Given its cast and director, it is disheartening that The Black Sleep isn't any better than it is. Basil Rathbone heads the cast as Sir Joel Cadman, who uses a mind-controlling drug known as "The Black Sleep" to place brilliant scientist Gordon Ramsay (Herbert Rudley) under his control. Cadman needs Ramsay's intellect and expertise to aid him in a series of mysterious, covert experiments involving brain transplants. Evidently Cadman has already endured a few failures, as witness the present feeble-minded state of his former "volunteer" Mungo (Lon Chaney Jr.). Ramsay and heroine Laurie Munro (Patricia Blake) finally learn what Cadman is up to when they stumble upon a dungeon full of his previous "experiments," including a demented, emaciated man (John Carradine) and a blank-eyed monstrosity (Tor Johnson). In his last mainstream film, Bela Lugosi essays the thankless role of Cadman's mute servant.
~ Hal Erickson, All Movie Guide