Tilda Swinton Tilda Swinton

Actress Tilda Swinton slept in a glass box Saturday as part of an art installation at the Museum of Modern Art in New York City, the New York Daily News reports.

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Swinton's appearance was unannounced, but she has performed the piece, titled "The Maybe," since 1995 at various museums around the world. She will perform the piece again at MoMA several times before the end of the year, though the specific dates won't be revealed ahead of time, according to the News. Each performance will last a full day during the hours the museum is open.

During Saturday's performance, Swinton, wearing jeans and a button-down shirt with her glasses beside her, snoozed on a mattress pad covered with a white sheet as museum-goers looked on.