26 Jan 2021

Why we need sleep

From Nine To Noon, 10:13 am on 26 January 2021

We all need sleep, but how much we need, how we do it, and why we need it differs across species.

Sleep researcher and UCLA Evolutionary Biology Professor Van Savage has been part of a research team included scientists with expertise in neuroscience, biology, statistics and physics, conducting a comprehensive statistical analysis of sleep, using data from more than 60 sleep studies involving humans and other mammals. 

Professor Savage tells Nine to Noon much of his work involves comparative studies across species and how things vary according to body size.

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Photo: 123rf

“For example, the heart rate of a whale is so much slower than that of a mouse… a whale can go one or two minutes between heartbeats.”

Other things, such as the turnover of cells or time spent pregnant also tend to be longer the larger an animal is.

“Looking across all species in all biology, why do we see the differences in times and timescales – that’s how I came to sleep, I looked at sleep as an example and it fascinated me because it’s almost the exact opposite to every other time.

“The bigger you are, the longer you live across species – some whales live 200 years – but if you look at sleep, a whale sleeps only around an hour and a half per day and it sleeps half the brain at a time.”

Savage says that gave him a clue as to how sleep was special and unusual.

For the past 20 years, there have been many theories about why we sleep including that it gives us time to repair the body and brain and allows the brain to reorganise for learning and memory. Other theories say it allows the brain to forget information, and others say it helps problem solving.

He says they went into the study with the hypothesis that sleep was for repair of body and brain, but when they began looking at babies, it no longer added up.

“What we found was that if we reformulated the different ideas and different hypotheses, sleep being for learning and changing connections in the brain became the most likely function in that early stage.

“In the first two or three years of life, you’re growing lots of connections and pruning lots of connections to learn how to walk and talk. It’s really a time of great fluidity or plasticity.”

So, does that mean the old adage, never wake a sleeping baby, is true?

“There are times in our lives when we can’t help but wake a sleeping baby, but I’d say if you can avoid it, you should. It is really valuable to their development. Getting a lot of sleep is helping them make the changes in their brain they need to make to develop.”

However, he says parents shouldn’t worry if their baby isn’t a good sleeper.

“I have a son who is now seven and he was not a good sleeper but he’s turned out well. So, I don’t want to make people worried if they’re not sleeping well. But, if they are, you should let them go at it and not disturb them.”

It’s around two and a half years old when children’s sleep function makes a big transition.

“Before two and half, it seems it’s mostly about changing connections in the brain and, after two and a half, it seems to quickly transition to be more about repairing neurons in the brain.”

Savage explains that we don’t replace neurons in the brain, the ones we have are the ones we are stuck with for life.

“We have to repair the ones that are there and it seems sleep is one of the ways we do that. I’m not surprised that adults need to repair neurons, but I was surprised that the switch happens so early.”

As any parent knows, sleep deprivation can be almost physically painful and mentally debilitating. Savage says it’s likely this is due to the brain being unable to do the repair and processing it needs to do to keep up with the energy we’re using.

“It does degrade and wear down our brain, it affects your reaction times, your learning, your ability to think clearly – it has a big effect.”