The Function of Sleep

“Among the open questions in neuroscience, I believe the function of sleep remains perhaps the most embarrassing. We don’t know why every night our brain needs us – and presumably itself – to sleep for several hours. There is every indication that sleep performs an essential function, if we only knew which one.

The indications are that sleep is, first of all, dangerous. Obviously you are made vulnerable to predators if you fall asleep and don’t respond to stimuli. It is pervasive; we do it from the cradle to the grave. It is universal: every animal that has been carefully studied does so – from fruit flies to ourselves. There is no single exception. It is also irresistible, as we all know. If you do experiments to keep animals awake – or humans awake, for that matter – there is no way that you can overcome the need for sleep. You can even use pain, shocks — at some point the animal will fall asleep. It is also tightly regulated, with a big portion of the brain stem, the hypothalamus, all kind of complicated connectivity being set up to put ourselves to sleep and then wake us. And finally, if you don’t sleep, or sleep too little, it is clear there are serious consequences. In fatal familial insomnia, after a few months of lack of sleep, you die. Rats die after two weeks. But the most obvious consequences are cognitive consequences. There are all kinds of problems. We become extremely bad cognitively. We make mistakes of all sorts, and we become extremely irritable…

The enigma of sleep function has been around for a long time. All kind of ideas have been proposed. The theory I propose is the synaptic homeostasis hypothesis, which is an attempt to understand what is the core function of sleep in every animal.

The idea here is, in short, that sleep is the price we pay for plasticity, which of course is a feature that is functional primarily when we’re awake, when we adapt to the external world.

In our brains, neurons either spike or they don’t — that’s how they communicate. Spikes are more expensive than non-spikes in terms of their synaptic consequences, in terms of the energy they use. So spikes should be reserved, and as far as we can tell they largely are, to signal important events — events that convey a lot of information. Now, when you need to adapt to a changing world — which is basically all the time, especially during development — what you need to do is shift strength of synapses or even add synapses to make sure you are firing for important events or important changes in the environment.

And every neuron does that, to make sure it signals important stuff downstream, in this complicated brain where every neuron is immersed in a sea of other neurons. It doesn’t know what it’s getting, or from where; it doesn’t know what it’s signaling, and it doesn’t know where it’s sending it.

So, in all of this uncertainty, for any learning system which is as complicated as a brain, there is a problem in the end, which is that neurons tend to strengthen to make sure they signal. But then that becomes biologically untenable, because stronger synapses consume more energy, occupy more space, require more supplies, and finally they saturate signal to noise.

Basically, then neurons start firing for everything, and that can’t be good. So there is a need for renormalization to make sure that total synaptic strength is constant.

And we think that that renormalization is not only essential, but it better happen offline, when you actually can sample in an unbiased way the environment of a neuron. And every neuron, by itself, does this in the course of a night’s sleep.

That’s what we think sleep is fundamentally for.”

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From Dr. Guilio Tononi’s recent talk on The Function of Sleep at the Allen Institute for Brain Science Symposium.

I had never before seriously considered this question of the purpose of sleep, or the sheer strangeness of the fact that we don’t yet have a firm understanding of its biological function. It’s nothing less than absurd to ponder what happens to me, to you, to everyone as we spend one third of our lives unconscious.

As Neil deGrass Tyson observed, “Aliens might be surprised to learn that humans must lay semi-comatose on cushions for nearly a third of every Earth rotation.”

Craig Raine, in his poem “A Martian Send A Postcard Home,” mimics the voice of a martian who is noting different facets of human life and reporting them back to his kind. These take the form of riddles, the last one being a coded reference to human sleep and dreaming:

At night when all the colours die,
they hide in pairs

and read about themselves –
in colour, with their eyelids shut.

Watch Tononi’s entire talk below.