Pokemon Asleep Status
From the rules, p15:


  Between turns, flip a coin. If you flip heads, the Pokémon wakes up.


So you flip a coin to wake up your pokemon after every turn - both yours and your opponent's. This means that even if the opponent can put your active pokemon to sleep every turn and you have no other way of cancelling it (like evolving the pokemon, or sending it to the Bench with a Switch), there's still a half chance that by the time your turn starts it'll be awake and able to act.