Wikipedia:

On October 16, 2011, Marconi Union created an eight minute track, titled “Weightless”, in collaboration with the British Academy of Sound Therapy.
According to scientists at the Mindlab institution (a commercial ‘neuromarketing’ company) it induced a 65% reduction in overall anxiety and brought test subjects’ resting pulse rates to 35% of their usual resting rates. The song features guitar, piano and manipulated field recordings. It is punctuated throughout by low tones that supposedly induce a trance-like state.

Is this the most relaxing song in the world? You’ll be the judge:

Seen here.