This move effect may be in need of research. Reason:Does the Shed Skin bug occur in Pokémon Colosseum and XD? How does the Shed Skin bug work in link battles? You can discuss this on the talk page.
If the target is asleep, Nightmare afflicts the target with a status that causes it to take damage equal to one quarter of its maximum HP each turn. If the Pokémon wakes up, the effect of Nightmare ends and does not reoccur if put back to sleep.
Nightmare inflicts damage equal to one quarter of the Pokémon's maximum HP every turn, after it attacks or its Trainer uses an item, or at the end of the turn if it did not attack and its Trainer did not use an item. If a Pokémon in a Nightmare causes an opponent to faint, it will not take damage that turn. Nightmare bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of Dig or Fly.
In Generation II only, due to an oversight, if an opponent cures their Pokémon of sleep with a Full Heal or Full Restore, it will not be cured of Nightmare even after waking up; this does not occur if the player cures their own Pokémon of sleep.
Nightmare inflicts damage equal to one quarter of the Pokémon's maximum HP at the end of each turn.