The benefits of sleep seem obvious. And yet scientists have long debated precisely how it improves brain performance at the cellular level. One camp argues that sleep reduces the unimportant connections between neurons, preventing brain overload. Another camp maintains that sleep consolidates memories from the previous day.

A group of researchers recently tried to settle this debate by studying the larvae of a common see-through aquarium pet, the zebrafish. Like humans, zebrafish are active during the day and sleep at night. Unlike humans, zebrafish larvae are transparent, which allowed researchers to watch their brains as they slept. The researchers, led by Lior Appelbaum and Philippe Mourrain of Stanford University, tagged the larvae neurons with a dye so that active neuron connections, or synapses, appeared green, whereas inactive ones appeared black. Decreased synaptic activity would show that sleep pruned unnecessary memory connections, whereas memory consolidation would have a different pattern. After following the fluctuations of these synapses over the course of a day, the team found that the zebrafish did indeed have lower overall synapse activity during sleep. The researchers published their results in the journal Neuron, becoming the first to show the effects of sleep/wake cycles and time of day on the synapses of a living vertebrate. “Sleep is an active process that reduces the activity in the brain,” Mourrain says. “It allows the brain to recover from past experiences.” Without the synapse reduction that happens during sleep, he notes, the brain would not have the ability to continually take in and store new information.

But the debate is not yet settled. Among the team’s other findings was that not all neural circuits are affected by sleep in the same way. Learning and memory may benefit the most, Mourrain says. For this reason, the two hypotheses about sleep “may not be mutually exclusive,” says neuroscientist Jan Born of the University of Lübeck in Germany.  A resolution may not be far off; Mourrain and Applebaum’s new imaging technique will allow for more detailed study of the brain during sleep in years to come.