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The wielder cannot be put to sleep or be rendered drowsy via Yawn. Rest will fail when used by this Pokemon. If the wielder is sleeping, it is instantly cured of sleep. A Pokemon with Mold Breaker can target the wielder with Yawn and make it drowsy, but the wielder will fail to fall asleep (unless it no longer has Insomnia the next turn).
Like Vital Spirit, Insomnia grants the wielder the ability to switch into sleep-inducing moves fearlessly. Banette and Honchkrow can take advantage of this fact to switch in and fire off powerful attacks, and the former can also Skill Swap its own Insomnia to screw over Rest users. Noctowl can become virtually immune to status with Psycho Shift in hand, and Hypno can wall special assaults all day long without fearing Hypnosis or Sleep Powder putting it to an early sleep.
Found in Dream World