Light therapeutics: How light (or darkness) affects our circadian clock, sleep, and mood

Author: Satchin Panda, PhD

When we travel from one time zone to another, our sleep-wake cycle slowly readjusts to the new time zone. It’s as if our sleep-wake cycle figures out when the sun rises and sets in our new location. Similarly, during a Seattle or Melbourne winter it may be easy to fall asleep at 9pm, but during the summer, when these cities are bathed in sunlight at 9pm, it’s not as easy to enjoy an early bedtime. This has led to the idea that light affects our circadian clock (i.e., our sleep-wake rhythm). But the mechanism connecting light and sleep are not so simple.

One confusing observation concerning the relationship between light and sleep is that many blind individuals (not all) can still align their sleep-wake cycle with the day-night cycle. In contrast, blind individuals who have completely lost both eyes (e.g., through an accident or cancer) cannot align their sleep-wake rhythm with the natural day-night cycle. This holds true for experimental animals, as genetically blind animals align their sleep-wake cycle with the light-dark cycle, whereas animals subjected to the surgical removal of both eyes cannot. Based on these observations, scientists reasoned that some sort of light sensor must still be present in many blind individuals. It seems that this light sensor can tell our brain clock whether it’s day or night, but it doesn’t provide enough detail for us to see the world around us.

This mystery was solved around the turn of this century, when scientists discovered a novel light sensor that is present in only a tiny fraction of nerve cells (less than 0.1%) in each of our eyes. This light sensor is called melanopsin, and it senses blue light. You may wonder why blue? Pure sunlight is a mix of seven colors of light, which you can clearly see in a rainbow. Blue is one of those colors (between indigo and green), so natural sunlight is a rich source of blue light. To see at night, humans have traditionally used amber firelight, which contains almost no blue light. By  using a blue light sensor, our circadian clock recognizes daylight, but ignores the dim firelight/candlelight that is prevalent at night.

After discovering that our circadian clock uses a blue-light sensor to distinguish between day and night, scientists began to ask whether melanopsin may be affecting our brain in additional ways. Why did they ask this?  Biology is smart, and bio-molecules rarely have just one job, rather they multi-task. Over the past several years we have learned a great deal more about melanopsin and its relevance to health.

Cells in our eyes that contain melanopsin make contact with the master circadian clock in our brain, but they also make direct or indirect connections with more than a dozen other parts of our brain. These brain regions in turn control our sleep hormone melatonin, alertness, depression, stress hormones, and many more aspects of our health and mood.

In lieu of what we know about how profoundly light can affect our brain and overall health, every time we light up a room, step outside to daylight, draw the curtain, or put our sunglasses on, we are sending either good or bad signal to our brain. Light truly is as powerful as any drug in its potential to affect our brain and overall health. If light is a drug, then we must figure out how much light and what kind of light (i.e., what color) should be consumed when to promote mental health and general well-being.

While basic science research on the impact of light on brain function has grown rapidly, clinical research on the health effect of the levels of light at different time of the day is still evolving. Even before the discovery of melanopsin, scientists knew that too little bright light during the day (as occurs during the winter in extreme northern or southern countries) can trigger seasonal affective disorder or winter blues. Therefore, the delivery of bright light via light boxes is used to reduce winter blues and lift mood.

Scientists have long wondered how much light (what intensity and for how long) must a person be exposed to in the morning or afternoon to align our circadian clock with the day-night cycle. Answering this question has been difficult for practical reasons. It is extremely difficult to perform controlled clinical studies in which volunteers are confined to a room and given precise amounts of light to determine the effects on their circadian rhythm, mood, and sleep. Even if you could perform such studies, its’ unclear how they would relate to everyday life. Despite these limitations, results from some of these controlled studies can be used as guides for our everyday life.

We know that bright light during the day is good for us, as it resets our clock, suppresses sleep, and improves alertness. While a few minutes of bright light may be enough to nudge our clock and synchronize our internal clocks with the morning light, we need an hour of bright light to suppress melatonin (and sleep), and we may need more than an hour of light to stave off depression. In the evening it works differently. As we wind down the day and prepare for sleep, we need less light or even no light. Bright light in the evening confuses our circadian clock, making our bodies think that the evening has not yet arrived. Even a modest amount of light may prevent melatonin from accumulating enough to make us sleepy. Of course, in our modern lifestyles we cannot switch off all the lights and spend evenings in complete darkness. But, we can avoid bright lights, or light that is rich in blue light, by choosing to work under dim light or light that is orange or red in color. Similarly, exposing ourselves to bright computer screens, smartphone, or TV screens, even in an otherwise darkroom, can modestly suppress sleep.

What can you do to optimize light for better health? The simple rule of thumb is to expose yourself to as much daylight as possible during the day (without looking directly at the sun!), as little light as possible in the evening, and absolutely no light when you sleep (i.e., sleep in complete darkness).

Even if you live in Florida, California, Spain, or the south of Italy (extremely sunny regions), you may not be getting enough bright light if your daytime job keeps you inside a windowless office. Taking a walk outside or having breakfast or lunch next to a large window or on a patio is a good way to get an hour or two of bright light. Having bright LED light in the office also helps. Towards evening and night, the same bright light can work against your health. Using a dim, orange-tinted light is much better. While choosing a light bulb may seem like an easy solution, many of us move from one room to another with different lighting, or run errands in the evening and therefore do not have complete control over our light environment. To solve this problem, some people have started to use blue filtering glasses to reduce their evening light exposure. These eyeglasses make everything look slightly yellow, orange, or even red. Although many users report they feel sleepy sooner, or that the glasses reduce eye strain at night, rigorous research on these glasses is still lacking. Using blue filtering glasses (or a blue filtering coating on regular prescription glasses) in the evening may have some benefits, but they should not be used continuously, as wearing them during the day may deprive us of the blue light needed to suppress sleep, improve alertness, and reduce depression.

Finally, when it comes to bedtime, people often use a nightlight to feel safe or to avoid tripping during a late-night visit to the bathroom. For a good night’s sleep, however, nothing beats a completely dark bedroom. Consider replacing the night light with an easily accessible small flash light (e.g., your cell phone screen) or an easily accessible light switch. If you want to invest a little more, placing motion-sensing lights in the hallway and bathroom is also a good alternative. Complete darkness in the bedroom should be the goal if you want to enjoy a truly rejuvenating night’s sleep.

Soumya
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