SOLVED. Sci-fi: aliens have red-hot bodies, and must sleep in pairs.

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SOLVED: see answer below.

This sci-fi has aliens that have red-hot bodies, too hot to have liquid water. They're basically heat-engines, that move by expanding air to change the volume of fluid chambers in their bodies. When they sleep, their bodies cool down enough for liquid water, and a symbiotic species swims around fixing damage inside their bodies. Because they are completely immobilized, they instinctively sleep in pairs, and feel nervous sleeping alone.

They might also excrete material that is useful as starship fuel or faster-than-light fuel, but that might be a different novel. I heard about this book on (I think) a Cortex podcast where CGP Grey mentioned it. Or some other random podcast. Either way, I haven't read it, and already tried searching TV Tropes for Aliens Never Invented The Wheel. ^^;

Thanks so much!

EDIT:
I actually found this myself. The book is Project Hail Mary, the alien species is the eridians, and I'd heard about it on Conversations With Joe, where Joe Scott interviewed the author.