- Exposure to very dim light during sleep — even if it does not noticeably impair your sleep — may affect your brain function and cognition during the day
- Sleeping under 10 lux light conditions decreased activation in a brain region involved in response inhibition, attentional control and the detection of relevant cues when performing a task the following day
- Animal research found that nighttime exposure to 5 lux for three weeks in a row produced depression-like symptoms and impaired cognition
By Dr. Mercola
Inside the suprachiasmatic nucleus (SCN) of your brain, which is part of your hypothalamus, resides your master biological clock. Based on signals of light and darkness, your SCN tells your pineal gland when it’s time to secrete melatonin, and when to turn it off.
Your melatonin level inversely rises and falls with light and darkness, and both your physical and mental health is intricately tied to this rhythm of light and dark.
When it’s dark, your melatonin levels increase, which is why you may feel tired when the sun starts to set. Conversely, when you’re exposed to bright artificial lighting at night, including blue light emitted from TVs and electronic screens, you may have trouble falling asleep due to suppressed melatonin levels.
Many sleep problems can be resolved by making sure you avoid blue light exposure after sunset and sleep in total darkness.
Interestingly, being exposed to very dim light during sleep — even if it does not noticeably seem to impair your sleep — may also affect your brain function and cognition during the day.
Minute Amounts of Light During Sleep Can Affect Cognition
I’ve been a long-time advocate of sleeping in TOTAL darkness, and an interesting study1 published in Scientific Reports highlights the importance of this recommendation — not just for solid sleep, but also for cognitive health.
In this study, 20 healthy men slept in a laboratory shrouded in complete darkness for two nights in a row. On the third night, they were exposed to a dim light of either 5 or 10 lux while sleeping.
To get an idea of how dim a light intensity of 5 or 10 lux is, 1 lux is equal to the brightness of a surface illuminated by one candle, placed 1 meter (3.28 feet) away from the surface. Twilight is just below 11 lux, whereas an object illuminated by the light of the full moon is about one-tenth of a lux.2
After the second and third nights, the participants performed working memory tests (so-called n-back tests) while undergoing functional magnetic resonance imaging (fMRI). The goal was to evaluate the effects of dim light exposure during sleep on functional brain activation during a working memory task the next day.
When sleeping under 10 lux light conditions, there was decreased activation in the right inferior frontal gyrus, an area of your brain involved in response inhibition, attentional control and the detection of relevant cues when performing a task.3
Exposure to 5-lux light had no statistically significant effect on the participants’ brain activity. In other words, past a certain point of very dim light, nighttime light exposure can have a direct influence on your brain function, specifically your cognition and working memory.
Nighttime Light — A Hazardous ‘Pollutant’
According to the authors of this study:
“Nighttime light is now considered to be one of the fastest growing pollutants, and the invasion of artificial light into previously unlit areas is threatening the soundness of human health and sleep.
Nighttime artificial lighting in cities is divided into three types: sky glow, trespass and glow. Light trespass refers to unwanted direct lighting of an area, and it occurs when unwanted light spills over into another property or dwelling and causes sleep interference, negative influence on one’s well-being …
Several studies have also shown that light pollution and shift work are tentative risk factors for cardiovascular disease, breast cancer, ovarian cancer, gastrointestinal disease and metabolic syndrome …”
Fortunately, the detrimental effects of nighttime light pollution are starting to gain recognition, and some countries have even adopted regulations to reduce nighttime light in residential areas.
Guidelines issued by the Commission Internationale de l’Eclairage (CIE), Illuminating Engineering Society of North America (IESNA) and Institution of Lighting Engineers (ILE), have an upper brightness limit for light trespass of 2, 3 and 5 lux in in residential areas respectively.
Chronic Exposure to Light During Sleep May Cause Pronounced Effects on Cognition
The study in question was done to investigate whether these limits are sufficient to reduce sleep and cognitive problems associated with nighttime light pollution.
While limits of 5 lux or less appear sufficient, they discovered that exposure to 10 lux may produce adverse brain effects even if there are no subjective, outward symptoms of impairment. As noted by the authors:
“This study is meaningful because it is the first to scientifically identify the effect of the dim light at night on human brain function and cognition. It is noteworthy that the brain activation was altered after only a single night of light exposure.
This suggests that the chronic exposure to the light at night for many nights might have caused more pronounced effects on the brain and cognition … The interesting finding in the 10 lux group … was the discrepancy between the n-back task and fMRI results.
The decrease of the brain activation in fMRI in the frontal lobe without significant finding in the n-back task of 10 lux group suggests that the absence of evidence of subjective or objective cognitive dysfunction does not necessarily mean that the brain is functioning normally.
