This psychological horror imagines a reality where sleep turns terrifying

Jason Yu attended the best film school in the world. No, not NYU or Łódź, but the set of Okja, where Yu served as an assistant director to Bong Joon-ho. “In Korea, the director and assistant director have a mentor/mentee relationship,” says Yu over a video call, in early July, from Bucheon. “You learn the trade from him or her, and they teach you how to be a director. Bong was the first person I gave my screenplay to. He told me it should be my debut feature.”

As Yu, who’s 34, really did skip film school, his idea of how movies are created was entirely formed by his time with Bong. Unsurprisingly, Yu’s aforementioned script, Sleep, is like Parasite, Okja, and other Bong films in that it smartly and unpredictably blends genres. On the surface, Sleep is a body-horror about an actor, Hyeon-soo (Lee Sun-kyun), who sleepwalks and self-harms at night, much to the distress of his pregnant wife, Soo-jin (Jung Yu-mi). Yet in between the jump-scares and hints of ghostly interference, Sleep is also a nuanced, darkly comedic marital-drama that satirises middle-class health crazes and superstitions. On second viewing, upon knowing the ending, it’s an entirely different movie. In other words, if it’s playing near you, sleeprun, don’t sleepwalk.

Still, Sleep is fundamentally a frightening movie, eking out an ongoing dread about the vulnerability of surrendering your body on a nightly basis. The other option, insomnia to the point of insanity, might be even more alarming. “Sleep can bring you peace, tranquillity, and happiness,” says Yu. “But you have to have complete faith that while you do this act, you’ll be safe. So when you flip it around and make it horrific, that’s really devastating. It works well for horror because you’re unconscious and at the mercy of your surroundings.”

While Hyeon-soo and Soo-jin should feel protected inside their flat in South Korea, the phone call is coming from inside the bedroom: in an unconscious state, Hyeon-soo scratches his face until it bleeds; he wanders around and gulps down raw meat; he even attempts to leap out of a window, until Soo-jin intervenes. Divided into three chapters, the film presents a trio of extreme measures, including Soo-jin’s panicked paranoia that a vengeful ghost has possessed her husband and will murder their baby. Wherever you think the plot is going, you’re wrong. “Some people call the film open-ended,” says Yu. “But when people discuss it with friends, they realise it’s close-ended. I would love for people to watch the film twice, not because of monetary reasons, but it’s structured for you to get something out of a rewatch.”

Almost entirely a two-hander, Sleep is led by two titans of Korean cinema in Lee Sun-kyun, who played the affluent father in Parasite, and Jung Yu-mi, the star of numerous Hong Sang-soo comedies. In fact, Lee and Jung had previously acted together in three films by Hong: Our Sunhi, Oki’s Movie, and Lost in the Mountain. Subsequently, the actors rejected Yu’s request for rehearsals. “What they relied on, rightfully, was their lifelong friendship,” says Yu. “On their Hong Sang-soo films, they were mostly acting as a couple, and their chemistry was already there.”

During the shoot, Lee came to set with a script full of notes, including ideas for individual line readings. “He would give me ideas for what Hyeon-soo would or wouldn’t do,” says Yu. “He knew everything about the character. Even though he was a veteran, he was a very hard worker.” In contrast, Jung would turn up with an open mind. “She said to me, ‘Director Yu, just tell me what to do, give me instructions from one to ten, and I’ll do it.’ I would tell her something like, ‘For this scene, you’re 20 per cent angry with him, and for this, you look at him twice.’ She ingested it like a supercomputer. I was watching two styles of genius working their craft.”

Yu, too, had a specific method of working, even if it was borrowed from his mentor. On Okja, Yu noticed that Bong not only storyboarded the entire film before production, he hand-drew it all himself. “I didn’t work for other directors extensively,” says Yu. “I thought that was normal.” After writing Sleep, Yu went straight into storyboarding every scene by hand. “My peers thought I was crazy and wasting my time. It’s a one-in-a-million shot to get a film made, and they thought I should be honing the script. But I said: no, when Director Bong finishes a screenplay, he goes straight into the storyboard. I’m going to do what I learned.”

Far more of a popcorn flick than an arthouse slow-burner, Sleep is slick, efficient, and, judging from the niftiness of its transitions, adhering to careful planning. “You shoot what you prepare,” says Yu. “That’s what I learned under my tutelage with Director Bong.” Yu reveals that Bong offered notes throughout the process, some of which he applied, some he didn’t. I immediately ask about the disagreements. “This is the first time I’m saying it,” Yu says, laughing. “I’m hoping he won’t read this!”

When chapter one segues into chapter two, a piece of music accompanies an optimistic montage of Soo-jin giving birth and the couple starting afresh. “I worked hard with the composers on creating that score,” says Yu. “But Director Bong was concerned it would harm the momentum of the film, and that the score should be darker to give this feeling of dread that’s about to come. It was a good point, but I was so in love with the score.” In the end, Yu didn’t change the music, but Bong was forgiving. “When we watched the final cut together in the theatre, he said he really liked it. He was very proud of what I did.”

In his twenties, Yu worked as a subtitles translator for a number of Korean films, most notably Lee Chang-dong’s Burning. He’s thus fastidious about the English subtitles for Sleep: the rhythm, the line lengths, the best wording. “I was obsessive about how to time lines to get the biggest shock or laugh,” he says. “I was very anal, and I’m sure the subtitle creator hated me.” Given Yu’s fascination with how audiences receive the film, I ask him how he’d feel if they dozed off during Sleep. After all, some directors, like Apichatpong Weerasethakul, consider it a compliment. “I’m horrified by the idea,” Yu says. “I would be pretty hurt.”

For his next feature, Yu is working on two scripts: a mystery-thriller in the vein of Sleep, and a romantic comedy that, he admits, “nobody seems to be very excited about”. After all, Yu has already proven himself as a horror filmmaker: in Korea, Sleep topped the box-office the weekend it hit theatres. However, the UK release of Sleep arrives with a tinge of melancholy: in December 2023, Lee Sun-kyun died, aged 48, from carbon monoxide poisoning, shortly after media scrutiny and a police investigation about alleged drug use. In January 2024, Bong led a press conference in Seoul to question how future deaths of this manner could be prevented.

“Technically, Sleep isn’t Lee Sun-kyun’s last film,” says Yu. “There are two more films in his slate that are going to be released this year, which I’ve heard great things about. I’m eternally grateful that such a veteran actor was so generous to give me the opportunity to create my own film.” He adds, “His death was a great tragedy. Not only for his friends and family, including myself, but for the whole country, because he was such a great actor. It was like everybody lost their best friend on TV, almost. We haven’t processed it yet.”

Sleep is in UK cinemas on July 12