From left to right: Dr. Richard Hansler, Mr. Vilnis Kubulins and Dr. Edward Carome.

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How Light at Night Is Stealing Our Melatonin

How serious is this problem of using light at night? Why hasn’t this problem shown up much earlier? We have had artificial light for well over 100 years and we didn’t seem to have any problems. Partly it was because the early forms of artificial light were flames (torches, candles, gas lights) that produced golden-colored light. It was almost devoid of the blue rays now known to be most effective in suppressing melatonin. Edison’s original incandescent lamps had carbon filaments that could not be operated at very high temperature. Again their light had relatively little blue in it.

Each new generation of light bulbs has had more blue rays in their output. When the carbon filaments were replaced by tungsten, the temperature could be increased to make whiter light. Next came fluorescent lights, many of which were very blue. White LED lights really are a different kind of fluorescent light. The LED chip produces blue light, but it is coated with a layer of phosphor that absorbs much of the blue light, which excites the phosphor to emit green, yellow, and red light. The combination appears as white light. The thickness of the phosphor coating controls how much blue light escapes. In general, LED light bulbs produce a larger fraction of blue light than incandescent bulbs.

The color temperature of a lamp is a way of rating how yellow or blue the light appears. Incandescent lamps appear yellow with a color temperature of about 2700K. Many LEDs are available that are more blue and rate at 3000K. It is a confusing measurement since it is opposite to ordinary temperature measured with a thermometer. Cool-colored light with a lot of blue corresponds to high color temperature, and warm colors with a lot of yellow and red correspond to low color temperature.

We measured the energy output of a number of modern light bulbs and multiplied by the response curve of the sensors in the eye that control melatonin production to get the amount of blue light and by the response curve of the combination of the rods and cones (parts of the eye) that produce vision to get total light. The ratio is what we report as the percent of melatonin-suppressing blue light.

Source                          Percent Melatonin                        Lumen

Type                              Suppressing Light                        Output


Ecosmart 14W CFL                  41%                                     922

5000K 60W Equiv.


GE Soft White 41W Halogen   31%                                     812

2700K 60W Equiv.


GE Soft White 60W                  29%                                     840

Incandescent 2800K


LS “Goodnight” 12W LED        22%                                     918

2500K 60W Equiv.


LowBlueLights 7W LED              4%                                   371

1500K 25W Equiv.

Note that even some of the bulbs that claim to be “low blue,” such as the “Goodnight” bulb from Lighting Sciences, really are not much lower than an ordinary incandescent bulb. The real question is whether the lighting found in most homes is capable of suppressing melatonin.

One of the most important studies (PMID22017511) was financed by Philips, the world’s largest producer of lights of all types, and carried out by the University of Surrey in the United Kingdom. They found that exposure to the types of lighting in the typical English home caused significant increase in time to fall asleep and caused significant delays in the timing of the circadian rhythm. They confirmed that having more blue light increased the effect. This implies a decrease in melatonin production.

Harvard Medical School has been the place where the most impressive studies of the effect of light on humans have been done. Searching in PubMed with the three words “Harvard light melatonin” finds 85 papers starting in 1991. Charles Czeisler, MD, is the author of more than 230 papers and is probably the most prominent sleep scientist in the world. He is the head of the committee at the National Institutes of Health (NIH) concerned with sleep. My efforts to get NIH to conduct studies on the benefit of blocking blue light with orange glasses in the evening have not yet been successful.

My purpose next is to review the evidence that low levels of light as found in a typical home are sufficient to significantly reduce both the amount of melatonin and the time when it is present in the bloodstream and brain.

The basic unit of light is the lumen. It has the same dimensions as the watt. It is the rate of delivery of light energy. A burning candle produces four Pi (12.566) lumens. A lux is one lumen per square meter. If you hold a white paper one meter (39.37 inches) from a candle, it will give you an idea of what one lux looks like. It’s pretty dim.

In a 1998 study (PMID9579664), it was discovered that ordinary room light (180 lux) was sufficient to phase shift the circadian rhythm (change the setting of the internal clock) of melatonin and cortisol production and phase shift by the same amount the core body temperature rhythm. The body temperature drops to a minimum during the night, controlled by melatonin.

In 2000 a study (PMID10922269) of the effect of light intensity on circadian phase shift and melatonin suppression found intensity below about 80 lux had no effect while above about 180 lux caused total suppression and significant phase shifting. This was before it was known that it is primarily blue light that suppresses melatonin, so we don’t know how much blue light was in the sources used for these experiments. It must have been a relatively large fraction, as the light was from cool white fluorescent lamps.

In a 2007 study (PMID17502598), it was found that entrainment (locking in the circadian rhythm) to a daily schedule that was shifted by one hour could be accomplished by a daily exposure to as little as 100 lux of white light. Intensity of 25 lux had no effect. This shows again that the light levels found in homes are more than sufficient to have a major effect on the body.

