How Melatonin Promotes Sleep

Melatonin is a naturally produced molecule that has long been suspected to play a role in healthy sleep, but it has been unclear how it does so. Now, Caltech researchers have discovered a mechanism through which melatonin promotes sleep, using zebrafish models in the laboratory.

The research was conducted in the lab of Professor of Biology David Prober and is described in a paper appearing in Current Biology on April 20.

Sleep is a vital and evolutionarily ancient behavioral state, yet there are still many open scientific questions about how sleep is regulated by the body, and there are few effective therapies for sleep disorders.

To understand the mechanisms by which sleep is regulated, the Prober lab is using an unusual lab animal: zebrafish. There are several advantages for using zebrafish as a sleep model, including that their brains are simpler than ours but still similar. They also follow a diurnal pattern of sleep—meaning, they sleep at night and are awake during the day, similar to humans, as opposed to nocturnal lab animals like mice.

The circadian clock is a molecular mechanism that oscillates with a 24-hour rhythm, regulating many different aspects of physiology and behavior through gene expression, hormone synthesis, and several other outputs—of which melatonin production is one. In 2015, Prober's team discovered that zebrafish that lack the ability to produce melatonin have normal circadian rhythms but lack circadian regulation of sleep, showing that melatonin conveys circadian rhythm information to mechanisms that regulate sleep. In other words, getting rid of melatonin removes the circadian clock's influence on sleep but has no effect on the circadian clock itself, so a lack of melatonin does not affect other aspects of physiology and behavior that are regulated by the circadian clock.

Melatonin clearly has an important role in regulating sleep, but the question remained: How does it work?

Led by former graduate student Andrew Hill (PhD '24), the team discovered that melatonin promotes sleep by binding to certain receptors, called MT1, found on the surface of some brain cells. Through meticulous genetic experiments, Hill showed that mutant zebrafish that lacked MT1 receptors also lacked circadian regulation of sleep, similar to fish that lack melatonin. Hill then aimed to discover which neural cells had MT1 receptors and understand their functions in the brain.

Hill found that MT1 receptors are most prominently found in a brain region, the optic tectum, that plays an important role in processing sensory information, particularly visual stimuli. The team then hypothesized that melatonin promotes sleep by reducing the activity of these neurons, leading to reduced behavioral responsiveness to visual stimuli.

To test this idea, Hill analyzed groups of zebrafish during the daytime when they were awake. When the light in the laboratory was briefly dimmed—simulating the shadow of a predator passing overhead—the fish responded with rapid movement. However, after treatment with melatonin, the fish responded much less to the sudden light dimming, even if they were awake. Hill also tested fish in response to flickered light at night, which fish normally ignore. He found that mutant zebrafish that do not produce melatonin responded to the flickered light at night with the same rapid movement as if it were daytime and they were awake. These experiments suggest that melatonin does not simply put the animals to sleep, but rather it directly suppresses behavioral responses to visual stimuli.

"One of the hallmarks of sleep is reduced responses to environmental stimuli," Prober says. "Our findings fit with the experience of many people who take melatonin as a sleep aid. Unlike many commonly prescribed sleep aids, melatonin does not cause an irresistible drive to sleep. Rather, melatonin makes one more receptive to falling asleep, and our work suggests that it does so, at least in part, by turning down one's sensitivity to arousing stimuli from the environment."

The team next aims to determine whether melatonin prevents zebrafish from reacting to other types of sensory stimuli, such as sound and touch.

The paper is titled "Melatonin promotes sleep by suppressing responses to visual stimuli via MT1 receptors." Hill is the study's first author. In addition to Hill and Prober, additional co-authors are graduate students Olivia Eliopoulos and Jasmine Emtage, and staff scientist Grigorios Oikonomou. Funding was provided by the National Institutes of Health. David Prober is an affiliated faculty member of the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.