When do baboons catch up on sleep?

Sleep appears to be life critical. Deprive a human of it and they suffer metabolic and psychological consequences. Some animals even die if they're sleep deprived for long enough. And, as most new parents know, given the chance, we can catch up on lost sleep by sleeping for longer later. But what about in the wild, where the pressure of daily life might not afford a sleep-robbed animal the opportunity to catch up? Do they go without? That's what Carter Loftus has been trying to find out in Kenya's baboon population...

Carter - There's this homeostatic sleep drive. After animals experience particularly poor quality sleep, they respond to that by sleeping better the following nights: longer and more intensely. But in the wild, animals face a lot of competing priorities; predation, they have to find resources, and we really don't know anything about how animals in the wild navigate between having to invest in sleep and social and ecological priorities.

Chris - So really, the gold standard is: you're robbed of sleep, you make up for it in some way. But, because we have very artificial circumstances when we're doing the sorts of studies that inform how animals and humans take sleep, we don't capture that tension in the way we do those studies?

Carter - Exactly. The vast majority of studies of sleep have occurred in the laboratory or at the bedside where these external factors that might shape our sleep are intentionally removed. But what they remove are the ecological and social pressures that may be really influential on the way that sleep has evolved in the environment in which it actually evolved.

Chris - Presumably, part of the reason why those are not captured is because it's actually quite hard to do this, I would think?

卡特——这是。和学习的黄金标准sleep research is polysomnography which requires intracranially or subdermally implanted electrodes that can measure brain activity. That's where we introduced a new method of using accelerometry to try to noninvasively answer some of these questions about how animals balance their need for sleep in the wild.

Chris - Accelerometry, that's basically capturing movement. Are you using movement as a proxy for how awake an animal is and, therefore, when they're not moving, they must be asleep?

Carter - Exactly. We're essentially looking for periods of sustained inactivity, and then we're validating this measure of sleep against thermal video that we have of sleeping baboons in the wild.

Chris - Which baboons were you studying and how do you capture that movement from them?

Carter - We were studying a group of wild olive baboons in Laikipia, Kenya, and collared greater than 80% of the adults and sub adults in the group with GPS collars that had an accelerometer unit implanted. Then, these accelerometers collect essentially all the movements that a baboon does for the duration of the study, which was around 30 days.

Chris - That's powerful because that tells you what they're doing and where they are when they're doing it. So, presumably, you can also get the proximity. You can look at where they are in relation to each other as well?

Carter - Exactly. That's something that we looked at as well as several other GPS derived metrics that we were able to pull from the data, such as how far they travelled on a given day, where they slept, where they slept in relation to their group mates, and how all of these variables actually had an influence on their sleep patterns.

Chris - The reason I brought that up is because, obviously when we do sleep studies, often we divorce people from other stimuli, including each other. So, being able to study how groups affect compensatory sleep, or sleep behaviour, is quite insightful. So, were you able to capture that in this study?

Carter - Yeah, we were. I think that was one of the most exciting findings of this study is that the collective dynamics that animals experience when they live in groups, really continue into their sleep period. We found that the baboons that we studied actually synchronised their periods of nocturnal awakening, which is actually quite in contrast to a lot of predictions which suggest that animals should really stagger their periods of nocturnal awakening to maximise their collective vigilance and be able to hear and see predators when they're coming as a group.

Chris - What did you actually see then? You tracked these baboons, you've got the whole lot over a really nice long period of time so if there are periods when they're robbed of sleep we should see compensation, etc. You've got the group dynamic: what trends and what patterns emerged which are not what we would predict based on previous studies of how individuals sleep?

Carter - Essentially, what we found is that animals in the wild can't afford to make up for lost sleep all of the time. The ecological and social pressures seem to be really the pressures that are driving their sleep patterns. When they sleep in a new and unfamiliar environment, they sleep quite a bit less and they sleep more in a more fragmented way. They also sleep quite a bit less when they are sleeping in proximity of more group members. But, surprisingly, how much they've slept in the recent history and how much physical activity they had on the preceding day had relatively minor influence.

Chris - Presumably there's some kind of cost attached to this loss of sleep?

Carter - Presumably, yes, but we don't have a lot of data on how exactly those costs manifest. The studies that have looked at this also found difficulties in finding the exact cost of lost sleep, which brings us back to one of the central questions of sleep research which is, "Why do we sleep if we can give a up sleep without major costs?" That's something that we still just don't really have a great answer to.

Chris - You don't think it's possible that, in your animals where they have been robbed of sleep, that in fact they are cat napping here and there? I know you studied baboons not cats, but you know what I'm saying. Perhaps there is a bit of catch up going on but, actually, it's below the resolution of your ability to catch it with the accelerometer. So, they're just dropping off periodically here and there throughout the day to compensate and you're missing it.

Carter - That might be possible. But one of the beauties of accelerometry data is that we can collect continuous and really high resolution data for relatively long periods of time. So, I would say that's rather unlikely. It could be that the physiological need for sleep is playing out at time scales that we're just not looking at. So maybe they've given up sleep for 10 days or something, and then they're catching up on it in the 10 days.