Zoom fatigue is vague
if you use the academic version of Google, Google Scholar, you will get only six results, and some are accidental: Google gives you something like "... Zoom. Fatigue ..." That's no surpise. Research takes time to be published, and Zoom fatigue is a recent phenomenon. There simply has not been time to do adequate research on it. While understandable, that also means alarm bells should start ringing when you read explanations for Zoom fatigue: How can scientists possibly know what it is and how it works, if they have not done the research?
Just to emphasize: the fact that we haven't studied it, does not mean Zoom fatigue is not real. It does mean that any explanation of how it works is tentative at best.
Do we know anything?
Clearly, video calls are not always a problem. Business, particularly those who engage extensively in what is called distributed work, have been using videoconferencing for decades. Despite that, there have been no previous large-scale complaints about video calls causing mental exhaustion. In fact, if we read the scientific literature on distributed work, we find that video calls are seen as an important tool to mitigate the psychological effects of social isolation. Working alone is hard for humans and video calls allow us to connect with people. Furthermore, companies like Google have had video portals for a while now. These are essentially "always on" video calls. But the way they are used, has not caused mental exhaustion.
The cause would seem to be the sheer number of video calls we are having right now. However, nobody knows what number that is. Not only that, there is a range of issues that are left unaddressed. How many video calls is too much? What is the first point where we start noticing it? When does it become so taxing that we just no longer have the energy to continue? How much does the type of call (work, teaching, informal) matter? I could keep asking these questions, but the point should be clear: we are trying to explain a phenomenon without having any real information.
Let's look at some psychology first. One explanation is that video calls require us to multi-task more than we can handle. We are looking at a lot of people at the same time (4 in Microsoft Teams, but a lot more in Zoom), when we should only focus on one. Normally all those other people would be in our peripheral vision, where they would be less distracting. However, at the same time, if only the speaker is visible, that's also no good, because then you cannot see what everybody else is doing, which again you would notice in your peripheral vision. And all this overwhelms the brain.
Now fun fact, it is entirely unclear if this indeed is what overwhelms the brain. While I don't doubt that multitasking is hard, there are no neurological studies that have measured how human brains react to these kinds of conversations. The anecdotal evidence actually suggests the opposite. All those family meetings with large groups of people run of well, and people are enjoying them, even people who go through a lot of Zoom meetings. Now I'm not saying that therefore video calls are not overtaxing our brain, but the evidence points both ways. Therefore, we cannot just extrapolate from what psychological theory tells us.
Let's unpack. The interactional explanations focus on a few aspects. One, the continuous delay (lag) in video calls makes it hard to have a smooth conversation, which can be annoying. Two, we cannot have eye contact with other people, since the camera is above or below the screen. Three, we cannot use all our non-verbal behaviour, but because we can see each other, our brains are still trying to use them.
All these points are definitely true. Research on silence in conversation goes back decades, and there is good evidence that we have more trouble with setting up a smooth conversation by video. Similarly, it is obvious that we struggle with where to look in a video call and our gestures cannot always be seen. However, how much this actually exhausts us is a secondary question. Nobody has ever investigated this.
The main problem here is that people are making claims about how our brain works, without having any way of connecting our behaviour to any responses in our brains. The explanations sound nice and plausible, but they are not supported by any evidence whatsoever. It makes for a nice story, but we should stick to what we know.
We need touch
We are living in a time of massive stress and anxiety. Some of us (your truly among them) live almost completely isolated from other people. I go out less than once a week for my shopping, and the face-to-face interaction with the cashier is all I have to sustain myself. Others live with partners and children, and are now forced to see each other all the time. This can create tensions even in good relationships, let alone when things are less then ideal, either in the relationship or in society at large. Anybody would feel exhaustion at some point.
Of course, I am just as most speculating about the cause of our exhaustion. In fact, that is the point. Without evidence, any explanation is as good as another - okay, any explanation that is not supernatural. Is Zoom Fatigue real? Possibly. But just because we feel exhausted now and are having a lot of video calls, there is no reason the two have to be causally related: Zoom does not have to be the cause of our fatigue. Ice cream consumption and murder both increase by about the same rate during the summer, but eating ice cream generally does not cause you to go into a murderous rage. Moral of the story: just because a scientist can provide a nice story and the BBC or National Geographic publishes it, that does not mean it's true.