In Part 4 of this article series, I summarize the lighting equipment I have experimented with in my efforts to expose my eyes to the color spectrum, intensity, timing, and duration of light that has been reported in scientific studies to enhance nighttime melatonin secretion, growth hormone secretion, slow-wave sleep, the waking cortisol response, and mood as described in Parts 1-3. I also make some equipment suggestions in the case readers want to experiment with enhancing their personal lighting environment.
Taking action: color spectrum, intensity, timing, and duration of retinal light exposure
From my research for this article series, I am convinced that humans have not made a concerted effort to determine the exact parameters of retinal light exposure with indoor lighting that will replicate the benefits of the sunlight exposure on clear, sunny days outdoors—the environment in which humans presumably evolved. More research may be needed on this topic. So, for now, we must self-experiment to determine which parameters optimize our biomarkers, function, and sense of well-being. In this section, I present my suggestions for starting places for your own self-experimentation, focusing on four parameters of retinal light exposure:
- Color spectrum (i.e. wavelength profile)
To re-word these four parameters in practical terms, one might ask the questions:
- “Which bulbs should I use?” (Color spectrum)
- “How bright should they be? (Intensity)
- “When should I be exposed to their light?” (Timing)
- “How long should my exposure be? (Duration)
Because the length of this article series is becoming considerable, and this section is dedicated to actionable details, I will endeavor communicate the following practical information concisely.
Color spectrum of light (I suggest 5000 K “daylight” temperature)
By the phrase “color spectrum”, I’m referring to the wavelength profile of the light emitted by the bulb. As you might know from your understanding of light, white light contains multiple different colors in the visible light spectrum, so one can say that white light has a color profile that includes many different specific colors or wavelength of light. However, when light does not look perfectly white, there is a greater intensity of some colors being emitted than other colors, and this difference causes the light to look like a color other than white.
“K” values on light bulbs refer to the color spectrum emitted by lighting equipment. Lower K values—such as “3000 K”—are called “warmer” because they emit more red light and less blue light. Higher K values (e.g. 6000 K) are called “cooler” because they emit more blue light and less red light. The color spectrum is sometimes referred to as color “temperature”. The Wikipedia article about color temperature offers a good explanation of color spectra and K values.
Many different wavelengths of light have been explored in scientific studies of animals and humans. I suggest self-experimenting with only one type of color spectrum: light bulbs rated at 5000 K color temperature. These are sometimes called “daylight” bulbs. This is the main color spectrum I tested in my personal experiments, and they appear to be consistently associated with desirable outcomes in scientific studies. The other color I experimented with was 460-470 nm (pure blue light) using LEDs, which I recommend against experimenting with (for now). I write more on that next.
A caution about blue light
While I currently recommend against self-experimenting with blue light, one remarkable observation is that pure blue light—at approximately 470 nanometers (nm)—appears to be the cause of at least some of the promising effects of retinal bright light exposure. As mentioned in Part 1, Figueiro and Rea (2012) reported that 80 minutes of exposure to 470 nm blue light at only 40 lux (relatively dim light) increased the cortisol awakening response in sleep-deprived adolescents by about 60%. In a different study, Cajochen et al. (2005) reported that blue light at 460 nm caused a greater suppression of melatonin, and significantly increased alerting response, core body temperature, and heart rate, than did 550 nm (green) light.
Personally, I experimented with 250 lux of blue light at 470 nm using the Ayo blue light therapy glasses. However, when using them daily for 40-80 minutes, my eyes would have a dull aching sensation around 6 AM the next day. This concerning experience takes a day of not using the device for it to go away. This is the primary reason why I recommend against self-experimenting with blue light. I didn’t find any studies offering convincing evidence that blue light therapy is harmful. This mildly negative personal experience might have been caused by my simply using too bright (250 lux) of blue light, and I might have avoided the aching sensation by exposing my eyes to only 50-100 lux of 470 nm (blue) light.
While blue light seems particularly efficient for achieving the benefits associated with retinal bright light exposure, I did find some concerning reports in the science literature about what has been called the “blue light hazard” (Wikipedia) under certain circumstances. For example, Nakamura et al. (2018) reported some concerning effects of retinal blue light exposure in mice, though the light exposure lasted 3 days at 1100 lux—quite extreme compared to the 40-80 minutes of 250 lux to which I was exposing my eyes. At first, I dismissed these concerns as not being relevant for humans. Most studies I found were in mice—usually albino mice whose eyes are less protected from light damage—and the studies often used extreme light exposure regimens, such as the 3 days of 1100 lux blue light used by Nakamura et al. (2018).
I suspect that the discomfort I experienced from blue-only light might be due to brightness insufficient to constrict the pupil—a method for the eye to protect the retina from excessive light exposure. However, a study by Cajochen et al. (2005) found that blue (470 nm) light caused a greater constriction of the pupil than did green (550 nm) light, so perhaps my suspicion is incorrect.
