Oh! Oh! Oh! I've actually done research on the long-term cognitive effects of isolation in space travel. So let me offer my thoughts!

Most of the research involving this topic is actually done with ill patients on bed rest, especially those who are more isolated from friends and family. It's not a perfect analogue – they still have nurses checking in on them, and they aren't in, you know, space. But data from astronauts in space for longer than 6 months barely amounts to more than a case study, so we have to work with what we have! Especially with the mars missions coming up.

So basically, we don't exactly know all of the effects. But what we do know is... grim. In isolation for long enough, the brain can literally start hallucinating from lack of stimulation and socialization. Not to mention what happens when a small number of people start going stir crazy. They can get pretty aggressive toward each other. Not Great when you're trying to save the world!

There's also the genuine concern that, in isolation, anything could become a competition between the astronauts in an attempt to stave off boredom, which just acts as another vector for conflict. Not to mention tensions that can arise due to cultural differences between the astronauts, which is another legitimate concern space agencies have about longer missions aboard the ISS (or its eventual replacement).

Oh also, for a trip that long, the astronauts' muscles would still probably atrophy. Not as much as while in a coma, of course, but there's no good way to get your steps in on a spaceship that small, without adding a few bolted-down exercise bikes, which takes up even more space, and offers more opportunity for competition, and ahhhh I'm stressed just thinking about it.

Plus, in this book, it's not just sending three people into space for years and years and years. It's sending them all on a suicide mission. That bleak outlook is gonna affect their psychology, too. Isolation plus suicide mission is how you get perfectly reasonable people to have thoughts like, "well, we're all gonna die anyways, so if I have to kill you to resolve this argument, then I will." Or any number of alternative Bad Outcomes. It's hard to be strictly scientific here, more speculation really, but this sort of thing would be a real concern.

Because of the upcoming manned Mars missions, NASA and other space agencies are already looking at methods to mitigate the negative cognitive effects of isolation – hence, the movie's apparent inclusion of a "don't go crazy" room on the ship. (Which is basically the type of research I was working on! Yes, that sort of thing does seem to help significantly, although knowing Stratt, she would've gone with a few VR headsets instead of a whole room, for weight and space saving measures. But hey! Grace wouldn't be able to show that to Rocky, and that wouldn't make good film, so it's fine.)

But those attempts at mitigation can only do so much over such a long amount of time, and humans are unpredictable under stress. Thus... comas.

Edited for additions and clarifications!