT, a gene controlling melatonin production, CC carriers for rs4238989 had an increased incidence of breast cancer associated with light exposure at night, whereas GG carriers for rs3760138 had a higher incidence of breast cancer (40%) with the highest (4 shifts) light exposure .
The SCN in the hypothalamus is a central command for sleep patterns or circadian rhythm entrainment. It is connected to the retina via the retinohypothalamic (RHT) tract. You can read more about the SCN’s involvement in the circadian rhythm here. The photopigment melanopsin, contained in the intrinsically photosensitive retinal ganglion cells of the eye, acts as an information gatherer/communicator. Melanopsin determines the body’s reflexive response to light such as pupil size change and release of melatonin from the pineal gland.
Internal body clocks are created through regulation of the genes CLOCK, ARNTL, and PER3 in neurons. These are adjusted by light input from the photoreceptors. Read How Your Genetics Affect Sleep for a more detailed description.