In any case, I have never experienced this aching sensation from exposure of up to 20,000 lux from 5000 K LED light for up to 4 hours per day while indoors.
Intensity of light (I suggest trying 2,000 – 20,000 lux)
Once we’ve chosen a color spectrum for the bulb, we’ll want to know how bright we want it to be. This involves knowing how much light reaches our eyes from the bulbs. The intensity of light striking a surface is often measured in units called “lux”. This is the unit I used to compare the intensities of light exposure during my self-experimentation and is the most common unit of measure I observed in the science literature.
However, light bulbs are often assessed in terms of how much light they emit—not how much light falls on a surface. The name for how much light is emitted from an object is called “luminous flux” (see Wikipedia for more details). Luminous flux is often reported in the unit “lumens”, and this unit is often used to rate the brightness of light bulbs.
It makes sense that bulbs are rated by how much light they emit because the alternative measure—how much light falls on a surface from the light source—depends on the distance that surface is from the light source. That can vary by application. When you shop for light bulbs, you will usually see the lumen rating on the packaging, and it is often printed directly on the bulb.
In deciding which intensity (in lux) to design for our personal environments, we’ll also want to know what light intensity at 5000 K will effectively suppress melatonin secretion during the time periods we want to be awake and stay alert. Fortunately, Hanford and Figueiro (2013) summarized some research literature on which colors of light suppress melatonin by 50% after 1 hour of exposure, and at what intensities of light those colors do so. These are summarized in Table 2 of their paper, presented below (also presented in Part 1).
In short, 470 nm blue light is by far the most efficient for suppressing melatonin levels, with only 50 lux needed to suppress melatonin by 50% after 1 hour of exposure. The next most efficient color spectrum in the above table is the 5200 K LED bulb, which only needed 430 lux to suppress melatonin by 50%. This result reported in this table is one of the main reasons I recommend using 5000 K light bulbs and why I used it in my personal experimentation.
Justification for 2,000-20,000 lux
Besides suppressing melatonin and enhancing the cortisol waking response, it is not clear from the scientific literature what intensity is optimal for indoor, daytime retinal light exposure. The above table reported what illuminance (lux) level was associated in a 50% suppression of melatonin secretion. But do we want to suppress melatonin secretion by more than 50%? Or less than that? I’m not sure. I have the impression that researchers have not attempted to test for the optimal indoor light intensity for human health. Here are some observations that have helped me to decide on a suggestion for personal experimentation of somewhere between 2,000 lux and 20,000 lux:
- From Table 2 above by Hanford and Figueiro (2013), approximately 5000 K light at 430 lux reduced melatonin secretion by 50% after 1 hour of exposure. I presume a higher percent reduction is desired during the day (especially shortly after waking), so I presume greater lux than 430 at 5000 K color spectrum is desired (additional data below suggest that higher is desirable).
- Recall from Part 1 that Mishima et al. (2001). used 2,500 lux to restore nighttime melatonin secretion in elderly insomniacs to youthful levels (see Figure 1).
- Leproult et al. (2001) reported that using 2,000-4,000 lux enhanced waking cortisol secretion and limited the decline in alertness associated with sleep deprivation.
- Penders et al., (2016) reported that he intensity of light reported to be effective in helping to improve depression was 5,000+ lux.
- From my measurement data, normal daytime light intensity on a cloudy day is approximately 4,000-8,000 lux.
- Normal daytime light intensity on a clear day in the shade is approximately 20,000 lux (see Wikipedia, and this is consistent with my measurement data).
A very important aspect of light intensity (lux) to keep in mind is that it declines rapidly as you move away from the source. Therefore, a light visor placed just inches away from the eyes can provide far higher lux values than standard overhead lighting with the same power (and electricity cost). The distance between the light source and the eyes is dramatically different between these two methods. During my self-experimentation, I tested lux values using two methods: an Android smartphone app called Lux Meter and a hand-held lux meter by Dr. Meter. I strongly suggest the latter device by Dr. Meter (or something like it), but the Lux Meter app seems to give measurements that are close to those given by the Dr. Meter.
I want to emphasize the importance of using a lux meter to assess the illuminance (lux) level of light at the level of your eyes. These figures can vary dramatically based on whether you are looking at the light, partially away from the light, or completely away from the light source. They also decline dramatically when moving away from the light source. This exponential decay is not a normal phenomenon we experience in what I call the “linear life” of most things we interact with. And if you’re determined to achieve some of the positive results I’ve reported in this article series, you will be rightly motivated to measure what you’re trying to manage (something emphasized here at Long Life Labs). If I were trying to lose weight, it would be much easier to measure my effectiveness of I weigh myself periodically. I strongly recommend taking lux measurements with a reliable and inexpensive device if you’re committed to experimenting for yourself.
