Without them, the irregular light schedule did not impair mood and cognitive (thinking) function, even though their vision and general light detection ability remained intact. This showed that light affects learning and mood directly through these special photosensitive retinal cells, Hattar said.
The researchers created light-exposure patterns for the mice that allowed the scientists to rule out the possibility that circadian rhythm and sleep disruption were responsible for the changes in mood and learning ability they observed.
Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism's environment, according to the U.S. National Institutes of Health.
One expert questioned whether the mice's normal circadian rhythm was indeed maintained. "Perhaps even though the overall sleep timing pattern remained intact, the quality of their sleep deteriorated," suggested Tony Tang, an adjunct professor in the department of psychology at Northwestern University, in Evanston, Ill.
Tang also found an important difference between how humans are exposed to light at night in modern life and how the reaction of mice to light was tested during the research.
"In the current study, the poor mice ended up having bright lights shining on them while they slept; but for humans in the past century, we've stayed up while we kept lights on, and then turned the lights off when we sleep," he said.
Scientists note that research with animals often fails to provide similar results in humans.
Study co-author Hattar said the study should be replicated in human subjects. "But even if it comes out not as clear as it did in mice, I think there will be some benefit for people to turn down their lights at night. I don't think there is any harm in it."
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