The term sleeper has different meanings for different people in the world of fantasy football. Some folks believe it's simply a player who will outperform his draft position. That would include players like Justin Herbert, James Robinson, and Justin Jefferson in 2020. Others believe it's someone who was a relative unknown who emerges into a useful fantasy option, such as Myles Gaskin, Curtis Samuel, or Robert Tonyan.
However, I think there are different levels of sleeper value among NFL players.
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For example, the athletes I labeled as traditional sleepers are likely to come off the board in the middle to late rounds.