Zebrafish lend clues to why humans need sleep

Oct. 6, 2010, 2:03 a.m.

Zebrafish and your PowerPoint presentation. What’s the connection?

According to a recent study led by researchers in the Stanford School of Medicine, zebrafish suggest that, rather than paging through your slides just a few more times as the clock ticks onward, or endlessly rehearsing your speech into the night, it might be wisest to get a few good hours of shut-eye before your test or presentation.

Zebrafish lend clues to why humans need sleep
(ANASTASIA YEE/The Stanford Daily)

Throughout the day, as animals and humans learn, communicate and perform their daily habits, the brain forms new synapses to handle the inputs of information. Now, Stanford scientists have used zebrafish, transparent freshwater fish related to minnows and commonly used as models in scientific studies, to actually trace the changes in the number of synapses between waking and sleep. According to a paper published in Wednesday’s issue of Neuron, synapses in the brain decrease as you sleep, consolidating important information and shedding irrelevant irritants.

“When you’re awake in the day, you create more and more synapses to do things, but at a certain point, this would be unsustainable,” said Emmanual Mignot, senior author of the paper and director of the Stanford Center for Sleep Sciences and Medicine. “Sleep is needed to clear all the synapses that are not helpful<\p>–<\p>to prune the synapses that have not been used much.”

In other words, the phone number that guy with bad breath at the bar gave you last night? Blissfully gone by morning. Knowledge of how to brush your teeth? Still there.

While similar theories had previously been established about invertebrates, scientists had never before concretely demonstrated this sleep-related synaptic change in vertebrates, a group into which humans fall.

“For the first time we demonstrated the role of synaptic downscaling on a living vertebrate,” said Philippe Mourrain, co-director of the study. “We can actually see the change in synaptic density within the animal.”

The study further showed that these synaptic changes depend on animals’ circadian clocks and do not adjust purely based on sleep.

In order to view the neurons through the transparent zebrafish, the Stanford team pioneered new imaging methods that would trace brain activity while avoiding affecting it.

“It was pretty special because nobody has ever been able to look in on an animal throughout its entire circadian cycle,” said Gordon Wang, who co-authored the paper and designed most of the imaging work. “You can actually embed the zebrafish in agarose without anesthesia, so there’s no issues with anesthesia affecting sleep, and hold it in space and take images throughout the day and night.”

Zebrafish, though not as pervasive as the mouse or fruit fly in lab studies, made this study possible, Mignot said.

“Its body plan and brain are much more similar to humans, it’s cheap<\p>.<\p>.<\p>.<\p>it has huge potential,” he said.

While zebrafish are more similar to humans than fruit flies are, their brains are still different enough that researchers are wary of drawing too many implications for human sleep patterns. Furthermore, because of the technological challenges of tracing human brain activity while maintaining normal sleeping patterns, it’s “definitely sci-fi” to imagine similar studies on humans, Mignot said.

“The real thing to keep in mind is that sleep is such a complicated phenomenon that encompasses all parts of the brain,” Wang said.

At the same time, the researches learned a little bit about their own sleeping needs.

“You need to have someone that will stay up for 24 or 36 hours…to take images every three hours,” Mourrain said. “Even if you sleep-deprive the fish, you do it to yourself as well, so it’s only fair.”

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