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New insights about 'the darkness hormone,' melatonin

Melatonin is central to our sleep and to our cycles of rest and activity.

Melatonin is central to our sleep and to our cycles of rest and activity. As much as we’ve learned about melatonin and its importance to sleep, there’s a great deal we still don’t yet know about the hormone’s biological functions and purpose. In the past few years, there have been some important breakthroughs in our understanding of how melatonin affects the body’s sleep-wake cycles, and how it may influence health and disease as well as sleep. We’ve also seen some discoveries about what may be melatonin’s possible role in the evolution the sleep itself.

Melatonin is a biologically ancient molecule, known to have existed in some of the earliest and most primitive living organisms. Recent research about melatonin has provided us with some fascinating new insights to the possible evolutionary beginnings of sleep. Scientists at the European Molecular Biology Laboratory in Heidelberg, Germany investigated how melatonin works in a form of marine zooplankton, a tiny, invertebrate ocean worm. The scientists’ discoveries may shed new light on the earliest biological origins of sleep.

Researchers studied marine zooplankton in their larval stage. These larvae have a regular, distinct day-and-night pattern of activity that unfolds in their natural ocean environment. As the sun sets, the larvae swim toward the surface of the water to feed under the cover of darkness. As the sun rises, they drop back to the ocean’s lower depths, where they remain—protected from predators and shielded from the sun’s UV rays—until the sun begins to go down and they start their upward move again. Scientists regard these cyclical patterns among zooplankton larvae as a possible link to an evolutionary ancient version of circadian rhythms, which exist not only in humans but also in nearly all animals and other living organisms.

Scientists wanted to see what role melatonin might play in the cyclical, night-day activity of the tiny larvae. Based on earlier research, scientists had already identified cells in the larvae that were sensitive to light and showed some basic similarities with the light-sensitive cells in the human brain that trigger melatonin production. Researchers investigated genetic make-up and activity of these light-sensitive cells, looking for evidence of melatonin and its possible relationship to the night-day routines of the larvae.

Researchers found that the zooplankton generated melatonin at night, and ceased melatonin production during the day. They also found that the nightly rise and fall of melatonin levels in the larvae was directly involved in helping the tiny organisms move up and down in the water. They even found that the larvae experienced their own version of circadian rhythm disruption, or jet lag—deprived of exposure to light, the larvae continued to produce the melatonin that regulated their movement in water according to their night-day cycle.

The researchers theorize that what they observed in the larvae may be a biologically ancient, primitive version of a sleep-wake cycle, one that traces back hundreds of millions of years. It’s an intriguing and compelling scenario about how our own sleep cycles may have originated.

In other research that suggests the deep evolutionary role of melatonin to sleep-wake cycles, the hormone’s fluctuating levels have also been linked—for the first time-- to the lunar cycle. Researchers at Switzerland’s University of Basel have documented the first evidence that our sleep-wake cycles are influenced by the shifting phases of the moon. The scientists didn’t set out to study the lunar influence over sleep. They used data from a previously completed study on circadian rhythms and the body’s internal, homeostatic sleep drive. Their study data included detailed sleep information about 33 healthy men and women, ages 20-74, who spent multiple stints sleeping in a laboratory setting that monitored and measured sleep and also controlled exposure to light, including moonlight and artificial light at night.

Researchers found that melatonin levels—along with several other important markers of sleep—shifted significantly along with the changes to the moon. At the time of the full moon, nighttime melatonin levels were at their lowest, having fallen steadily in the several days leading up to the full moon. After the full moon passed, nighttime melatonin levels rose, before beginning to drop again after passing the halfway mark toward the next full moon. In addition to the changes in melatonin, researchers also found that people slept less overall, spent less time in deep sleep, and took longer to fall asleep in the same corresponding pattern to the full moon. These findings surprised researchers and other scientists—we’ve never before seen reliable scientific evidence of a connection between sleep and the lunar cycle in humans. Their research—preliminary to be sure—suggests that our sleep-wake cycles may not only operate under the influence of the 24-hour circadian cycle of daylight and darkness, but under the effects of the roughly 29-day lunar cycle as well.

Next, we’ll take a look at research that explores in groundbreaking new detail ways melatonin may influence the body’s circadian clock.

Sweet Dreams,

Michael J. Breus, PhD

The Sleep Doctor™

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