Sunshine affects the brain via the interaction of the chemicals melatonin and serotonin, as well as vitamin D. When sunlight hits your eyes, your optic nerve sends a message to the gland in the brain that produces melatonin (a hormone that helps you sleep); the gland decreases its secretions of melatonin until the sun goes down again. The opposite happens with the chemical serotonin; when you're exposed to sun, your brain increases serotonin (a hormone connected with feelings of happiness and wakefulness) production. And when the ultraviolet rays from sunshine touch your skin, your body produces vitamin D, which helps you maintain serotonin levels. Generally, we're asleep or feeling slowed down during the dark hours, and physically and emotionally up during the day. This is the human circadian rhythm. We are able to function against these biological rhythms when we must (as night-shift workers do), but it can be hard on our bodies and minds. When we go without sunshine, we can even get seasonal affective disorder (SAD); people who suffer from this disorder get depressed during the times when there's not much sun, although they're typically fine in the warmer, sunnier months of the year. SAD is most prevalent in places where there are scant sunlit hours in the winter (such as Alaska) or where it's overcast for extended periods (parts of the U.S. Northwest). SAD can often be treated with phototherapy that exposes the patient to full-spectrum light, which may be sunlight or artificial light.
Being out in the sun isn't enough by itself, though. We have to soak in the sun's rays. But we've been taught to put on sunscreen whenever we go outside, and there are downsides to the use of sun-blocking chemicals. Our bodies need to be exposed to some full-spectrum sunshine - at least 15 minutes of undiluted sun three times a week [source: USA Today]. The ultraviolet rays of the sun make our bodies produce vitamin D, which is thought to help protect us from various types of cancer and helps us build a store of the vitamin to last through the dark winter months. In addition, vitamin D helps our brains make more serotonin. If we slather our skin with sunblock every time we step out the door, we're cutting down on vitamin D and its benefits.
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