Striking electron microscope pictures from inside the brains of mice suggest what happens in our own brain every day: Our synapses – the junctions between nerve cells – grow strong and large during the stimulation of daytime, then shrink by nearly 20 percent while we sleep, creating room for more growth and learning the next day.
The four-year research project published today in Science offers a direct visual proof of the “synaptic homeostasis hypothesis” (SHY) proposed by Drs. Chiara Cirelli and Giulio Tononi of the Wisconsin Center for Sleep and Consciousness.
This hypothesis holds that sleep is the price we pay for brains that are plastic and able to keep learning new things.
When a synapse is repeatedly activated during waking, it grows in strength, and this growth is believed to be important for learning and memory. According to SHY, however, this growth needs to be balanced to avoid the saturation of synapses and the obliteration of neural signaling and memories. Sleep is believed to be the best time for this process of renormalization, since when asleep we pay much less attention to the external world and are free from the “here and now.”