This indicates that certain exposure to dim light might influence brain function for cognition even if there is no significant impairment in subjective symptoms (or even in an objective neurocognitive function test).”
Lack of Symptoms Does Not Mean You’re Unaffected
In other words, what they discovered is that while you might not notice a problem, your brain is still not working normally or optimally. The reason for this is not entirely clear. One possibility is that the decrease in brain activity is related to a reduction in deep sleep, most likely brought on by disrupted melatonin secretion.
Another possibility is that light exposure at night somehow directly induces cognitive dysfunction (opposed to indirectly, via sleep disturbance). One mouse study found that aberrant light exposure caused learning impairments and mood disturbances by directly affecting melanopsin-expressing neurons.
These melanopsin-expressing neurons, also known as photosensitive retinal ganglion cells, found in the retina of the eye, are not involved in vision. Instead, they play a role in circadian rhythm synchronization and the suppression or release of melatonin.
These retinal cells are also linked to the hypothalamus and the limbic regions, including the amygdala. Other researchers have suggested dim light at night could have a direct influence on brain function via some process related to these photosensitive retinal ganglion cells.
Even 5 Lux Could Potentially Contribute to Depressed Mood
In one study, hamsters exposed to 5 lux at night for four weeks altered their neuronal structure, which in turn caused the hamsters to exhibit symptoms of depression. Another animal study also found that nighttime exposure to 5 lux — this time for three weeks in a row — produced both depression-like symptoms and impaired cognition.
Neurons in the hippocampus also shrunk in length, an effect primarily attributed to a decrease in brain-derived neurotrophic factor (BDNF).
BDNF is a remarkable rejuvenator in several respects. Not only does it preserve existing brain cells, it also activates brain stem cells to convert into new neurons, effectively making your brain grow larger.
The study in question basically showed that nighttime light exposure of just 5 lux effectively inhibited this important brain rejuvenator, causing neuronal shrinkage in the hippocampus, a brain region involved in both long-term memory storage and the regulation of emotions.
In light of such evidence (no pun intended), it would certainly be prudent to evaluate your nighttime light exposure if you “feel blue” or struggle with any kind of depression. Even a seemingly insignificant amount of light could be interfering with your melatonin and/or BDNF production, causing a mood imbalance.
Even the display on your alarm clock could be causing you trouble without you realizing it. I used to recommend covering up digital alarm clocks but know from personal experience how inconvenient that can be, especially if you have blackout drapes and sleep in pitch blackness like I do. I finally discovered a perfect solution — an alarm clock for blind people. It has a very large button that is easy to find, and when you tap it, it audibly tells you the time.
Light-Sensing Pigment in Your Eyes Help Direct Waking/Sleeping Cycles
The wavelength of light also matters to your health, not just the brightness itself. The wavelength gives light its color. Red and orange light have longer wavelengths while green and blue are shorter. The influence of varying wavelengths of light on brain function was demonstrated in a 2014 Belgian study,4 which showed that orange light serves as a powerful “wake-up call” for your entire body.
Again, the influence of light wavelengths has to do with the photosensitive retinal ganglion cells in your eye, which produce a light-sensing pigment called melanopsin. This pigment plays an important role in directing your waking and sleeping cycles. As reported by New Scientist:5
“To find out how melanopsin wakes up the brain, Gilles Vandewalle at the University of Liege, Belgium, and his team gave 16 people a 10-minute blast of blue or orange light while they performed a memory test in an fMRI scanner. They were then blindfolded for 70 minutes, before being retested under a green light.
People initially exposed to orange light had greater brain activity in several regions related to alertness and cognition when they were retested, compared with those pre-exposed to blue light. Vandewalle thinks that melanopsin is acting as a kind of switch, sending different signals to the brain depending on its state.
Orange light, which has the longer wavelength, is known to make the pigment more light-sensitive, but blue light has the opposite effect. Green light lies somewhere in the middle. The findings suggest that pre-exposure to orange light pushes the balance towards the more light-sensitive form of melanopsin, enhancing the response in the brain.”
This kind of information becomes particularly important if you work the night shift. By carefully selecting the type of artificial light you expose yourself to at different times, you can ameliorate at least some of the adverse effects associated with shift work. For more details, please see my previous article, “How to Counteract the Ill Effects of Working the Night Shift.”
How to Make Digital Screens Healthier
In addition to reducing the light in your sleeping environment it is also helpful to eliminate blue light from artificial sources like watching TV at night. You can do this be picking up a $9 pair of UVEX blue blockers on Amazon. It is far more convenient, though, to use blue light blocking software on your computer monitor after sunset.
