Ask a ScienceBlogger: A Sun Ray a Day....

For this round of Ask a ScienceBlogger, the question is "Is sunshine good for you?"

It's a beautiful sunny day outside. And, of course, you're stuck in the lab (or the office, classroom, or daily holding tank of your choice). Although you may thumb your nose at those who seem to have nothing better to do than have fun in the sun, don't be so quick to judge. In fact, your lack of sun exposure might be partially to blame for that not-so-sunny disposition of yours. Or worse.

Your body operates on a 24-hour circadian rhythm, and regular daily exposure to sunlight keeps it running smoothly. So, although extended unprotected exposure to the sun damages your skin and eyes, a small daily dose is important for maintaining general health and wellbeing. (And, in addition to its role in regulating circadian rhythms, sunlight is also essential for the body's production of vitamin D.)

Every time you step out into the sun, you unconsciously reset the clock located within the suprachiasmatic nucleus of your brain. Through the controlled periodic expression of several genes, this central clock maintains a daily circadian rhythm and synchronizes the myriad other clocks operating throughout the body. Not quite as accurate as a wristwatch, your central clock can run without outside influence but needs frequent resetting to keep it from slipping into its more natural but slightly offbeat 25-hour cycle. Jet lag throws this system out of whack, temporarily causing abnormal sleep patterns, fatigue, and decreased productivity.

Similarly, a lack of natural light comes with its own set of problems. In the dark winter months at higher latitudes, many people suffer from a depression-like illness called seasonal affective disorder. Not surprisingly, these SAD patients are treated with a regular schedule of exposure to high-intensity light. Throwing off your circadian rhythm through jet lag or a lack of sun can have more serious effects, including an increased risk of infection in the short term and worse in the long run. Links have emerged between offbeat circadian rhythms and cancer. For example, working night shifts can increase a woman's risk of breast cancer, and certain clock genes are downregulated in breast cancers.

Scientists now have a pretty good idea of how the body resets its central clock, and it does this through a pathway distinct from the visual pathway. When you observe an image, light is first detected by the photoreceptors (rods and cones) in your retina, and then the visual signal travels to the occipital lobe for processing. In their quest to figure out how some types of blind mice were still able to regulate their internal clocks with light, scientists discovered another light-detecting pathway, independent of the vision pathway. In this pathway, ganglion-cell photoreceptors in the retina detect light and transmit the signal directly to the suprachiasmatic nucleus. These ganglion-cell photoreceptors detect light with a unique type of opsin called melanopsin. (Opsins are proteins found in photoreceptors and are bound to retinaldehyde, a small molecule activated by light.) Melanopsin also has a unique light-driven ability to regenerate retinaldehyde, making it resistant to bleaching and making ganglion-cell photoreceptors particularly resilient. So, via the melanopsin pathway, light can hit your eyes and reset your internal clock, even if you can't actually "see" it.

So, knowing all of this, what can you do to keep your own internal clock running smoothly? An unexpected suggestion comes from a recent study from the lab of Diego Golombek of Universidad Nacional de Quilmes in Argentina. The authors found that sildenafil (a.k.a. Viagra) can decrease recovery time for jet lag by making the body's clock more sensitive to light, and they conclude that "sildenafil may be useful for treatment of circadian adaptation to environmental changes, including transmeridian eastbound flight schedules."

Viagra on intercontinental flights? I've got a feeling that the mile high club might be in for quite a boost in membership.

In all seriousness, though, the take-home message is this: go get some sun. It's good for you--with a couple of caveats. Skin cancer is the most common form of cancer, so wearing sunscreen for extended sun exposure is still a must. Ultraviolet light is quite harmful to the eyes as well, so wearing sunglasses is also advisable for long periods of exposure. You might naturally ask, then, if wearing sunglasses will inhibit your attempts to reset your internal clock. Although the answer is not quite so black and white, the short answer is that, no, wearing sunglasses will not negate the beneficial effects of sunlight. Melanopsin is selectively activated by light of wavelengths of about 360 to 430 nm. This puts it right at the edge of the visible spectrum, absorbing violet light and some low energy UV radiation (UVA). Sunglasses selectively absorb almost 100% of more dangerous and higher energy UVB radiation, and, depending on the glasses, about 70 to 90% of visible light passing through them. This still leaves quite a bit of light passing directly through the lenses, not to mention the large amount of light that makes it to the eyes around the sides of the sunglasses. Therefore, wearing sunglasses should not have a negative impact on your circadian rhythm, although if you are only going to be in the sun for a short period, you might be better off not wearing sunglasses.

So, get out there and enjoy the sun. Doctor's orders!

Thanks to David Earnest of Texas A&M University for helpful discussions.

For more information on biological clocks and circadian rhythms, check out Bora's Blog Around the Clock.