How Bright Light Therapy Helps with Low Mood, Sleep Problems and Jet Lag
By Daniel F. Kripke, M.D.
Last Revised November 2019
Happiness Needs Bright Light
Think about a person who has Seen the Light. Think about brilliance. Think how we describe the scintillating joy of love by singing, “You are my sunshine.” We know that light makes us happier.
Take a rose or a petunia indoors and it will wilt. Bring a peach or an apple tree indoors and you will get no fruit. Only a few houseplants will flourish indoors. People are similar. We wilt in the dark. Think about the dark dungeons of despair, the heart of darkness, the gloominess of a funereal mood. This language tells us what people have always known. Sadness rules where it is dark.
People were designed to be outside. It is part of what makes us human. Perhaps somewhere back in evolution, our ancestors may have resembled monkeys or chimpanzees that climbed trees and lived in dark forests. For example, the “Gorillas In The Mist” lived in a gloomy rainforest. In contrast, as our ancestors became human, they moved out more into equatorial fields and savannahs, became able to run further, and started looking about further for food. Some seasons had clear bright sun every day and some had rain, but it was always bright near the equator where they lived. Much of the time, there was hardly even partial shade. Our modern human ancestors became intelligent in places that were indeed very sunny. It is for such a life that our bodies are adapted.
When people settled down, about the time that recorded history began, our ancestors were still outdoors people. The men were outdoors hunting, fishing, farming, and fighting. The women were outdoors farming also, when they were not gathering food, grinding grains, weaving, cooking, or doing other things done mostly outside. Humans still spend much of their time outdoors where many people live: in China, in India, and in many of the tropical developing countries. Indoors, our moods start to wilt.
1.A. Modern people get less light
Only in recent generations, as society urbanized, did people begin spending most of their time indoors. Even in my lifetime, we have moved more indoors. As a little boy, I went outdoors every day to play, even in the winter. I walked to school and back, whereas more contemporary children ride. When television arrived, kids came indoors more, but we still played outside, because the attraction of the fuzzy black and white tube or the radio of my boyhood years was hardly comparable to entrancing modern television selections. How different it is today, when people of every age group spend so many hours watching television. With often hundreds of channels on the cable or satellite, there is always something fascinating for every taste. Big screen color has extra attraction, also. As if that weren’t enough, along came video games, and even more recently (for most of us), tablets, the Internet, and movie downloads. The result is that we spend more and more time in front of the tubes and less and less time in daylight. The tubes may seem bright, but the reason that we often watch them in dark rooms is that most give off very little light.
There is another problem in the cities of today. When I was a boy, I used to feel safe walking down Fifth Avenue in New York City. As a medical student, I walked at night in every part of the city. People no longer feel as safe. Older retired people, especially elderly women, may no longer feel safe walking around the big cities or going to the parks. The result is that older people are spending more time indoors. Construction of more indoor health and exercise facilities may be another factor keeping people indoors.
To gain more understanding of the lighting which contemporary Americans experience, my colleagues and I have measured how much time people spend outdoors in San Diego. We have studied a random population sample. It appears that in San Diego, California, people are outdoors in daylight less than one hour per day on average. It is true that some people are outdoors for hours, working outdoors, shopping, playing golf, strolling on the beach, or whatever, but these people are not average. There are an impressive number of people who are in daylight only ten or twenty minutes a day or even less. Some San Diegans go through most days experiencing no daylight at all, besides which, sometimes they experience only dim indoor lighting.
We have found several factors which determine how much bright light people experience. The proportion of time spent indoors or outdoors is the most important influence. Season is next important, influencing the duration and brightness of daylight and the comfort of outdoor activities. As might be expected, people in San Diego experience almost twice as much light in the summer as in the winter. Daylight savings time seems to be as important a factor as season itself, though season and the time standard are difficult to separate. We have found that volunteers from San Diego minority groups experienced somewhat less light than people of European ancestry, and women were outdoors less than men. However, somewhat to our surprise, we have found that in San Diego, women from age 20 to age 80 years seem to experience rather similar amounts of daily light.
