Oversomnia is a traditional fangame. You play as a boy named Irving, and your goal is to explore his dreams while collecting effects. You enter the dream world by sleeping in bed and save by using either the closet the phone in the room. The latter allows you also to view stats (how many times you slept, the number of effects got and the number of events you have triggered) and to change the menu theme.
Unlike many fangames, there is an NPC (seated behind a booth next to the Nexus doors) who will provide Irving with hints and temporary item abilities for a fee.
There is currently very little plot. According to the creator, Irving is part of a "dream club" whose three members gather or otherwise use other means to share the dreams they have. Therefore, unlike many protagonists Irving is willing and able to leave his room as he pleases.