An interesting point to keep in mind about light intensity (lux) is that outdoor sunshine on a sunny day can be as high as 100,000 lux, even without looking at the sun. Compare this to what I’ve observed for common indoor overhead fluorescent lighting at 20-200 lux, and you can see the extreme difference in ambient lighting intensity when comparing modern indoor lighting to natural outdoor light (the environment in which human eyes and brains evolved).
Timing of light exposure (I suggest within an hour of waking)
Earlier, I summarized one study reporting that light exposure around the normal waking time caused a substantial increase (> 50%) in the waking cortisol response, while being exposed to this light later resulted in no change in that response (Leproult et al, 2001). So there may be something remarkable about retinal bright light exposure shortly after the normal waking time (to coincide with the cortisol awakening response).
However, other research has reported some improvements with retinal bright light exposure at different times of the day. For example, Mishima et al. (2001) reported restoration of nocturnal melatonin secretion in elderly insomniacs used bright light exposure between 10 AM and 12 PM, then again between 2 PM and 4 PM. I presume 10 AM is not more than a few hours after waking. And Hanford and Figueiro (2013) summarized in Table 1, studies utilizing many times throughout the day, usually 2-3 hours of exposure at a time, starting at 7 AM, 8 AM, 9 AM, 9:30 AM, 4 PM, 5 PM 5:30 PM, and 6 PM. Again, the first exposure time in this experiment was significantly before noon (in this case, as early as 7 AM).
Given the above studies, combined with the assumption that humans evolved to function best under outdoor lighting conditions, it seems reasonable to begin light exposure shortly after normal waking time (within 60 minutes of waking).
Duration of light exposure (I recommend 1-6 hours/day)
I did not perform exhaustive research on what duration of light exposure would be optimal, partly because very few studies tested continuous bright light exposure during their experiments, even when trying to help people with a terminal illness (such as Alzheimer’s disease). I presume that a more aggressive protocol would be tested (i.e. 8-16 hours of bright retinal light exposure per day) for people who are otherwise going to die of their condition. But I did not find these studies. In other words, there appears to be very little data to inform the optimal duration of retinal bright light exposure. But I did find a few studies that tested different durations of light exposure.
Justification for 1-6 hours/day
Earlier, I summarized a study conducted by Wehr (1991), in which he exposed 8 volunteers to either 10 hours of light per day (simulating shorter winter daylight hours) or 16 hours of light (simulating the longer, summertime daylight hours). He found that the longer, 16-hour light period was associated with 7.7 hours of sleep per night, while the 10-hour light period was associated with a remarkable 11.0 hours of sleep per night—43% more than the summer simulation. This small study suggests that if we want to be awake, active, and alert for a longer period of time each day, we want to suppress melatonin with bright light for a longer period of time during the day. Mishima et al. (2001) reported restoration of nocturnal melatonin secretion in elderly insomniacs used bright light exposure between 10 AM and 12 PM, then again between 2 PM and 4 PM, for a total of 4 hours of exposure per day. And Hanford and Figueiro (2013) reported many durations utilizing several different durations tested in different studies, usually consisting of 2-3 hours of exposure at a time.
Given the above, and assuming that humans evolved to function optimally outdoors where light intensity was around 1,000 times higher than common indoor lighting and remained that bright for over 8 hours each day, I recommend readers self-experiment with light exposure periods starting with 1-6 hours. Given that natural, outdoor lighting from the sun can easily last much longer than 4 hours, I am curious to know how that intensity and duration of light exposure might affect a person when it is experienced via indoor lighting. But achieving indoor illuminance at this intensity and duration can be expensive and cumbersome. Most of the lighting equipment that can achieve the desired illuminance (lux) is expensive to purchase (a few hundred U.S. dollars for a single light source) and expensive to operate (approximately 200-250 watts using LEDs). Moreover, the equipment is often large and cumbersome to move.
During your personal experimentation with enhanced personal indoor lighting, you will find that unless you have a very good light visor (which I’m not sure exists yet), you’ll need to stay relatively stationary in front of your lighting device for a prolonged period of time. In my personal experience, sitting still for so long is not my favorite way of living. Having one lighting device that reaches 10,000 lux at my treadmill desk is helpful, but I do not take this lighting device with me around the house into the kitchen or living room. When experimenting with interior lighting enhancement, I concluded that the sun is indeed an excellent stationary lighting source and one we often take for granted, given how expensive and cumbersome it can be to re-create even a fraction of the illuminance the sun provides on most days.