San Diego is one of the most pleasant coastal cities in Southern California. We have more sunny days than 80% of American cities. Moreover, in San Diego, it is rarely too hot or too cold to be outdoors. Phoenix has more sunshine than San Diego, but if you ever walk around downtown Phoenix in the summer, you will find few people outdoors. In hot climates like Phoenix, people with air conditioning find it more comfortable indoors in the daytime. More widespread use of air conditioning may also be a new factor keeping more people indoors.
With colleagues, we compared people’s light exposure in San Diego with the pattern in Rochester, Minnesota. Rochester is a charming small city in a rural farming area. In the summer, people in Rochester were out in daylight a bit more than in San Diego, perhaps because summer days are longer further north, or perhaps because more were busy with farming. In winter, on the other hand, people in Rochester were in daylight less than one third as much as in San Diego. The shorter winter days in Rochester are not the main reason that Minnesota people experienced little daylight. Most of the winter, Minnesota was just too cold for most people to stay outdoors more than they absolutely had to. In larger cities, people might see even less daylight than in Rochester, because Northern big cities are now built for traveling underground. In cities like Toronto, a person can take the subway to work and walk for blocks underground to arrive at the office or shopping without ever setting foot outside.
1.B. Lighting measurement
Our eyes adjust over such an astonishing range of lighting, that it is hard for us to realize how much brighter it is outdoors than indoors. Because your sensation is a poor judge of brightness, modern cameras contain light meters called photometers to measure brightness. The camera photometer tells you that your camera might need a one-second exposure for an indoor shot (without flash), when the same camera would capture an outdoor scene in 1/1000th of a second. That would be a thousand-fold difference.
Consider what scientific photometers tell us about the brightness of our varied environments. Modern photometers measure lighting in a measure called lux. Lux is a measure of how brightly things are illuminated. One lux is approximately as bright as the illumination from a candle one meter away in a dark room. An older measure, one foot candle, is the illumination produced by a standard candle one foot away. Since lighting diminishes as the square of distance, and one meter is a bit more than three times one foot, illumination one meter from a candle is less than 1/10th of the illumination at one foot. Thus, one lux is .0929 foot candle. Similarly, illumination at two yards (six feet) is only 1/36th as bright as at one foot, and lighting ten feet from a candle is only about 1/100th of the lighting at one foot (or 0.01 foot candle). The brightness 30 feet from a candle in a dark room is about the same as 0.001 lux, which is the brightness of starlight on a moonless night. When we distinguish shapes in starlight, we are seeing in only 1/1000th of one lux, which is about the darkest light in which we see anything.
In contrast, on a bright summer day when the sun is overhead, the brightness looking horizontally toward the horizon may reach 10–20,000 lux. It may get a bit brighter on a sandy beach or a snowy ski slope, but few eyes can stand more than 20,000 lux even briefly. Many people will put on sunglasses before it gets to 10,000 lux, so such people never expose their eyes to more than 3000–5000 lux. You may have heard that daylight reaches approximately 100,000 lux at noon on a clear summer’s day, looking straight up at the sun, but if we value our eyes, we never look directly at the noonday sun. Therefore, 10–20,000 lux is about the maximum that our eyes experience in sunlight. In summary, the human eye has the adaptability to see in a remarkable range of lighting. We can see throughout a visual range of more than seven orders of magnitude (2 X 107) or 20 million fold, from 0.001 lux to as much as 20,000 lux falling on our eyes.
1.C. Our lighting experience
Many scientists do not realize the dimness of indoor lighting, because illumination engineers often teach that indoor lighting is typically 100 to 500 lux. The engineers are thinking of bright offices and shopping centers (which are often especially brightly lighted), and they are measuring the lighting with their photometers pointed toward the ceiling lights where we rarely look. If we measure in the directions in which eyes usually look, we have found that most indoor settings are much dimmer than 100 lux. A fashionable living room might be 20 to 50 lux or less. A fashionable bar or restaurant could be as dim as 1–2 lux. In a lecture hall where scientists watch slides, 1–2 lux is about average. Movie theaters are darker than one lux. Thus, it could be 1,000 or even 10,000 times brighter outdoors in daylight than indoors.
