Circadian Rhythm and Human Health

Joan E. Roberts 

Department of Natural Sciences, Room 813
Fordham University, 113 West 60th Street
New York City, NY 10023


All life on earth evolved under both a light and dark cycle (Santillo et al., 2006; Musio and Santillo, 2009). As the sun rises and reaches its peak at noon, the spectrum it emits is smooth throughout the visible spectrum with a high intensity in the blue region [400 - 500 nm]. As the sun sets, blue visible light is preferentially scattered (removed) from sunlight, leaving an emission appearing orange-red [600 - 700 nm]. At night, there is darkness with limited visible light emitted from the stars, with the exception of when there is a full moon. During the full moon, there is five times the amount of visible light emitted from the sky, and significant light emitted in the blue visible range (Ugolini, 2007). 

Humans evolved being exposed to different spectra of light in the morning, the late afternoon and evening.