If you always pick "sleep with the helmet on" or one of the situational nighttime options (the game will assume your default is sleeping with it on, if you did it the night before), then you won't get Corruption in some scenes. I guess the idea is that sleeping with it off even once weakens your psychic defenses, though as you point out the MC hasn't been wearing it the entire time he's in the monster world so it's a little weird that that hasn't made him vulnerable any. He can spend months in a world full of monsters unprotected and have no trouble, but sleeping one night in H.E.R.O.'s base without protection will leave him open to Corruption?