What is a sleeper in fantasy football?
A sleeper is a player drafted late or going under the radar who you expect to significantly outperform their draft cost.
A sleeper is a cheap, overlooked player you believe will break out and return far more value than his ADP suggests.
A rookie receiver going in the 12th round who's just been named a starter is a classic sleeper — low cost, high upside if the situation breaks right. The whole point is asymmetry: a sleeper costs you almost nothing if he busts but can win you a league if he hits. (Note: this is unrelated to the Sleeper app, the platform LeagueLogs is built on.)
Good sleepers usually share traits like an improving situation, opportunity opening up via injury or depth-chart change, or strong underlying metrics the market hasn't priced in yet.