A team of researchers spent four years studying electron microscope pictures to better understand the effects of sleep on brain cells. The team found that when we sleep, the connections between nerve cells shrink to help “renormalize” the synapses for the next day. The details are in a paper that was just published in the journal Science.
Considering all of the recent scientific advances, it’s surprising how little we know about sleep. As researchers have studied possible explanations for why our body needs sleep, one theory began to gain traction: the synaptic homeostasis hypothesis. Synapses connect brain cells called neurons, allowing them to communicate and store information. According to the synaptic homeostasis hypothesis, these synapses are activated over and over throughout the day—resulting in growth and saturation. Sleep allows the connections to “reset,” helping us start the next day refreshed and ready to absorb new information.
To test the synaptic homeostasis hypothesis, scientists from the Wisconsin Center for Sleep and Consciousness used a form of 3D electron microscopy to look for size differences in synapses between the conscious brain and the sleeping brain. The research team used a scanning method with extremely high spatial resolution, called serial scanning electron microscopy, to analyze 6,920 synapses in mouse brains. During analysis, the researchers were unaware of which scans were from sleeping mice, removing possible biases.
The team discovered that after a few hours of sleep, about 80% of the synapses shrunk by an average of 18%. The synapses that didn’t decrease in size were the largest and likely associated with the most stable memory traces. The research team was surprised that the synapses shrunk so much within hours of sleep. The data seems to confirm the synaptic homeostasis hypothesis; synapses decrease in size during sleep so that they can renormalize for the next morning.
The team’s findings finally provide an explanation for why sleep is so important to overall brain health. By scaling down the synapses while we sleep, the brain avoids oversaturation and gets a chance to rest. This process allows us to start fresh the next morning, ready to learn new information and form additional memories.
De Vivo et al. Ultrastructural evidence for synaptic scaling across the wake/sleep cycle. Science (2017).