[Learning strengthens some connections between neurons, known as synapses, and even forms new synapses. These synapses demand a lot of extra energy, though.] That means that at the end of the day, you have a brain that costs you more energy, ... That's where sleep would kick in.

[But why does consciousness fade during deep sleep early in the night?] You cannot say that consciousness fades because the brain shuts off. That's not the case. Scientists have long known that the brain remains active while we sleep, ... So what could be responsible?

During deep sleep early in the night, ... the response is short-lived and doesn't propagate at all.

The brain breaks down into little islands that can't talk to one another.

Essentially, we activate an area. We can do this anywhere in the brain. Once an area is activated, it responds by sending signals, waves that travel through the axons (nerve fibers) to other regions of the brain. At the same time, we can record how the rest of the brain is responding.

Essentially, we activate an area, ... We can do this anywhere in the brain. Once an area is activated, it responds by sending signals, waves that travel through the axons (nerve fibers) to other regions of the brain. At the same time, we can record how the rest of the brain is responding.