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Human Sleep Cycle May Have Roots in Ancient Oceans

A new study suggests that melatonin, the hormone that regulates our sleep, may have first evolved hundreds of millions of years ago in tiny ocean creatures.

There’s no denying the importance of melatonin in the daily and nightly rhythms of humans, also known as circadian rhythms. As day turns to night, and our eyes detect changing patterns in light and dark, a tiny gland at the center of the brain called the pineal gland begins to secrete melatonin. Melatonin production, which spikes during the nighttime hours, interferes with the activity of the thalamus, the part of the brain that receives and processes signals from our eyes and other senses. This induces certain physiological changes, including decreased body temperature and respiration rate, that prepare the body for sleep. During daylight hours, melatonin production decreases, stimulating wakefulness and helping us remain alert.

As reported in the New York Times, researchers from the European Molecular Biology Laboratory in Germany have spent the past several years comparing the activity of genes responsible for producing melatonin in vertebrates like humans with that of similar genes in a distantly related invertebrate, the marine worm Platynereis dumerilii. They focused on the worms at the larval stage, when they were part of the teeming mass of plankton swarming in ocean waters.

According to the group’s findings, published in a recent issue of the journal Cell, Platynereis dumerilii larvae migrate every day towards the surface of the ocean by beating microscopic hairs on their midsections, known as cilia. They reach the surface at dusk, then sink throughout the night to deeper waters, which offer them protection from predators as well as from the harmful UV rays of the sun at the height of the day. This movement of plankton in the ocean has been described as the planet’s biggest migration, in terms of sheer biomass.

When the scientists examined how genes were activated in the worm larvae, they discovered that certain cells in their brains can sense light, in the same way that cells in the human eye do. Just as our eyes signal the pineal gland to switch melatonin production on and off, they found, these cells are able to activate genes that produce melatonin in the larvae.

By tracking the activity of these melatonin-producing genes over a 24-hour period, the scientists also discovered that–like us–the worms don’t produce melatonin all the time, but only at night. As melatonin production spikes, the hormone latches itself to the neurons controlling the beating motion of the cilia, causing long pauses in that motion. These long pauses, in turn, cause the worm larvae to sink. When dawn comes and melatonin decreases, the worms begin rising toward the surface again.

The similarities don’t end there: Just like humans, worms can get jet lag if their melatonin cycles are thrown out of whack. As study co-author Maria Antonietta Tosches explained in a statement: “When we exposed the larvae to melatonin during the day, they switched towards night-time behavior….It’s as if they were jet lagged.”

According to the study, such striking parallels suggest that the biological processes that produce melatonin may have evolved first in an ancient ocean-dwelling ancestor shared by both humans and Platynereis dumerilii. Lead researcher Detlev Arendt put it this way to the Times: “It could have been the first form of sleeping.”

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