Almost a century of research has established that sleep aids in the formation of memory. The question that scientists are now examining is how sleep accomplishes this. When you’re sleeping, the neurons in your brain fire almost as much as they do when you’re awake. For many years, it was thought that this sleeping brain activity helped learning by strengthening the connections between neurons. However, the role sleep plays in memory may be different than previously thought. Psychiatrists Guilin Tononi and Chiara Cirelli offer a somewhat controversial new theory in the August issue of Scientific American, called synaptic homeostasis hypothesis (SHY). SHY states that sleep aids memory not by strengthening the connections between neurons, but rather by weakening them. With twenty years of research to back them up, it’s a theory that’s rapidly gaining attention.
We know that sleep is vital for life. Its universality supports its importance—all creatures, from whales to fruit flies, assume the risks involved with sleeping (paralysis, unconsciousness, inadvertently becoming some other animal’s midnight snack, etc.) in order to acquire its benefits. Like many scientists, Tononi and Cirelli wondered what function of sleep made it so crucial to survival that these risks become worthwhile?
Memory or learning occurs when neurons are activated in groups:
“As linked neurons fire repeatedly, the synapses connecting them more readily convey signals from one neuron to another, helping neuronal circuits to encode memories in the brain. This process of selective strengthening is known as synaptic potentiation.”
But this ability to learn comes at a price. Not only does brain functioning require more than 20% of the body’s energy, but also the building and strengthening of these synaptic connections puts a huge amount of stress on nerve cells themselves.
Tononi and Cirelli’s theory of synaptic homeostasis suggests that sleep restores the brain to a baseline state after a day’s worth of activity, allowing it to create new memories the next day (and throughout a lifetime) without burning out or destroying older memories.
The authors stress that in their hypothesis, learning still occurs mainly through synaptic potentiation; however, SHY proposes that this strengthening does not occur during sleep. Instead, when we are asleep, the brain is weakening synaptic connections in a process called “down selection.” This theory runs directly against conventional wisdom, which states that sleeping brain activity strengthens the neuronal links of new memories, by “replaying” or re-firing the neurons involved in the memory.
Think about all the information your brain manages in a single day, both the conscious efforts to create memory (learning a new language or how to play an instrument) and all the unconscious information it processes (the color of a passing car, identifying the smell of your morning coffee). Tononi and Cirelli suggest, “to improve memory, the sleeping brain must distinguish the ’noise’ of irrelevant information from the ‘signal’ of significant happenings.” By being unconscious, as during sleep, the brain is able to step back, sift through the day’s bombardment of information, weed out the trash, and preserve the important memories. All so that it can start the process over again upon waking.
If the brain never reset, it would not be able to maintain its daily high-octane functioning. SHY proposes that “sleep restores the brain to a state where it can learn and adapt when we are awake...Sleep is the price we pay for the brain’s plasticity—its ability to modify its own wiring in response to experience.”
Tononi and Cirelli used electroencephalograms (EEGs) on both sleeping subjects and awake, to test this sleeping brain activity. From humans to flies, the results supported synaptic weakening during sleep. Through that selective weakening, they believe the brain is thinning out the insignificant links, and making sure the important ones stay intact.
In addition to raising questions about the importance of sleep in childhood and adolescence, and the lasting effects sleep deprivation can have on developing brains, it seems like SHY could also impact research and treatment for Alzheimer’s disease and memory disorders like retrograde amnesia and anterograde amnesia. Tononi and Cirelli are excited to continue testing synaptic homeostasis hypothesis and its predictions.
Tononi further discusses SHY and other aspects of human consciousness in his new book Phi: A Voyage from the Brain to the Soul. Also, head to Scientific American to watch Tononi discussing synaptic homeostasis theory at a recent Allen Institute for Brain Science symposium.