Cited research: Sci. Transl. Med. 2, 31ra33 (2010)

Light is an important regulator of the body's circadian rhythms. In humans, this is thought to be mediated by light-sensing cells in the eye known as intrinsically photosensitive retinal ganglion cells (ipRGCs). Steven Lockley at Brigham and Women's Hospital in Boston, Massachusetts, and his team have shown that the retina's colour-detecting cones are also involved.

They shone blue or green light of varying intensities directly into the eyes of awake volunteers for 6.5 hours during the night. Cones are most sensitive to green light, whereas ipRGCs mainly detect blue light. The researchers also monitored changes in the volunteers' circadian responses.

The team found that cones contribute to circadian responses on initial exposure to light and in dim light conditions, but that ipRGCs are the main photoreceptors when light is bright and long-lasting. The authors suggest their findings could help enhance light therapy for conditions such as sleep disorders. C.L.