1. Some event or situation in which people spend a lot of time sleeping. My son is having some friends over for the night. They're always really rowdy until the early hours, but then it's usually a sleep fest the whole next day.
2. Some event or situation that causes one to fall asleep because it is so boring or unstimulating. This director's work is usually really interesting, but I found his latest film to be a total sleep fest.
n. something, such as a dull lecture, that induces a long period of sleep. The history lecture today was a real sleepfest.