Light governs how we sleep or emote

How light is perceived by the eye does govern the sleeping patterns of people and those suffering from seasonal mood disorders, a John Hopkins University team has found.

The eye uses light to reset the biological clock through a mechanism separate from the ability to see, which could have implications for people with seasonal affective disorder and insomnia.

These findings suggest that patients with trouble sleeping or seasonal depression - disorders that can be linked to lack of exposure to daylight - could benefit from development of easier, more available tests to determine if they are able to detect light properly for functions distinct from normal sight, said Samer Hattar of John Hopkins University.

“It seems that even if individuals have normal sight, they might be having a malfunction that is contributing to their inability to detect light, which can adversely affect their biological clocks,” Hattar said.

Hattar and his team are convinced, on the basis of a long line of work by other researchers, that daily exposure to natural light enhances memory, mood and learning.

“Our tips are simple: Get out in the sun for at least a little while each day,” Hattar said. “There’s a reason why we seek the sun and the beach and we feel better when we can sit in the sun and bask.”

Hattar and colleagues reported that they genetically modified mice so that a particular set of retinal ganglion cells no longer functioned.

The mice were still able to use light to see normally, but had great difficulty synchronising their circadian rhythms to light/dark cycles, the constant lengthening or shortening of daylight hours that occurs depending on the time of year.

Prior research in the field leads the researchers to believe that because the rodents’ internal, biological “clocks” are out of sync with the solar day, the rodents would have difficulty learning and sleeping on a regular, 24-hour cycle. The team has not yet tested that hypothesis.