Being a resident of the mental plane also gives Sleepwalker a resistance, but not an immunity, to various mental attacks. When battling the monster Psyko, a creature that had the power to instill madness in his opponents, Sleepwalker briefly suffered from insanity, although he managed to resist and overcome Psyko's madness to finally defeat him. At the same time, Sleepwalker was shown as able to absorb the energy of the attack and use it to increase his own strength and power, although he still suffered its effects.