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

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/

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/