Why sleep is so important for cognition

What's going on in the brain when we nod off? And how does the brain know to wake up again? Adam asked this of Jason Rihel from UCL, who's looking at sleep in the context of dementia...

Jason - In addition to your body clock, you also have a timer that's telling you how long you've been awake, and that builds up as sleep pressure and that then signals onto neurons in your brain that control when you're going to be asleep and awake. These are neurons that also live in your hypothalamus and they're sensitive to some signal in your brain that's accumulating when your brain is active. We don't know what that signal is, but we do know that once these neurons get tripped to become active, they signal to the brain, it's time to be asleep.

Adam - And then on the other side of that, when I've been asleep, how does the brain know to wake up?

Jason - Once you've gone to sleep, this pressure to build up, whatever it is, it's either the depletion of something or the accumulation of something. Then that can reset during sleep. By the time you are ready to wake up in the morning and your body clock is saying, Hey, it's time to be awake. You also no longer have this sleep pressure.

Adam - It's nice to wake up naturally. Usually it's my cat deciding he wants more food, but when I don't sleep well the night before, I feel really awful. So what is not sleeping enough doing to my brain, to my cognition?

Jason - It's very clear that after a night of sleep deprivation or poor sleep quality, you have immediate effects on your cognitive ability. For example, if I give you a vigilance test where you have to respond to maybe a flashing light on a screen, you will make more mistakes and you'll have a slower reaction time. But we even see this in statistics in society where for example, the number of car crashes due to sleepy driving is estimated to be in the thousands with hundreds of deaths estimated a year.

Adam - What about something like dementia, a disease that affects cognition? How does not sleeping well enough affect something like that.

Jason - We've been studying some aspect of this through a grant that I got from the Alzheimer's Research UK, in which we've been studying some of the molecules that build up in human brains of patients with Alzheimer's disease. There are two main molecules. One is called amyloid beta and another one is called tau and these form these plaques, proteinaceous tangles that build up in your brain when you have Alzheimer's disease. And what people have shown, in both animal models and in humans, is that these molecules are secreted by neurons when you're awake, and so the levels of these molecules go up during wakefulness, and then they dissipate during sleep. One idea is that this is through a kind of washing machine of the brain, in which the interstitial space, that's the space between the neurons becomes more confluent with the cerebral spinal fluid that bathes the brain, and that allows for the flushing out of some of these toxic byproducts.

Now, one potential consequence of that idea, is that the longer you are awake over your lifetime, the chance of building up more and more of these byproducts goes up, so that the chance that you will make more of these damaging plaques will increase the longer you are awake over, of course, a lifetime. And so then one possibility that will happen, is these plaques will then begin to damage some of those neurons that are sensitive to the sleep pressure. That's the signals of the brain to say, it's time to go to sleep. You then might get into this vicious cycle situation, this build up of the amyloid beta and tau while you're awake. The buildup of this into these damaging plaques, they then damage some of the sleep promoting centers of your brain, causing you to be awake more often once you start to get dementia. And then that leads to the buildup of more of these materials, and the buildup of more plaques which then lead to more damage and so on and so forth.

Adam - Does that mean we might be able to use sleep to say prevent dementia or even treat it?

Jason - It's certainly a tantalising possibility that good sleep hygiene will improve your longterm cognitive health. Maybe slow the progression of dementia. Certainly in animal models, people have shown that if you force an animal, the model of dementia, to sleep more over its lifetime, they will accumulate fewer of these damaging plaques and show a slower cognitive decline. Now, whether or not that will work in humans, we don't really know. Basically every neuropsychiatric and neurodevelopmental disorder that you can name, that these also have, sleep problems accompanying them.

And it's still an open question on: Is sleep a contributing factor to this or is it a consequence?

Adam - So, having reflected on the programme, what do you suggest we do to better our own health, through our body clock?

Jason - Just having good sleep hygiene helps a lot. Having a set bedtime where you stick to it, having a time that you get up in the morning and sticking to it, and preparing yourself for a good night's sleep by beginning to wind down and sort of shut yourself off from the world prior to your natural bedtime, for example, shutting off some of those screens, turning off your computer and your phone, and kind of having a relaxed period before you try to go to sleep.