Sunlight and Melatonin
Humans are programmed to be outdoors while the sun is shining and in the dark at night. This is why melatonin is produced during the dark hours and stops upon optic exposure to daylight. This pineal hormone is a key pacesetter for many of the body’s circadian rhythms. It also plays an important role in countering infection, inflammation, cancer, and auto-immunity, according to a review in the May 2006 issue of Current Opinion in Investigational Drugs. Finally, melatonin suppresses UVR-induced skin damage, according to research in the July 2005 issue of Endocrine.
When people are exposed to sunlight or very bright artificial light in the morning, their nocturnal melatonin production occurs sooner, and they enter into sleep more easily at night. Melatonin production also shows a seasonal variation relative to the availability of light, with the hormone produced for a longer period in the winter than in the summer. The melatonin rhythm phase advancement caused by exposure to bright morning light has been effective against insomnia, premenstrual syndrome, and seasonal affective disorder (SAD).
The melatonin precursor, serotonin, is also affected by exposure to daylight. Normally produced during the day, serotonin is only converted to melatonin in darkness. Whereas high melatonin levels correspond to long nights and short days, high serotonin levels in the presence of melatonin reflect short nights and long days (i.e., longer UVR exposure). Moderately high serotonin levels result in more positive moods and a calm yet focused mental outlook. Indeed, SAD has been linked with low serotonin levels during the day as well as with a phase delay in nighttime melatonin production. It was recently found that mammalian skin can produce serotonin and transform it into melatonin, and that many types of skin cells express receptors for both serotonin and melatonin.
With our modern-day penchant for indoor activity and staying up well past dusk, nocturnal melatonin production is typically far from robust. “The light we get from being outside on a summer day can be a thousand times brighter than we’re ever likely to experience indoors,” says melatonin researcher Russel J. Reiter of the University of Texas Health Science Center. “For this reason, it’s important that people who work indoors get outside periodically, and moreover that we all try to sleep in total darkness. This can have a major impact on melatonin rhythms and can result in improvements in mood, energy, and sleep quality.”
For people in jobs in which sunlight exposure is limited, full-spectrum lighting may be helpful. Sunglasses may further limit the eyes’ access to full sunlight, thereby altering melatonin rhythms. Going shades-free in the daylight, even for just 10–15 minutes, could confer significant health benefits.