Inspired by her relationship with artist Matthew Barney, Björk wanted to create a record with a domestic mood.

Working with Knak, she wrote "Cocoon", a song which is lyrically about a woman who describes making love with her lover during their post-coital hibernation, and includes frank sexual narrative related both explicitly and through over-sharing and metaphor.

The song experienced moderate commercial success on record charts in the United Kingdom, Australia, and France but reached the top ten in Spain.

It premiered at the Raindance Film Festival in October 2001, and was made available online through the singer's official website in February 2002, closer to the song's release as a single.

Discussing the glitch sound of the song, Björk said: "when you take technology and use the areas where it breaks, where it's faulty, you're entering a mystery zone where you can't control it.

[5] Lyrically, for Michael Cragg from The Guardian newspaper, "it feels almost intrusive, like reading someone's diary as they write about a new love", as Björk had just started the relationship with Barney.

[6] Seth Stevenson from Slate magazine gave a positive review to it, commenting that because of the song, Björk is "actually at her best either barely murmuring or full-out yelling, and she may be the most stylized vocalist in music today".

While the first single Hidden Place, the choir-assisted Undo and the Matmos collaboration Aurora are among the highlights, it's Cocoon that best represents the album's sense of heavy-lidded, post-coital hibernation".

[5] David Fricke from Rolling Stone commented that "the flurry of rhythm" at the start of the song felt like "the gravity of a spider scurrying across linoleum".

[8] Ian Gittins, author of Björk: Human Behaviour - the Stories Behind Every Song, referred to "Cocoon" as the eyes of many of the most significant moments of Vespertine, as well as the most complete display and literal philosophy that the singer had taken for the album.

[10] Stephen Dalton from the same publication was less positive, and stated: "Then there is the uncomfortably intimate, tremble-whisper Björk voice of 'Cocoon' where she relates the joy of shutting herself away with her lover with a broken music box and some mouldy old string".

[11] Greg Kot from Blender magazine commented that album openers "Hidden Place" and "Cocoon" "live up to their billing as sound sanctuaries, with Björk singing a barely-above-a-whisper lullaby enhanced by the plush embrace of a choir".

[12] The Wire opined that the track "nearly matches the macabre tone of Dancer in the Dark, particularly the trembling moment three minutes in, when her barely audible whisper conjures a feeling beyond sadness as she pleads, 'Who would have known?

'"[13] In the United Kingdom, "Cocoon" debuted at its peak position of number 35 on the UK Singles Chart issued on 17 March 2002.

[19][20] Its treatment was described as playing with "minimalist white for both costume and bleached eyebrows, treating Björk as a geisha whose makeup extends over her entire nude body".

Jeremy Allen from NME magazine included it on his list of "10 Greatest Musical Moments" on the show, commenting: "with her fingers tinted like icicles and the background sonics just a sparse and glitchy soundscape with tinkles of warm xylophone like the inside of some ethereal cave, the singer takes a little bit of Iceland to the USA – via heaven – with her voice as commanding as it ever was".