A 2011 study (PMID21193540) found that evening exposure to moderate room light intensity (200 lux) experienced a delay in the start of the flow of melatonin and a reduction of about 90 minutes in the time that melatonin was present in the bloodstream.

A 2015 paper (PMID25535358) reported that reading an e-book on a light emitting reader in the period before bedtime resulted in suppression of melatonin, delay in falling asleep, and reduced alertness in the morning.

A 2009 paper (PMID19675108) reported the results of a study of how foods affect the concentration of melatonin in overnight urine. The only result of interest was that red meat appeared to reduce the amount of melatonin by about 20% between the highest and lowest quartiles of consumption. This may explain the observed increase in breast cancer related to increased consumption of red meat. Compared to the very strong effect of exposure to light, where only 180 lux caused compete suppression, red meat eating may be negligible.

These are all papers coming from the group at Harvard Medical School. Searching PubMed for “Surrey light melatonin” finds 80 papers (compared to Harvard’s 85 papers) beginning in 1987. The University of Surrey is probably Europe’s most notable center for sleep research. The names of Arendt and Skeen are the ones most quoted.

A 1987 paper (PMID3692439) reported that 300 lux of white light significantly suppressed melatonin. There is no description of the type of source used, so we don’t know whether it was rich or poor in blue light.

In 1995 the group at Surrey reported (PMID8795806) the results of a significant study in which six male subjects were kept in continuous dim light for 21 days without contact with their surroundings, except for a digital clock. All six became free running (their circadian rhythm was no longer in synchrony with the sun) as measured by when melatonin flow started and by when lowest core body temperature occurred. The average day length was found to be 24.26 +/- 0.05 hours. This study firmly established that without the daily morning light signal, the circadian cycle will go out of synchronization with the earth’s rotation.

A game-changing study (PMID11507175) that is the basis of all of my LowBlueLight activities for the past 10 years was published in 2001. The ability of monochromatic light of different wavelengths to suppress melatonin was measured in 22 volunteers. The response curve differed from that of either the rods or the cones, so it was concluded that there is another photoreceptor system in humans that responds most strongly to blue light.

An almost identical, larger (37 females, 35 males) study (PMID11487664) was done at Thomas Jefferson University and reported at the same time. It also found that melatonin is most strongly suppressed by blue light and that formerly-unknown sensors that differ from the rods and the cones are responsible.

A 2007 study (PMID18075803) compared the ability of 479nm (nanometer) monochromatic (blue) light to suppress melatonin with the ability of white light having the same amount of 479nm light to suppress melatonin. The white light was more effective. Researchers interpreted this to mean the other sensors (rods and cones) also contribute to melatonin suppression. This agrees with the results of a 2006 study (PMID16687299) by Brainard et al. that found that red light (640nm) was able to weakly suppress melatonin. One may argue that the pigment that activated the retinal ganglion cells, melanopsin, may weakly absorb red light and therefore leave the rods and cones out of it.

An interesting 2012 paper (PMID22306975) reported the melatonin suppressing ability of monochromatic blue light (479nm) by itself or in combination with red light (627nm). Adding the red light did not have any effect on the suppression caused by the blue light, nor did the red light by itself.

This was the last of the papers from the University of Surrey.

A 2007 study from Japan (PMID17364578) looked at whether there was a change in the melatonin suppressing ability of 1,000 lux in the winter and in the summer in a climate where there was a large change in the day length (factor of three). Researchers observed approximately twice the effect in winter as in summer. This is in agreement with other studies showing that prior exposure to bright light reduces the melatonin suppression effect of light.

In the above reported studies, the light was typically coming from overhead-mounted light fixtures. In recent years more and more light exposure has been coming from TV and computer screens. The big difference here is that one stares directly at the light source at close range. Even our smartphones have bright screens that we hold at only inches from our eyes.

A study at the Lighting Research Center in 2013 (PMID22850476) looked at the melatonin suppressing ability of an iPad tablet computer set at full brightness that delivered 40 lux at the eyes of 13 young-adult volunteers. After one hour there was a decrease in melatonin, but it did not reach statistical significance until after two hours. When the experiment was repeated with the subjects wearing orange glasses that block blue light, no melatonin suppression was observed.

A 2015 study (PMID25535358) at Harvard Medical School compared reading a book on a light-emitting e-reader with reading a traditional book in the hours before bedtime. The e-reader subjects experienced less same-evening sleepiness, took longer to fall asleep, and experienced phase delay in their circadian rhythm, suppression of melatonin, and decreased alertness the following morning.

Hopefully listing all these studies has convinced the reader that even very moderate exposure to light in the hours before bedtime is able to significantly reduce the concentration of melatonin in the bloodstream and brain. It also significantly reduces the time when it is available. Combining this conclusion with the conclusion of chapter 1, that melatonin maximization may reduce the risk of dementia and Alzheimer’s disease, we are forced to ask the question, “What can I do to avoid the damaging effect of light at night?”

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