Why do we spend a large chunk of our lives asleep, disconnected from, and vulnerable to, the outside world? We know that poor sleep has negative impacts on our daily functioning and health. When we miss out on sleep, cognitive functions like attention and memory suffer. After a night of quality sleep, our attention and memory can recover. Memory depends on sleep. While some believe that our brains can recover during resting wakefulness, research shows otherwise. Chiara Cirelli and Giulio Tononi, expert sleep scientists, argue that our brains can only recover when we are completely disconnected from what’s going on around us. And this only happens when we are sleeping.
Cirelli and Tononi have spent the last twenty years developing their theory. It is called the Synaptic Homeostasis Hypothesis. While we are awake, we are constantly having new experiences. But our brains have limited capacity and cannot store all the information to which we are exposed. Some of our experiences are important for us to learn and remember, whereas others are not so important. According to Cirelli and Tononi, our brains must go completely offline to keep important information and shed the less important information.
Brain cells, or neurons, communicate with one another through connections, called synapses. Each neuron in the brain has thousands of synapses. Synapses connect neurons to a number of other neurons to form circuits. Synapses can change over time. A synapse can get stronger so that one neuron has a stronger effect on the activity in another neuron in the circuit. After a number of changes in the strengths of the synapses connecting a bunch of neurons to one another, it is easy to imagine how the activity of an entire circuit of neurons can change. Neuroscientists agree that these changes in synapses are critical for learning and memory. Some research suggests that our memories are none-other than the strengths of our synapses. Each new experience can cause changes in a large number of synapses of our brains. So, some synapses can continue to grow stronger while we are awake. But there’s a limit to how strong a synapse can be.
Cirelli and Tononi suggest that our brains reorganize the synapses while we are asleep, through a process called “synaptic renormalization”. Synaptic renormalization works like this. If the synapses in a particular circuit have become stronger on average, then they are all made to be a bit weaker. In doing so, the synapses will be capable of changing in response to new experiences the next day. Without synaptic renormalization, the synapses in our brains could get stuck at the limits - some synapses would be unable to get any stronger – and they would no longer function properly.
To test their theory, Cirelli and Tononi along with several other researchers spent four years measuring 7,000 synapses from mice. Changes in the strength of a synapse are sometimes visible as the actual size of the contact between a pair of neurons. The researchers found that after 6 to 8 hours of sleep, over two-thirds of the synapses decreased in size by 18 percent compared to 6 to 8 hours of wakefulness. Another study found that a gene called Homer1a plays an important role in reorganizing synapses during sleep. Together, these two studies begin to offer an explanation for how learning and memory rely on sleep.