Additional research, (including and especially personal experimentation) should help us optimize the optimal duration of daily bright light exposure indoors. And I strongly suspect a bright, light visor with a very long battery life might make these experiments much more convenient to perform because that light source would be very mobile and enable one to move about one’s daily life without moving or re-orienting cumbersome lighting equipment. Moreover, because a visor can be positioned so close to the eyes, the electricity cost of its operation (recharging its batteries) could be far below the larger LED devices I primarily used during my experimentation.
My personal retinal light exposure regimen
Over the past several months, I have been experimenting with all four of the above variables to optimize my levels of energy, alertness, emotional affect, and subjective sleep quality. After many personal experiments, my favorite daily protocol is as follows:
- Within an hour of waking, I expose my eyes to about 10,000 lux of 5000 K light from a 240 watt LED stadium light mounted on a tripod stand. This must stay within approximately 5 feet of my face to achieve 10,000 lux.
- Continue the light exposure for 2-4 hours per day, usually about 30-90 minutes at a time (I take breaks from computer work after about 90 minutes).
One interesting observation about how this protocol makes me feel is how I feel the most tired about 30 minutes after lunch. This seems like a common occurrence for many people. However, the interesting part is that the 30 minutes before lunch is the most reliable period when I am not exposed to bright light. So, 30 minutes of meal preparation combined with 20 minutes of eating adds up to nearly an hour in very dim lighting (< 25 lux).
With a colleague, I explored the possibility that the afternoon sleepiness that so commonly affects humans might be at least in part due to the reduced light exposure when preparing and eating the lunch meal (usually indoors). I have noticed that when I prepare a meal ahead of time, and eat the meal in front of the bright light, I rarely feel tired enough that I want to nap.
Below are some more observations about my personal lighting enhancement experiments which you might be interested in.
Subjective effects of my indoor lighting experiments
Since self-experimenting, I have associated multiple improvements in my quality of life with the above retinal light exposure regimen.
- Reversal of lethargy
- Before light experimentation, I would find it difficult to do more work than my paid work responsibilities for my clients. Once that work was done, I had little energy to do any additional work. Since using the above light exposure regimen, I find that I can work several hours longer each day, and the quality of my work—and the speed at which I work—have all increased. I suspect this is caused by the increased alertness and reduced fatigue associated with retinal bright light exposure.
- Improved cognitive function (see improved work quality above)
- Better sleep: sleeping 7.0 to 8.5 hours in one session, rather than broken up sleep
- Less sleep: sleeping 7.0-8.5 hours/day, instead of 8.5 to 10 hours
- I now notice that I feel better rested on less sleep each night. I now nap less frequently, but when I do nap, it’s for a shorter period of time (40 minutes instead of 90).
- Reduced sleep latency (falling asleep faster)
- I have noticed that I am tired enough at bedtime that it takes me almost no time to fall asleep. It now feels satisfying to feel so tired when I go to sleep now.
- Earlier waking time (6-8 AM instead of 8-9 AM)
- Easier awakening (less grogginess).
- Before light experimentation, on about 50% of my days, I would experience grogginess and lethargy for 1-3 hours after awakening. The incidence of this lethargy has been reduced to less than 25% of my days, and it is shorter when I do experience it.
- Reduction of afternoon sleepiness
- I still experience afternoon sleepiness about every third day, but both its frequency and intensity has been reduced. I now rarely take naps (maybe one day per week).
- Increased rate of dreaming I can recall the next day (3-4 days/week instead of 0)
- For the past ~10 years, I am accustomed to experiencing dreams that I can recall only very rarely. But over the past few months of bright light exposure, the frequency of recallable dreams has increased dramatically to 3-4 nights per week.
- Improved mood (happier, more patient)
- I have always been a relatively happy, patient person. But the last few months of light exposure experimentation has been associated with a qualitative improvement in my mood. I feel better able to happily persist at an essentially infinite list of work projects and ambitions. I feel I am even more patient with the irrational behavior and emotional outbursts of other people. My life feels at least some degree happier and more satisfying.
I cannot think of any negative effects that might be associated with my retinal light exposure experimentation, except eye aches from pure blue light exposure.
This concludes this article series on the prospect that humans are capable of experiencing and suffering from a deficiency of retinal light exposure. There appears to be a significant volume of research that supports this hypothesis, and my personal experiences with enhancing my personal lighting environment have had multiple positive effects with no noticeable negative effects on my health.
Origin of and support for this article
Interest in this article arose from a combination of my ongoing interest in this topic and conversations with Dave Gobel, CEO of Methuselah Foundation. This article series is based on research that was sponsored by Methuselah Foundation.
I remind readers that the above report includes my personal research and self-experimentation, i.e. an “n-of-1” experiment. Your experience with the same parameters may give you very different results. Nothing here is to be taken as a recommendation in any particular person’s case. This report is intended to inspire a starting point for your own personal experimentation or as a basis for designing clinical or experimental research.