Most animals are sensitive to light. Their biological processes are profoundly affected, sometimes even controlled, by the cycles of sunlight and darkness that characterize day and night. These relationships are called circadian rhythms, and they help maintain cycles like sleep and wakefulness and are connected with seasonal fertility in "lower" animals.
In humans, the connection between light and biology is somewhat looser. People don't typically mate seasonally, for instance -- although conception peaks in Finland in the summer months, when the sun shines for up to 20 hours a day, so there's still a connection there [source: Brody]. And as for sleep patterns, while most people sleep at night and are active in the day, there are plenty of people who reverse that to work the graveyard shift.
But evidence shows that those graveyard shifters may be suffering from the reversal in unexpected ways. One area where humans may not be able to escape the sun's biological effects is on their moods. It all comes down to a complex relationship among sunlight, melatonin and serotonin that we're only just starting to understand.
Melatonin is a hormone that controls sleep, and serotonin in a neurotransmitter that is tied to states of wakefulness and being in a "good mood." Serotonin is the chemical targeted by a class of anti-depressants called SSRIs, which keep higher levels of serotonin in the bloodstream to help elevate mood.
The web connecting sunlight, melatonin and serotonin goes something like this: When the sun comes up again, and sunlight hits the optic nerve, some of that light is sent to the gland in the brain in charge of melatonin. In response, melatonin secretion decreases. When the sun goes down, the body increases its secretion of melatonin.
At the same time, when the body perceives sunlight, serotonin levels increase. And the more sunlight the human body is exposed to, the more serotonin the brain produces [source: ScienceBlog]. So in effect, melatonin and serotonin have an inverse-proportional relationship that is guided by the body's perception of sunlight. The overall effect is "downtime" at night and "uptime" during the day.
There's another factor involved in sunlight's affect on mood, though: vitamin D. The body actually creates its supply of vitamin D from the sun's ultraviolet rays hitting the skin, and high levels of vitamin help the body maintain high levels of serotonin [source: Collinge].
So what does this mean for the daily sunscreen routine? Are we bringing on sadness by protecting our skin from UV light?