Sleep review – marriage unravels in gleeful Korean somnambulist psycho-chiller

The sleep of reason is what’s supposed to produce monsters … but not as many as sleep deprivation. That is the awful paradox driving this elegant, intimate, gleefully brash Korean chiller from feature first-timer Jason Yu, a former assistant to the Oscar-winning director Bong Joon-ho. It’s a horror movie about sleepwalking and the unquiet sleep of a fraught marriage, with hints of The Exorcist, Sleeping With the Enemy and Steven Soderbergh’s neglected Hitchcockian thriller Side Effects.

Korean leading man Lee Sun-kyun appears posthumously in this movie – having heartbreakingly killed himself last December following a tabloid media scandal about his personal life. Poignantly, he gives one of his best performances as Hyun-su, an up-and-coming actor who has already won some kind of indie award and landed a minor role in a big studio film. He is happily married to Soo-jin, played by Jung Yu-mi, an estate agent who is heavily pregnant with their first child; she has evidently put off maternity leave until the last moment, perhaps because they are reliant on her steady pay. They live blissfully in a small flat, with Hyun-su using his acting talent to do a hilarious impression of the grumpy old guy in the downstairs flat who complains about being kept awake by their lovemaking. They have a sweet pomeranian dog called Pepper, but all horror devotees naturally worry about the safety of any pet in this kind of film.

Is Hyun-su secretly anxious about becoming a father? Or about his precarious career? Is he intuiting, perhaps, the possibility of postpartum depression in his wife? It could be the explanation for his strange somnambulist behaviour. Soo-jin is exasperated by her husband’s snoring, but more worried still when it stops; she wakes to find him sitting upright, quietly saying: “Someone’s inside.” His sleepwalking becomes concerning, then terrifying, after the baby is born. Soo-jin doesn’t trust herself, or her husband, to go to sleep alongside him and addresses a timid question to her unconscious husband: “Would you harm our child?” He murmurs: “I don’t think so.”

He’s right: someone is inside. An unconscious presence, an id, an alternative self that we are powerless to stop emerging when we are asleep. And the condition of marriage makes our relationship with this hidden someone more complicated still. When Hyun-su’s problem first emerges, he offers to live apart from his wife for a while in a hotel room but she says no; a committed loving couple stays together, tackles their problems together. They are one unit, and as they lie together in the marriage bed, only dimly aware of each other’s sleeping or even waking selves but always unconsciously sensing something, they are each other’s ids. And so the nightmare begins: Hyun-su entering the twilit somnambulist world of non-sleep and Soo-jin unable to sleep at all.

Kim Gook-hee gives an amusing, unsettling performance as their neighbour Min jeong who has a brattish son: she gets a dog exactly like theirs, something which Soo-jin interprets as an intolerable insult, for reasons she can’t fully explain. The movie expertly hits funny notes and light-relief moments that give us a break from fear while promising more to come, while the couple’s relationship with this ambiguously friendly neighbour is effectively managed. It is only with the explicit possibility of a supernatural explanation, combined with full-on psychiatric breakdown, that the movie loses its light touch and its plausible detail. Yet there’s always a hyper-vigilant twinge of fear.

• Sleep is in UK cinemas from 12 July.