A big surprise for us was to discover in what very dim light people often watch TV. In a recent study, the mid-range subject averaged 31 lux in the evening, but some people’s TV rooms are only about 1 lux (about the same as the light of a full moon). That is dark! Please do not think that the TV screen itself is very bright. Most TVs may appear bright, but they do not light up a room. Usually, you could not read comfortably by your TV’s light alone.
Because of the huge range of different lighting conditions in which we can function, an extremely wide range of lighting experiences is possible. We used a special instrument called an Actillume to measure how much illumination people actually experienced. Worn on a wrist, Actillumes measured both activity and illumination (Acti… + illum…) It was invented in our San Diego laboratory with the assistance of the late Mr. William Gruen of Ambulatory Monitoring, the company which manufactured and sold this instrument. The Actillumes could measure lighting (exactly like a camera light meter or photometer) and store the results every minute for several weeks in its internal computer memory. In measuring a randomly-selected group of 318 San Diego volunteers from 40 to 64 years of age, we found that in the course of a two-day recording, the volunteers who received the brightest illumination (spending bright days outdoors) were experiencing as much as 1,000 times the 24-hour light exposure of those who received the dimmest lighting. Evidently, those receiving the dimmest lighting never went out in daylight, and even indoors, their rooms were lit more for TV than for reading. We were quite astonished to learn that different people experience a thousand-fold range of lighting. Such differences must have many effects.
More recently, quite a few wrist mounted and body-mounted instruments have been developed to record the illumination that a person is experiencing. You can monitor your light exposure with some special smart watches as you can now monitor your pulse rate. There are some problems because a wrist-mounted light sensor is often not aimed in the same direction as the eyes. Also, at night, either the light sensor or the eyes may be covered by pillows or bedclothes.
The averaged light exposure over 24 hours for the median San Diego man was 350 lux, with only 278 lux for women. Since much of the night was spent near zero lux, time awake may have averaged close to 500 lux. That does not mean that most of the day was spent in 500 lux – to the contrary, to achieve an average of 278–350 lux, a short time (e.g., usually under one hour) was spent in bright daylight of several thousand lux, and much of the rest of waking time (even during the daylight hours) was spent in less than 100 lux.
Below the median daytime illumination, we have found that more people report depression. For some people, less than an hour of daylight seems insufficient to uphold their mood and may be a cause of depression.
Endnotes for Chapter 1
1. Espiritu, RC et al. Low illumination by San Diego adults: association with atypical depressive symptoms. Biol.Psychiatry. 1994;35:403-407; Kripke, DF et al. Adult illumination exposures and some correlations with symptoms, in Hiroshige T, Honma K (eds): Evolution of Circadian Clock. Sapporo, Hokkaido University Press; 1994:349-360. [return]
2. Cole, RJ et al. Seasonal variation in human illumination exposure at two different latitudes. J.Biol.Rhythms. 1995;10:325-336. [return]
3. Jean-Louis, G et al. Sleep duration, illumination, and activity patterns in a population sample: effects of gender and ethnicity. Biol.Psychiatry. 2000;47:921-927. [return]
4. Jean-Louis, G et al. Circadian sleep, illumination, and activity patterns in women: influences of aging and time reference. Physiology & Behavior. 2000; 68:347-352. [return]
5. Cole, RJ et al. Seasonal variation in human illumination exposure at two different latitudes. J.Biol.Rhythms. 1995;10:325-336. [return]
6. Youngstedt, SD et al. Light exposure, sleep quality, and depression in older adults. In Holick MF, Jung EG (eds): Biologic Effects of Light 1998. Boston, Kluwer Academic Publishers; 1999:427-435. [return]
7. Jean-Louis, G et al. Relationships among illumination, activity, and sleep patterns. In Holick MF, Jung EG (eds): Biological Effects of Light 1998. Boston, Kluwer Academic Publishers; 1999:37-39. [return]
Table of Contents
Brighten Your Life, in all its formats, including this ebook, copyright ©1997-2019 by Daniel F. Kripke, M.D. All rights reserved.