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How Does Sunlight Affect Our Mood?

  • Published7 Dec 2020
  • Author Hannah Zuckerman
  • Source BrainFacts/SfN
Woman wearing mask outside
Istock.com/Eva Blanco

In addition to friends and family, you may be missing another partner in your daily life — sunshine. Social distancing and quarantining reduce the risk of contracting or spreading COVID-19. But, these precautions can also limit our outdoor activities.

Sunlight lets us see and assess our surroundings. It also regulates our sleep cycle. Recently, researchers at the National Institute for Mental Health (NIMH) found light affects brain areas that regulate mood. Exploring the link between light and mood may help us better understand seasonal affective disorder (SAD), a form of depression that often affects people in the fall and winter when there’s less sunlight.

Samer Hattar
Bryan W. Jones

Samer Hattar was part of the team that identified light’s effect on the mood-regulating areas of the brain. The NIMH scientist has studied the relationship between light and mood since the 1990s and uncovered light’s complicated role in the brain. We asked him how light impacts the brain and our mood, and he suggests ways to safely get more sunlight.

How does light affect the brain?

Until the early 2000s, most researchers believed only two kinds of cells responded to light striking the retina — rods and cones. We discovered a third photoreceptor that responds to light but isn’t part of the image-forming system. These photoreceptors — intrinsically photosensitive retinal ganglion cells (ipRGCs) — contain melanopsin, a light-sensitive protein not found in rods and cones.

The ipRGCs send light information to the brain to regulate functions that aren’t concerned with images. One function is adjusting our internal biological clock — our circadian rhythms. It adjusts our days to 24 hours to match the solar day.

Until recently, we thought people developed SAD because of a disruption in their circadian clock. My team wanted to understand why people develop SAD when humans shouldn’t have circadian problems. When you live in an area with a shorter day length, the total day/night length is still 24 hours. You haven’t traveled between time zones, so the circadian clock should still be attuned to the light-dark cycle, even with less light.

In 2018, we found light can reach and affect the brain’s mood-regulating areas via the ipRGCs, projecting from the retina to the peri-habenular region, to the nucleus accumbens and the ventromedial prefrontal cortex (vmPFC). These structures are involved in emotion, reward, and decision-making.

How does this research impact our understanding of SAD?

Knowing how light affects mood may help us understand why SAD affects people the way it does. From studying mice, we know it takes time for changes in the light environment to begin affecting mood. We noticed mood changes in the mice after two weeks of disrupting their light environment. This could explain why people with SAD don’t consciously feel their moods changing, and why it takes time before they feel depressive light effects.

Depression is complex. Studies show light therapy can treat people with SAD but can also help people suffering from general depression and bipolar disorder. We’re far from pinning the reason down.

How does artificial light compare to natural light?

Artificial light is sufficient with enough intensity, but sunlight is more powerful. Think about it like this — it’s great to exercise for 30 minutes or two hours. But going for two hours, without hurting yourself, is better. That’s how I feel about the sun. There’s some evidence that getting enough sunlight during the day makes it easier to resist bright light at night. This could help you sleep better and balance your circadian clock, but we don’t have much data right now.

I’m convinced artificial light has limitations compensating for all aspects of vision and non-image forming systems. For example, people suffer from SAD even when they’re exposed to artificial light. This is admittedly a weak example. I believe sunlight can enhance mood better than artificial light, but I don’t have evidence yet.
How do you recommend getting enough sunlight during quarantine — especially during the fall and winter?

I’m not a physician so take my suggestions with a grain of salt [and consult your doctor]: You need as much sunlight as you can get. If you can, go outside even if it’s cold. Don’t expose yourself to bright light at night. Being exposed to bright light at night will block melatonin production, a hormone your brain produces in response to darkness. Melatonin helps with the timing of your circadian rhythms and with sleep. Switching off table lamps or decreasing the power from light sources helps the hormone melatonin rise before you sleep, letting your body know it’s nighttime.

Fernandez, D. C., Fogerson, P. M., Lazzerini Ospri, L., Thomsen, M. B., Layne, R. M., Severin, D., … Hattar, S. (2018). Light Affects Mood and Learning through Distinct Retina-Brain Pathways. Cell, 175(1), 71-84.e18. doi: 10.1016/j.cell.2018.08.004

Scientists Find A Brain Circuit That Could Explain Seasonal Depression. (n.d.). Retrieved July 13, 2020, from WAMU website: https://wamu.org/story/18/12/21/scientists-find-a-brain-circuit-that-could-explain-seasonal-depression/

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