When you're really tired, your brain deals with the problem by putting parts of itself to sleep. Scientists say partially-asleep brains could explain everything from sleepwalking and talking, to why you make so many mistakes when you're exhausted.

University of Wisconsin neuroscientist Giulio Tononi and his colleagues already knew that sleep deprivation causes people to make more errors. The question was why. He and his colleagues decided to investigate how rat brains respond to sleep deprivation by inserting electrodes into a couple regions of the rodents' brains while keeping their subjects awake for several hours longer than normal.


How do you keep a rat awake? Apparently by playing with them - the researchers distracted the sleepy rats with "new objects, then tested the rats' abilities to perform complicated tasks. Tononi and his team also measured electrical activity in the rats' brains. What they found was that the sleepier the rats got, the more of their neurons switched into an "off" or sleep state. Parts of their brains were falling asleep, while others stayed awake.

In a Nature reviews article, UCLA sleep researcher Christopher Colwell explains the difference between on and off neurons: P

atterns of neural activity in the brain change across the sleep–wake cycle. These temporal changes can be measured as electric field potentials, and are picked up as ‘slowwave activity' (SWA) using electroencephalography (EEG). Physiologically, neurons in the cortex seem to exist in two stable configurations: an ON state in which the neurons are depolarized and generate spontaneous spikes of membrane potential (known as action potentials), and an OFF state in which the neurons are hyperpolarized and electrically silent (see image at left). During SWA, the neurons oscillate in a synchronized pattern between ON and OFF states, generating changes in the surface potential of the cortex that can be monitored using EEG. During the awake state, the firing of cortical neurons is more active and less synchronized than in SWA. One of the key premises of [Tononi,] Vyazovskiy and colleagues' study, and of prior work, is that the OFF state is the hallmark of sleep in cortical neurons.

Tononi and his colleagues shed light on why sleep is not a full brain experience - indeed, the researchers suggest that "the basic unit of sleep is the electrical activity of a single cortical neuron." Some animals, like fish, never fall completely asleep - half their brains are always awake - because their bodies need to keep moving. So we've known for a while that brains could be half-asleep and half-awake, but this study shows that the partial sleep can affect just a tiny part of your brain - a few extra neurons here and there.


It's possible, say the researchers, that sleepwalking and talking happen when some of the neurons in your brain remain on while most of the others are off. And all those mistakes you make when you're tired could be ascribed to the fact that you aren't running on all cylinders, at least neurologically speaking.

But for those of us who would love to stay awake longer, drugs that tinker with this system could perhaps one day allow us to stay awake for years at a time just by carefully shutting down and reawakening a few sleepy neurons every second.

Read the full scientific paper via Nature

Chart via Nature. Photo by Andrea Danti/Shutterstock