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Asked by anon-251613 on 1 May 2020.
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Kim Liu answered on 1 May 2020: last edited 1 May 2020 12:45 pm
This is a cool question, which I don’t know too much about so I’m looking forward to seeing other answers. The brain, you may know, can only use sugar as an energy source and cannot directly use fat reserves like the rest of the body. Sugar is stored in the body as glycogen, which acts a bit like rechargeable batteries and allows easy energy acccess. A prevalent idea is that sleep is necessary for the brain to restore its glycogen resources from food eaten during the day, so that it can continue functioning well with its very high energy consumption during the day.
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Edit: @Andrew – I’ve read a little bit fact-checking this glycogen idea I picked up from somewhere; is it in fact outdated now? 🙂
Edit: @Andrew – ok cool; since I wrote this, I’ve actually found a bunch of literature which suggests glycogen store is more of a marker – classic cause/effect challenge haha. Definitely agree with the energy in/out argument though; I’ll edit to clarify ~
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Andrew Beale answered on 1 May 2020: last edited 1 May 2020 12:22 pm
Hi abiiiii,
That is a good question – we don’t actually fully know the answer!A few ideas relate to how the brain builds connections. During the day when we learn, the brain builds connections. During sleep important connections are strengthened and unimportant ones are pruned – this is called consolidation.
Another idea is that sleep when the cells of the brain do their “cleaning up”. The neurons in the brain do a lot of work (sending signals, making connections) during wake times and one idea is that it is most efficient to restrict the limited resources that the cell has to the daytime so that neurons can do this. But cells also need to get rid of broken bits of proteins, repair DNA and all sorts of other maintenance jobs to stay alive. Because neurons are so focused on their daytime role, they can’t do their maintenance in the day. Therefore they only do those jobs when they are not active – that is when we are asleep.
We ‘gain energy’ then because the cells recuperate from the effort they put in during the day. (We don’t really gain energy from sleeping because only eating puts energy into out bodies.)
I highly recommend Russel Foster’s TED talk on this topic https://www.ted.com/talks/russell_foster_why_do_we_sleep?language=en
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@Kim – glycogen idea sounds good I think. I just mean that if we treat our bodies as a system like you do in physics, you can’t gain energy from within – it needs to come from outside i.e. from a food! -
Nina Rzechorzek answered on 1 May 2020:
Hi abiiii – great question!
We actually still use energy whilst we sleep – we use about 1kcal/min during deep sleep (about 70 Watts). That energy cost is in addition to something known as our basal metabolic rate (BMR) = about 1.1kcal/min (77 Watts). We use a little more energy when simply sitting that when in deep sleep; when sitting we use about 1.2-1.4kcal/min (85-100W) on top of BMR.
The BMR is the energy needed to maintain the basics of life: maintenance of cell function in heart, lung, kidney, and brain. 30-40% of the BMR is guzzled by the brain and spinal cord though (central nervous system). It is estimated that 40-60% of the BMR may be used in the pumping of ions (mostly sodium) across cell membranes – something that brain cells do a lot of!
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