Def: Back-to-school blues
Actually, since la rentrée is more like New Year's, despite being timed when everyone is shoved off to work and school after a whole lot of summer vacationing, there's greater significance to it. It's exciting and bustling and new for some; overwhelming, depressing and a reason to crawl under the covers for others. This phrase is for the latter group.
Bon courage, mes amis!