'Barely Breathing' Singer Duncan Sheik Dead at 56

Duncan Sheik -- a singer-songwriter whose debut hit "Barely Breathing" made him a household name -- has died ... according to a post on his social media.

The post went up just a few minutes ago ... featuring a smiling Duncan holding his guitar. The first line of the caption announced his passing, which happened on Thursday.

The caption reads, in part, "The world has a lost (sic) a singular talent, and deeply influential voice in music and theater," adding, "Duncan’s artistry touched many and leaves an indelible mark on the cultural landscape."

The post notes Duncan was a father, husband, brother and friend ... and he will be remembered personally for his incredible wit, intelligence and "depth of spirit" he brought to those around him. A cause of death was not identified in the post.

Sources with direct knowledge tell TMZ ... the Grammy-nominated singer-songwriter had been receiving care in the intensive care unit of a New York City hospital earlier this week.

Duncan broke onto the music scene in 1996 with his self-titled debut album. The album went gold in the United States ... and "Barely Breathing" -- the breezy pop-rock single on the album -- netted him a Grammy nomination.

He released a few follow-ups in the next few years -- though none of them achieved as much success as his debut -- all while collaborating with Steven Sater on the Broadway rock musical "Spring Awakening."

The hit show -- which took 8 years to write -- won Duncan two Tony Awards ... Best Orchestrations and Best Original Score. It also won a Grammy for Best Musical Show Album ... and helped launch the careers of "Glee" stars Lea Michele and Jonathan Groff. His new show -- "Memoirs of an Amorous Gentleman" -- is set to begin previews next week.

Duncan was 56.