Scientists Have Worked Out Why Darkness Makes Us Sleepy

There’s nothing like the frustration of being woken up from a deep slumber because someone decided to switch on the light in the middle of the night.

But why does a bright light effect our ability to sleep when our eyes are closed?

Now scientists think they have worked out how darkness and light interact with our ability to doze off, and ultimately make humans diurnal animals who sleep at night and wake during the day.

Professor David Prober and his team from California Institute of Technology have been studying the neural relationship between light conditions and sleep, as light has long been known to directly affect (and modify the length) of our circadian rhythms.

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

The team found the correlation is due to a specific protein in the brain that responds to light and darkness and sets the correct balance between sleep and wakefulness.

They used zebrafish for the study, as they have a similar sleeping pattern to people, Prober explained:  “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.”

Researcher Wendy Chen genetically engineered the fish to overproduce a protein called prokineticin 2 (Prok2), and found that in contrast to normal fish, the fish with more protein were more likely to fall asleep during the day and wake at night.

And this shift depended only on whether lights were turned on or off, not on their normal sleeping habits.

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These observations suggest that an excess of Prok2 suppresses both the usual awakening effect of light and the sedating effect of darkness. This is because the sedating effect of Prok2 overexpression in the presence of light requires galanin, a known sleep-promoting protein.

“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,” said Prober. 

Further work is needed to fully explain how light and dark directly affect sleeping and waking, and to determine whether Prok2 has a similar function in humans.

If it does, this work might eventually lead to new sleep- and wake-promoting drugs.

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