Giorgio GilestroThis is a cute elephant. Elephants spend up to 18 hours a day eating grass, bushes, roots, shrubs to maintain their appropriate calorie intake. They sleep only for 1 or 2 hours a day.
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This is a cute bat. They, on the other hand, are believed to sleep more than 20 hours a day. 2/25
This is a cute Great Frigatebird. They would normally sleep 9-10 hours a day and you would have hard time trying to get them to sleep less than that. Unless it's migratory season. In that case, they sleep 40 minutes a day, while they fly for days and days in a row. 3/25
Evolution is one of the great mysteries of sleep. Why do some animals require 20 h, while others can cope with 1? Whatever sleep function is, how can it be accomplished in 10 hours in one season and 40 minutes in another, as it happens in migratory birds? We won't really understand what sleep is and what it does if we keep thinking about it in an anthropocentric way. We need to look at it from the evolutionary standpoint and only then we will be able to grasp what its role in nature is. 5/24
That is why I am chuffed to introduce our most recent work, about the evolution of sleep in the Drosophila genus. Non peer-reviewed pre print available on biorXiv. Let me tell you what we did and what we found. 6/24
We did not compare sleep between elephants and bats. Too tricky to keep in the test tube and too evolutionary distant. Instead, we used 7 species of Drosophila spanning an evolutionary distance of 5-50 Million years and with different ancestral origins and adaptation niches 7/24
In all of them, we measure sleep using a computerised video tracking system based on Raspberry PIs which can be linked to robots to deliver sensory stimuli in real-time, such as puffs of air or automatic rotations of the test tube to keep them awake 8/24
We combined those with the excellent hidden Markov chain model from the GriffithFlyLab
at Brandeis and were able to confirm that different sleep stages as detected by the Markov chain do indeed coincide with different arousabilities. Deep sleeping flies are harder to wake up! 9/
at Brandeis and were able to confirm that different sleep stages as detected by the Markov chain do indeed coincide with different arousabilities. Deep sleeping flies are harder to wake up! 9/
We found that all species sleep in a similar way, although for very different lengths of time. In almost all species, sleep is sexually dimorphic: females sleep only at night and males sleep in the afternoon too. Except for D. virilis: a cosmopolitan species believed to have arisen in the Miocene in the deserts of Afghanistan. Interestingly, this is something found in other desertic species. You probably don't want to be flying around in the desertic afternoon! 11/ https://www.biorxiv.org/content/10.1101/2023.05.27.542279v1
So, sleep amount is generally conserved and obviously it adapts to species-specific ecological conditions, exactly as for the elephant and the bat. But what about sleep homeostasis? How do these exotic flies react when we try to keep them awake? For this, we turned to our trusty robots and kept flies awake for 24 hours in a row by rotating their little world around every time the fell asleep. A bit like in the Inception movie. Watch the first tube from the left to see the robot in action.
When you deprive an animal of sleep, it tries to recover some of it ASAP. This is a hallmark of sleep homeostasis and what we observed in D.melanogaster: but not in any of the other species! Like the migratory birds, they suddenly seemed ok not sleeping. And even making our robot work for 7 days in a row - 168 hours - did nothing to them! These other species could stay awake just fine and showed no signs of tiredness. Except for melanogaster, which showed a steady increase in sleep pressure.
We see the same effect in at least four of our 7 species: male-male interaction does lead to sleep rebound in melanogaster, simulans, sechelia, yakuba. Still no signs of homeostasis in the remaining three though. What decides whether an animal will show homestasis? It seems the answer is in their brains. We found that, in general, sleep rebound correlated with an increase in synaptic strength. All the flies that showed rebound also showed a larger amount of a specific synaptic protein 19/
And conversely, when we remove synaptic proteins from specific parts of the brain involved in learning and memory in D. melanogaster we get a similar effect: no tiredness after sleep deprivation
What is the take-home message? Well, we try to figure out what all this means in evolutionary terms. We think sleep has different functions in different species (doh!) and some functions therefore evolved for some species but not others.
The one common thing all animals have in common is they all sit on the same planet which has been rotating at the same speed for a very long time. We believe this adaptation created sleep in the first place giving animals a chance to optimise their activities to days & nights
Then, other sleep functions kicked in. Some animals need sleep to cope with stress. Some others need sleep to learn better. To memorize. To fight bacteria. Who knows how many different functions there are? Some need sleep for multiple reasons at once. 24/
This makes sense on multiple levels and can ultimately explain why elephants can do in 1 hour what bats seem to take 20 hours for! This work was led by Michaela Joyce, with wonderful Alice French, phd students Laurence and Fede, and Lucia Prieto Godino at The Crick.
Ben Longdon@giorgiogilestro nice stuff! Re "fight bactera" can you point me to studies looking at how (lack of?) sleep affects susceptibility to pathogens in flies?
MAHanson@b_longdon @giorgiogilestro the role of nemuri in survival here is likely primarily with relation to its role in promoting sleep. Though the paper took the tack of its mild similarity to a vertebrate AMP to entertain otherwise. Jury's still out, but a good ref and should also be good as a source for further refs if interested!
David Duneau@b_longdon @giorgiogilestro I am not aware either. This new amazing device would allow to test that nicely with Drosophila though.
Hi @b_longdon I was mostly writing in general terms actually, beyond the Drosophila realm.
@giorgiogilestro ah my misunderstanding - would be cool to look at in flies given reported circadian effects on immunity. And if you ever want any more fly species let us know (50+ in the lab atm).
David@giorgiogilestro So this looks like evidence that sleep contributes to better cognition through synapse creation or maintenance. Is there evidence that it serves as garbage collection as it does in mammals? I'm not up on the literature so please excuse the ignorance.