stanford medicine



A Fish Story

Photo: Duncan Stewart
Tohei Yokogawa among fish tanks. He stayed up nights to learn whether zebrafish sleep.
Tohei Yokogawa among fish tanks. He stayed up nights to learn whether zebrafish sleep.

Graduate student Tohei Yokogawa never expected the study of sleep would cause him to lose so much of it himself. Or that videos of tiny swimming zebrafish would be what kept him awake. But by screening these fin flicks, Yokogawa sought to answer a basic question: Do fish sleep?

Yokogawa knew he wanted to study sleep when he came to Stanford from the University of Tokyo. He joined the lab of Emmanuel Mignot, MD, PhD, director of the Center for Narcolepsy. Mignot had uncovered the genetic cause of narcolepsy in dogs and wanted to find more genes involved in sleep regulation. For this, Mignot needed an inexpensive, quick-breeding genetic model. Enter zebrafish.

The problem was that no one knew for sure if fish slept. In fact, most fish can’t close their eyes. Divers have reported seeing sleeping fish, Yokogawa says. But for a scientific experiment, researchers needed more convincing evidence.

So Yokogawa set out to discover the secret life of zebrafish, common aquarium pets named for their horizontal black stripes. The fish didn’t seem to sleep when lights were on, so Yokogawa set up infrared lights and cameras to observe zebrafish during the night. He soon discovered that in darkness, zebrafish spend about 60 percent of the time hanging motionless with their tail fins drooping.

If these motionless fish were truly sleeping, then they should experience sleep rebound. In other words, when deprived of sleep they should compensate by napping more later. But Yokogawa discovered that depriving a zebrafish of sleep was no easy task.

At first Yokogawa tried tapping on the tank or playing sounds underwater. The noise would startle the fish awake, but as they got more tired, they slept right through the disturbance. Finally Yokogawa discovered that a gentle electrical pulse through the water would keep them active. He had to play with the voltage to get just the right amount of stimulation. Too little voltage, and the fish would snooze through it; too much, and they would awake with a jump.

Once the system worked, Yokogawa spent two nights per week on zebrafish duty. He studied eight zebrafish at a time, each swimming in a tiny plastic cubicle with electrified walls. The cells, stacked together like a fish apartment complex, were arrayed in front of a video camera. From 11 p.m. to 9 a.m. Yokogawa watched the zebrafish cells by video. If a fish remained motionless for more than 6 seconds, it got an electrical wake-up call.

Although 10 hours of watching fish swim doesn’t seem like a recipe for alertness, Yokogawa says he didn’t use any special techniques to keep himself awake. He relied only on the power of concentration. One night his fiancé came to the lab to keep him company. “She tried to keep me awake, and I tried to keep the fish awake,” Yokogawa says. “After several hours, she was getting sleepy, so only the fish and I stayed awake.”

Yokogawa’s painstaking hours paid off: His work demonstrated that fish do experience sleep rebound, and laid the way for further genetic research into sleep. Even better for Yokogawa, a collaborator wrote a software program to automatically stimulate the fish with electricity whenever they dozed off, and Yokogawa went back to sleeping nights. — Madolyn Bowman Rogers






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