Spontaneous logout while playing a game on steam

I was playing *Don't starve together* with a friend on Steam on my MacBook Pro (15-inch, 2019, Mojave 10.14.6) when I was spontaneously logged out of my account at exactly 11pm. The same thing occurred the following evening at precisely 11pm. I'd been playing the game in full screen mode for at over an hour on both occasions. Any help would be appreciated.