Many use f.lux to do this, but I have a great surprise for you as I have found a FAR better alternative that was created by Daniel Georgiev, a 22-year-old Bulgarian programmer that Ben Greenfield introduced to me.
He is one of the rare people that already knew most of the information in this article. He was using f.lux but was very frustrated with the controls. He attempted to contact the f.lux programmers but they never got back to him. So, he created a massively superior alternative called Iris. It is free, but you’ll want to pay the $2 and reward Daniel with the donation. You can purchase the $2 Iris software here.
Iris is better because it has three levels of blue blocking below f.lux: dim incandescent, candle and ember. I have been using ember after sunset and measured the spectrum and it blocked nearly all light below 550 nanometers (nm), which is spectacular, as you can see in the image below when I measured it on my monitor in the ember setting.
When I measured the f.lux at its lowest setting of incandescent it showed loads of blue light coming through, all the way down to as you can clearly see in the images below.
So, if you are serious about protecting your vision you will abandon f.lux software and switch to Iris. I have been using it for about three months now, and even though I have very good vision at the age of 62 and don’t require reading glasses, my visual acuity seems to have dramatically increased. I believe this is because I am not exposing my retina to the damaging effects of blue light after sunset.
Nighttime LED Light Pollution May Be Particularly Harmful
As detailed in my interview with Dr. Alexander Wunsch, a world class expert on photobiology, lighting is an important health consideration. Natural sunlight simply cannot be beat, but unless you spend a majority of your time outside, you’ll need to give some serious consideration to the kind of artificial lighting you use at home and at work.
Light-emitting diodes (LEDs) have now become a standard indoor light source, thanks to their energy efficiency. However, the price society will have to pay in terms of health could end up being enormous. If you missed this interview, I strongly recommend taking the time to listen to it, and read through the accompanying article, “How LED Lighting May Compromise Your Health.” It’s a really crucial issue.
In summary, light-emitting diode (LED) lighting may promote age-related macular degeneration (AMD), the leading cause of blindness, and exacerbate health problems rooted in mitochondrial dysfunction, including obesity, diabetes, heart disease and cancer. For this reason, LEDs are best avoided.
One rare exception is if you work the night shift. In this case, to help establish a new circadian rhythm you’ll want a small amount (just 15 to 30 minutes’ worth) of blue light exposure first thing upon waking (which if you work nights will typically be in the evening, when it’s dark out), along with incandescent light for the longer wavelengths, which include near-infrared. I describe all of this in more detail in the shift work article hyperlinked above. For all others, LED lighting is simply not a good idea.
Environmental Near-Infrared Light Exposure Is Important for Health
As explained by Wunsch, the vast majority of the energy your body needs to maintain systemic equilibrium actually comes from environmental infrared light exposure. The near-infrared range of light found not only in natural sunlight but also in incandescent light bulbs and halogens benefit your health in a number of important ways, including priming the cells in your retina for repair and regeneration.
LEDs emit primarily blue light, which reduces melatonin production in both your pineal gland and in your retina. In your retina, melatonin helps with regeneration, which is why LEDs are so harmful to your vision. Blue light also creates reactive oxygen species (ROS) that, when generated in excess, cause damage. So, when using LEDs, you end up with increased damage and decreased repair and regeneration throughout your body, not just in your eyes.
LED light exposure that is not balanced with full sunlight loaded with the red parts of the spectrum is always damaging to your biology, but even more so at night. Hence lighting your living room, kitchen and dining room — any room where you spend most of your evening — is best done using good old-fashioned incandescent light bulbs, halogens and candles.
Save the energy-saving LEDs for your garage, closets and hallways where exposure is minimal. More detailed information on how to identify the healthiest light bulbs can be found in “How LED Lighting May Compromise Your Health.”
To Optimize Your Sleep and Protect Your Brain Health, Sleep in Total Darkness
When your circadian rhythm is disrupted, your body produces less melatonin, which means it has less ability to fight cancer, and less protection against free radicals that may accelerate aging and disease. So if you’re having even slight trouble sleeping, I suggest you review my 33 Secrets to a Good Night’s Sleep for more guidance on how to improve your sleep-wake cycle.
Even if you think you’re sleeping OK, but know you have light pollution entering your room at night, consider taking steps to block it, since being asymptomatic does not mean your brain is unaffected and functioning normally. Also consider cleaning up the lighting sources in your home and office to avoid unnecessary harm.
As mentioned, AMD is a very real and serious side effect of being chronically exposed to LED lighting, especially if you’re also getting very little natural sunlight exposure.