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Sunlight, Serotonin and Your Sleep Cycle

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morning sun over foggy field with trees in background
Credit : Tomasz Baranowski

Why Seasons Affect Mood

If you feel like winter always puts you in a gloomy mood, you’re not alone. During the winter months, days are shorter and most of us are exposed to less sunlight than during other parts of the year.

Being exposed to less sunlight doesn’t just affect your tan lines, it can affect how much of the hormone serotonin your body produces.

What is serotonin?

Serotonin is a neurotransmitter hormone that’s produced by the intestines and the brain. It’s scientific name, 5-hydroxytryptamine, references its base which you might be familiar with: tryptophan (that amino acid everyone mistakenly blames their post-Thanksgiving nap on).

When sunlight enters your eyes, it stimulates the parts of your retina that then cue your brain to produce serotonin. Serotonin appears to play a role in regulating mood, emotions, appetite, and digestion. The body uses serotonin to send messages between nerve cells, but it cannot cross the blood-brain barrier, which means that the brain has to produce any serotonin it needs to use.

Scientists don’t know exactly what causes depression, but one theory is that it’s due to an imbalance of neurotransmitters, like serotonin, in the body. A common variety of antidepressant medication is a selective serotonin re-uptake inhibitor (SSRI) which keeps serotonin from being reabsorbed after it’s used, leaving higher levels circulating in the body. SSRI’s appear to relieve the symptoms of many people with depression, but it’s still unclear how they work since we can measure serotonin levels in the bloodstream but not in the living brain. Medical studies have found conflicting results regarding how affective they are at alleviating depression, and studies show that it takes weeks for an SSRI to have a positive effect on mood – suggesting that increasing serotonin isn’t what treats depression. Instead, research has suggested that SSRI’s increase positive emotional processing over time, resulting in a mood shift, and they also reduce inflammation in the body, which has been linked with depression.

Serotonin and your sleep cycle

While going out into the sunlight might help you wake up in the morning, it’s also going to affect how well you sleep at night. Melatonin is another hormone your body produces, and levels naturally rise in the evening as it gets darker, putting you “into a state of quiet wakefulness that helps promote sleep,” explains Johns Hopkins sleep expert Luis F. Buenaver, Ph.D., C.B.S.M.

It turns out your body needs serotonin to produce melatonin. Your pineal gland (located deep in your brain in an area called the epithalamus) chemically alters one hormone to create the other. Your body’s sleep-wake cycle is affected by how much light (natural and unnatural) you’re exposed to, and the related production of these hormones. That’s why sleep experts recommend keeping the lights low as you approach bed time and avoiding exposure to the blue and green toned light that computers, smartphones and TVs produce within a few hours of going to sleep. If you find you have trouble sleeping, Dr. Buenaver recommends exposure to daylight during the morning and afternoon to sync your body’s clock with the sun’s cycle.

It’s cold out this time of year, so going for a walk might not always be an ideal choice, but even sitting in a sunny window can be enough to stimulate the production of serotonin. And the good news is, one Australian study that measured levels of brain chemicals flowing directly out of the brain found that people had higher serotonin levels on bright sunny days than on cloudy ones, no matter how cold or hot the weather was.

So, if you can bundle up and get outside a few times a day, it just might help you sleep better at night. But if not, there’s always indoor light therapy boxes that mimic the effects of sunlight.