Comment by schmidtleonard
4 days ago
Exactly! Going offline for hours every day in an adversarial world is positively nuts! The reason can't be idiosyncratic. No gentle gradient of comparative advantage can rationalize it. It must be something severe and nigh impossible to do any other way.
Furthermore, sleep is very specifically about the taking the brain offline: that's what deteriorates first in the absence of sleep and the tortured workarounds for animals that absolutely must avoid sleep (e.g. migratory birds) involve sleeping part of the brain at a time. Any explanation that isn't highly specific to the brain's responsibilities has the immediate hurdle of explaining this away, and for that reason I don't buy the mitochondrial explanation. Mitochondria are too universal and sleep is too specific to the brain. Energy is fungible, so I don't buy that nature wouldn't figure out the "trick" of having a subset of the mitochondrial population sleep at a time.
My money is on the "brain algorithm" requiring an online/offline phase, whether that's contrastive learning or memory consolidation or something else. There are lots of candidates for fundamental brain algorithms with the "feature" that they require an offline phase that cannot be incrementally worked around, and these fit the observations much better.
> Mitochondria are too universal and sleep is too specific to the brain
The brain has uniquely high specific power requirements per gram of dry weight. Not even the heart is this power-hungry. This surely places a lot of uniquely high metabolic stress on the neural cells.
And neural cells are long-living, so they can't be easily replaced throughout the lifetime. So their housekeeping has to be very thorough, carefully cleaning up all the waste products.
So this hypothesis actually makes a lot of sense.
Mitochondrial dysfunction literally leads to Alzheimers, dementia, etc. The link is clear as day - don't sleep, lose your mind. Put a different way, life rusts (oxidizes) your brain, and sleep de-rusts it. And unfortunately I'm still someone who regularly pulls all nighters because of a combination of disorders, ADHD and sleep cycle issues. I'm killing myself rather prematurely. But then, all addictions tend to do that and tend to be things the addict has trouble controlling. :(
Also, the theory would better be expressed as "all mitochondria require rest, neuronal rest in the brain looks like sleep (but many cells in the body also get quite a bit of rest during this time)" - so many people here seem to be getting this backwards thinking sleep is the special thing - it's one way large scale mitochondrial rest (as well as lots of other important co-occuring processes) presents in the brain.
The really interesting question is... how do heart cells do it? Because they're a clear exception to this theory... Lactate?
Maybe the heart can just handle the stress longer before something visible breaks? It has a simpler function than the brain, which probably helps. If people tend to die of cancer first (or whatever), the heart only needs to withstand about 80 years of gunk buildup to not be the weakest link.
> Furthermore, sleep is very specifically about the taking the brain offline
Animals that lack a nervous system, like sponges appear to sleep. It may be that becoming unconscious is a symptom of sleep, not a cause of it. Not much point being awake if your body is effectively shut down.
> Going offline for hours every day in an adversarial world is positively nuts!
It's no more nuts than being awake given how much energy vigilance costs.
I mean, it still can be idiosyncratic if the local maximum is steep enough. Identifying and signalling subgroups of mitochondria in a cell to put on pause might be prohibitive, for instance, and would still reduce the power available to that cell.
Or maybe going all the way on and mostly-off with your mitochondria, even specifically with your brain mitochondria, really is that much more efficient than having half of them offline (but still consuming energy for upkeep) at any time. The brain is a big ol energy hog, after all.