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Puts the target to sleep. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves. Pokemon with the abilities Insomnia or Vital Spirit are immune. Pokemon with the ability Soundproof are immune.
GrassWhistle is unreliable due to poor distribution and accuracy. Most prominent users also have access to the much more reliable Sleep Powder, anyways. All things considered it's clear that GrassWhistle is completely outclassed.
Sing is a Normal-type equivalent.
Hypnosis is a slightly more accurate Psychic-type equivalent.
Sleep Powder is a more accurate equivalent.
Lovely Kiss is a more accurate Normal-type equivalent.
Dark Void is a more accurate Dark-type equivalent that hits both Pokemon in double battles.
Spore is a much more accurate equivalent.