There is a term for this in literature...

Hello! Back when I was in a very good literature class in college my professor talked about how literature often ebbs and flows with life. So when war and strife is happening, literature becomes darker and more realistic. Then, when life is better, literature follows suit and becomes lighter and delves into comedy more.

Does anyone know what this is called? Can you help me remember? There are clear peaks and valleys that follow history a lot in all forms of entertainment, but definitely, literature is where it is most prevalent. I've thought about this a lot since college. Afterall, it seems we are in one of those valleys now where everything is darker, more visceral, and "real."