[londons_explore]

It's notable that almost every animal sleeps in some way, even organisms that are away from sunlight (ie. creatures that live underground).

If you sleep 30% of the time, that's 30% of the time you aren't eating, mating, etc. Also, during sleep you are more vulnerable to predators. One would expect evolution to get rid of sleep in creatures who don't rely on sunlight cycles.

So, there must be some really good evolutionary reason for sleep.

[JumpCrisscross]

> during sleep you are more vulnerable to predators

Marine mammals sleep unihemispherally [1]. Land mammals can burrow, et cetera, which explains why predation and injury risk decrease during sleep. (Counterfactual: "large animals that are not at risk for predation, such as big cats and bears, can sleep for long periods, often in unprotected sites and appear to sleep deeply" (Id.).

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC4948738/

[2] https://www.semel.ucla.edu/sites/default/files/publications/...

[carlosjobim]

You've got it backwards, the awake part of life is the sacrifice all living beings have to make to support the primary function of dreaming.

[gavmor]

This is a fun thought experiment, but our working definition of life is something along the lines of heritable metabolizing, which dreaming doesn't so obviously serve.

[davisp]

A wild tangent but reading “heritable metabolizing” really hit me on the “are viruses alive” question.

I’ve been around enough biotech to have considered the differences between plasmids and viruses versus archaea, bacteria, and eukaryotes. I’ve always considered “heritable change” as the base definition of “life”. As in, “life” is progeny resemble their parent(s)? Or “heritable change”.

“Heritable metabolizing” quite nicely captures that difference between the levels of single molecule “life” and singular/multicellular “life”.

Apologies for the random aside, it was just one of those random “I have a vague idea of why mitochondria are important, but I don’t see them as fundamental” parts of my “What is life?” definition being refined.

[mncharity]

> “heritable change” as the base definition of “life”

I years ago saw a research talk by someone doing, IIRC, regional-sized evolutionary-time-duration multi-scale ecosystem simulation - they made the same call.

[mncharity]

> heritable metabolizing

That's a really nice phrase - thank you.

Hmm, searches for `"heritable metabolizing"` have like no hits, *bolism only a few.

[gavmor]

For a broader definition, I'll refer you to Dr. Michael Wong, astrobiologist and planetary scientist at the Carnegie Institution for Science's Earth & Planets Laboratory: https://pca.st/episode/40b841c8-6f31-4bbb-88e0-c834ae319ee4

[lukas099]

> So, there must be some really good evolutionary reason for sleep.

Just one, but partitioning activities in time allows for further specialization. For example, instead of having generalist eyeballs that are okay at seeing in light and dark, you can have specialist ones that are really good at one or the other. Then another species in the same area that utilizes similar resources can specialize in the opposite, reducing competition between the two species (this is called resource partitioning).

Then, once there's a chunk of the day that is less productive than the other, it makes more sense for the body to devote that time to various recovery mechanisms. Then that chunk of time is even less productive, making using that time for recovery even more attractive.

[jlpom]

Repairing cellular damage (mitochondrial and main DNA) from oxidation thanks to slower metabolism is the main reason. On a side note taking vitamin E (an antioxidant that passes the blood-brain barrier) seems to have slightly reduced the need for sleep for me.

[Cotterzz]

It has a lot to do with diet and metabolism. Cat's sleep 2/3 of the time, have a very high metabolism and spend a very short amount of time eating or catching prey. (compared to say, Cows who are chewing for as long as humans are sleeping) So it's likely to conserve energy in situations where there is a limited food supply or constraints on what and when you can eat. Then there's animals that hibernate, for similar reasons, over winter. But that's only one aspect of sleep, there's also the file indexing and disk defragging that goes on when you're not running any applications.

[shepherdjerred]

I don't think this is necessarily true. Is there some really important reason many animals have brains in their head (versus elsewhere in their body), or is it that many species happen to be derived from some ancestor who had that trait.

Another way to ask this would be how much of a benefit would be not sleeping? It's difficult to preserve food, but I guess there is no limit on how much energy you can expend pursuing a mate.

[lukas099]

> Is there some really important reason many animals have brains in their head

I'm guessing proximity to sensory organs.

[shepherdjerred]

Maybe; my point though is that there are a lot of these common traits. I don't think these traits (e.g. need for sleep) being shared is necessarily because it's globally optimal.

[ajsisbckjx]

> So, there must be some really good evolutionary reason for sleep.

Is this how evolution works? Might it be that animals sleeping is good on the macro level but bad for the animal?

[dahart]

Parent is suggesting that sleeping must be good on the macro level. That does imply that it must also be good for the animal, right? In order to statistically benefit the population, the benefits to any given animal must statistically outweigh the costs. It can’t be all bad for all animals and still be good for the whole.

But sleep can be partially bad for some animals, they can be more prone to becoming prey while sleeping (though Google tells me sleep also prevents nighttime activity where predators have the advantage). There are definitely evolutionary adaptations around sleep to make it safer, sleeping in trees, sleep patterns & duration, hypnic jerk, etc.

[lukas099]

Not only that, there are 'parasitic genes' that are wholly bad for the animal and the species.

[jampekka]

I don't know why this is downvoted. It's entirely feasible that weaknesses in a species (or its individuals) may be beneficial for the ecosystem and hence survival of the species in the long term.

E.g. rabbit population being culled by predators may prevent the rabbits from eating their feed plants to extinction.