The Intensity and Duration of Exposure to Light Can Affect the Circadian Rhythm

Circadian rhythms are the physical, mental, and behavioral changes that occur over the course of a day, responding primarily to light and darkness in the environment. Special non-image forming cells in the eye’s retina that contain melanopsin (a pigment involved in the regulation of nonvisual responses) and are sensitive to blue light are known to affect these daily rhythms. These nonvisual responses also have been noted with higher intensity light levels. Now, new research reveals that green light and the cones and rods in the retina used for vision are also involved. The research, published in Science Translational Medicine, examined how melatonin (a major hormone associated with human sleeping patterns) responds to different wavelengths of light and suggests that light therapy for sleep disorders may be optimized by stimulating both visual and nonvisual systems.

The researchers assigned 52 healthy participants into two groups and exposed them to two different wavelengths of light. Half of the participants received long-wavelength “green” light (designed to be detected by the visual response system), and half received short-wavelength “blue” light (designed to be detected by melanopsin-containing cells in the nonvisual response system). The participants were exposed to their designated light wavelength continuously for 6.5 hours. Melatonin levels were recorded before and after light exposure.

Results showed that at the beginning of the illumination, the “green” light was just as effective at suppressing melatonin levels as the “blue” light; however, the “green” light did not sustain this effect over the course of exposure. In addition, at low-light levels, the melatonin-suppressing effects of “green” light exposure were significant. Taken together, these findings indicate that the visual system contributes substantially to circadian rhythms at the beginning of a light exposure and at low-light levels, while the nonvisual system (melanopsin) is involved in long duration of light exposure and at high-light levels.

The researchers suggested that these findings have implications for developing and enhancing light therapies for conditions such as sleep disorders and seasonal affect disorder. They also indicated that devices that block blue light may not always effectively hinder circadian responses. They also suggested that low-light exposure late at night, such as from computers, television, or indoor lighting, could interfere with circadian rhythms and make it difficult to wake in the morning. Further research is needed in order to better understand the subtle nuances of light therapy and its effect on circadian rhythms.