The connection between light and sleep

Why do we wake up in the middle of the night when lights are suddenly turned on? Or why is it that we have trouble sleeping when the room gets flooded with white light?

These questions and more are the focal point of a study conducted by Professor of Biology David Prober who discovered part of the answer: a specific protein in our brain responding to darkness and light to put in place the right balance between sleep and wakefulness.

“Researchers had previously identified the photoreceptors in the eye that are required for the direct effect of light on wakefulness and sleep,” Prober said in a statement. “But we wanted to know how the brain uses this visual information to affect sleep.”

To discover the connection between light and sleep, researchers used zebrafish that has diurnal sleep patterns like that of humans and optically transparent that allows for noninvasive imaging of its neurons.

Wendy Chen, formerly a graduate student at Prober’s laboratory, pried the protein called prokineticin 2 (Prok2) into the brain of the zebrafish, genetically engineering the latter to overexpress Prok2 that resulted in an abundance of it.

She discovered that as opposed to common zebrafish, the animals in study sleep in the day and wake up in the evening.

Additionally, she observed that they didn’t really depend on their normal sleep and wake cycle but they merely depended on whether the lights are on or off around them.

Chen discovered that the excess of Prok2 in their brain quells both the normal awakening effect of light and the sedating effect of darkness.

She later produced zebrafish that has mutated forms of Prok2 and its receptor, and looked at light-dependent defects in sleep of these animals.

For instance, Chen discovered that such zebrafish was deemed to be more active when lights were on as compared to when the lights were off, which was the opposite of what she had found in animals with overexpressed Prok2 and functional Prok2 receptors.

“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.”

They also found that Prok2 overexpression increased the level of galanin expression in the anterior hypothalamus, a key sleep-promoting center in the brain.

But in animals that were engineered to lack galanin, overexpression of Prok2 did not increase sleep.

The researchers also wanted to know the effects of Prok2on sleep, so they decided to look at whether other proteins in the brain known to affect sleep were necessary for the effects of Prok2 on sleep behavior.

They discovered that the sedating effect of Prok2 overexpression in the presence of light demands galanin, a protein known for promoting sleep.

Plus they also discovered that overexpression of Prok2 boosted the level of galanin expression in the brain’s anterior hypothalamus, a key sleep-promoting center.

Prok2 overexpression, however, didn’t increase sleep in animals engineered to lack galanin.

These findings are just the start of an examination that looks into the connection between light and sleep. Further studies are needed to fully explain this connection.