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News & Events for the Smith College Community
Research & Inquiry March 28, 2022

Guiding Light

Psychology professor Mary Harrington explains how light influences our circadian rhythms

Marker illustration of a large pink and yellow starburst dividing day and night
Illustration by Elisabetta Bianchi

The Tippit Professor in the Life Sciences at Smith, Mary Harrington teaches courses in biological rhythms, Alzheimer’s disease, and experimental methods in neuroscience. With her students, she conducts research into brain circuits regulating circadian rhythms, those daily internal processes that influence our sleep-wake cycles. It should come as no surprise then that as the seasons change, Harrington is always attuned to the light. Here, she discusses five questions that intrigue her—and should intrigue us all—as she investigates the effects of light on our own circadian rhythms.


No, not at all. For example, an early study from 1938 showed that people living deep in Mammoth Cave in Kentucky were found to have circadian rhythms in body temperature cycles. While these people were not in constant darkness—they tried to live on a 28-hour day using small desk lamps they could turn on—researchers have studied other animals living in total darkness and these also show very regular repeating circadian rhythms. The cycle length is close to, but not exactly, 24 hours, and thus these are circadian rhythms. These rhythms are generated from daily clocks present in individual cells in our bodies, arising from cycles in clock genes being turned on and off every day.


Yes! Light changes the speed of our circadian rhythms. Light during the middle of the day has little effect on clock speed, but light in the morning speeds up our internal clock. On the other hand, light in the evening slows our internal clock. These effects of light are what allow us to adjust our internal rhythms—which are not exactly 24-hour cycles—to perfectly match the 24-hour cycle of the Earth’s rotation. Light can alter internal time to perfectly match external time by changing the speed of the internal cellular clocks. One thing I think is especially intriguing is that this pattern—morning light speeds up the clocks, evening light slows them down—is the same for all plants and animals.

Mary Harrington



The ability to have dark nights is a social-justice issue, Harrington says.




Yes! Most of us have internal circadian clocks that naturally run with a cycle a bit longer than 24 hours. Left to our own devices and without light, we would naturally get up a bit later each day. You might have noticed this on the weekends, for example. But we can correct for this. Light in the morning has a brief effect on circadian clock speed, pushing it to run a bit faster. This can adjust for the natural lag in your internal clock and will lead to you waking up earlier than usual the next day and feeling more awake earlier in the day. Do you have an important morning meeting? Aim to get bright light in the morning of the day before your meeting, so you will be wide awake for your meeting. On the other hand, evening light will slow your clock, having the opposite effect. With bright light in the evening, you will tend to wake up later than usual the next day and will feel more drowsy the next morning.


Sunlight is much brighter than indoor light. Less obviously, sunlight has different wavelengths than indoor light. Different colored lights have different wavelengths. For example, blue light has a shorter wavelength than red light. White light is a mixture of many wavelengths. Sunlight has a big mix of many wavelengths of light. Indoor light sources, on the other hand, can be a lot more restricted in the wavelengths of light they emit, even though, due to careful mixing, they appear white.

This is important because our biology is tuned to specific light intensities and wavelengths for specific responses. So, for example, the psychologically arousing effects of light are mainly due to the blue wavelengths. Circadian clock resetting effects of light are caused by green and other wavelengths and are very sensitive to the brightness of the light. Some body tissues, such as the skin and eye, have circadian clocks most sensitive to ultraviolet wavelengths of light. It’s complicated, but if you get outdoors each day, sunlight will naturally provide the signals needed for robust circadian clock timing. This is why outdoor light can be much better than indoor light to keep you sleeping well and feeling happy.


This is important for two reasons. First, it is important for human welfare. We are healthier if we can have darkness at night. For many people this is a challenge, given where they live. Light pollution is associated with poverty. It is an issue of social justice we don’t often think about—the ability to have dark nights.

It is also important for all the plants and animals that share the planet. Artificial light at night can mess up the ability of plants and animals to determine the seasons. Day length used to be a very predictable cue for the changing seasons. Now, with light pollution, these ancient mechanisms can be seriously impacted.

Fortunately, this is a simple form of pollution to reverse: Look over the lights around your home at night—including the blue light of personal electronic devices—and talk to others about how these might be reduced or eliminated.

This story appears in the Spring 2022 issue of the Smith Alumnae Quarterly.