Light

Light controls our circadian rhythms.  These determine our daily pattern of waking and sleeping, and probably how our energy levels vary throughout the day as well.  In the modern environment, most of us spend most of our time indoors with artificial light.   The signals our brains get from light are no longer triggered exclusively by the rising and setting of the sun.  A number of health effects from altered light exposure have been noted including

• Increased incidence of cancer (esp. breast and colorectal cancer in shift workers)
• Insomnia
• Obesity
• Diabetes
• Depression
• Timing of onset of puberty

For more detailed discussion, references to specific papers and the above image see: http://ehp03.niehs.nih.gov/article/info:doi/10.1289/ehp.117-a20

AND

http://ehp03.niehs.nih.gov/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1289%2Fehp.118-a22 for a more detailed discussion with emphasis on blue light and photoreceptors.