Soo-jin (Jung Yu-mi) and Hyun-su (Lee Sun-kyun) are the sweetest young couple, full of promise and hope. Soo-jin works in business, hoping to make her way to the executive branch. Hyun-su is an actor; while he's only had small roles, they are both hopeful that he will find the success he deserves. They also have a baby on the way - pressure, to be sure, but their love should carry them through. That is, until Hyun-su starts sleepwalking - and more to the point, starts doing strange and increasingly dangerous things while sleepwalking.
In his feature directorial debut, Jason Yu weaves a tale of increasing dread and the limits of unconditional spousal support, of how desperate a person can become when their loved ones are a danger to themselves and those around them, and how a forgotten slight can manifest as terror. Sleep has a healthy dose of humour, but it also earns its scares, both jump and slow-building.
Being sensible young people, the first thing Soo-jin and Hyun-su do is see a medical doctor - he diagnoses Hyun-su with a sleep disorder, and the couple proceed to change their eating, drinking, and other habits to make sure he gets a decent night. But a terrible self-inflicted wound leads to putting oven mits on his hands, and still even higher doses of sleep medication is not enough - Hyun-su is becoming more violent; once that violence is turned on a living creature, Soo-jin suspects she might not be able to trust him with their newborn baby.
Yu focuses most of the time on the apartment setting: large enough to allow for movement and places to hide, small enough that Soo-jin feels (understandably) trapped: she loves her husband, but she must protect her baby. Hyun-su seems to be taking to fatherhood like a duck to water - is the problem, then, Soo-in? Yu allows us to understand the postpartum depression that affects many new mothers, and compound that with a husband who might be deadly, and that depression could turn to psychosis.
Her husband did say, on his first sleepwalking night, 'Someone's inside' - but Soon-jin didn't find anyone (except the dog, Pepper). But is someone there? A passing comment about a recently deceased neighbour who hated the barking dog, was not happy about the possibility of a crying baby, and seemed to stare at Soo-jin a little too long comes into play (maybe a little later than it should have), as Soo-jin searches for ANY answer that could stop the danger. The love of these two is so adorable when they are awake. But once asleep, Soon-jin does not know what will happen, and so her lack of sleep, and what it does to her mental state, begins to mirror Hyun-su's.
Yu keeps the pacing tight as he racks up the tension; we see first as Hyun-su's state deteriorates, at least while he's asleep; and Soo-jin rapidly follows as she tries to protect their baby. The climax is perhaps somewhat predictable, but Yu makes sure to keep the couple, and their love, front and centre: whatever monster is trying to hurt them, whatever spell is cast that could kill them, their love is at the centre of their world.
Sleep doesn't exactly reinvent the wheel of haunted house films, but the strength of the actors and the tense, slow-build structure make it an enjoyable rise. There's plenty of laughs and lots of earned scares to keep the pace tight and the story interesting.