June 23 (UPI) -- Many studies have explored the effects of light on the circadian rhythms of plants and animals, including humans. But few have looked at the direct effects of light on sleep.
Researchers at the California Institute of Technology set out to determine how and why light directly impacts sleep. Scientists wanted to know why darkness is soporific, and why brightness can disrupt deep sleep.
Testing in Caltech labs revealed a light-sensitive neural protein key in maintaining the proper balance between wakefulness and sleep.
"Researchers had previously identified the photoreceptors in the eye that are required for the direct effect of light on wakefulness and sleep," researcher David Prober, a professor of biology at Caltech, said in a news release. "But we wanted to know how the brain uses this visual information to affect sleep."
In the lab, researchers tagged the brain protein prokineticin 2, or Prok2, with bioluminescent markers in zebrafish models. Zebrafish follow a diurnal sleep-wake pattern similar to humans. They're also translucent, making it easy to monitor neural activity.
Scientists engineered several zebrafish to over-express the Prok2 protein. The fish with an excess of Prok2 were more likely to fall asleep during the day and wake up at night. These effects were dependent on external light, not circadian rhythm.
Zebrafish with mutated Prok2 receptors showed the opposite effects. Their sleep-wake patterns were less influenced by light.
"Though diurnal animals such as zebrafish spend most of their time asleep at night and awake during the day, they also take naps during the day and occasionally wake up at night, similar to many humans," Prober said. "Our study's results suggest that levels of Prok2 play a critical role in setting the correct balance between sleep and wakefulness during both the day and the night."
In follow up tests, researchers found Prok2 influences other proteins important to sleep patterns -- including the protein galanin, which is active in a sleep-promoting brian region called the anterior hypothalamus.
Light and darkness proved less disruptive to the sleep patterns zebrafish that were engineered to over-express Prok2 but under-express galanin.
Prober and his colleagues shared the results of their experiments in the journal Neuron.