Early Bird causes quick awakening from sleep. In the turn the Pokémon is put to sleep, the number of turns it will remain asleep is preset. If it is 6 turns, it will be halved to 3. If 3, it will be rounded to 1. If only 1 turn is preset, it is rounded to 0, causing the Pokémon to instantly wake up at the end of the same turn. This also applies to the self-induced sleep from Rest, reducing its length from 2 turns to 1 turn asleep (not counting the turn in which Rest is used).
Outside of battle
Early Bird has no effect outside of battle.