This makes Styles the latest in a list of celebrities such as Tom Hardy, Eddie Redmayne and Paul Mescal to gently send the nation to sleep with their soothing tones.
On Monday, the app followed up with an official teaser video that ended with Harry channelling his inner bedtime story-teller, saying: "Hello".
The story will be available from tomorrow - but obviously fans can not wait, and quickly the hashtag was trending with some enthusiastic, and some thirsty, tweets. The bedtime story is set to be called "Dream with Me" and it drops on Wednesday 8th July.
Military reservist in armed Rideau Hall incident in court, charges expected
CBC has confirmed Hurren is a member of the Canadian Rangers , an organization within the Canadian Armed Forces reserves. Mr Trudeau was not at home when the incident occurred at the entrance to the Ottawa estate on Thursday morning.
He said: "Sleep and meditation are a huge part of my routine, whether I'm at home, in the studio, or out on the road".
Some fans even said they had been reduced to tears, just from the seconds-long clip. "I'm Harry Styles" in that dulcet voice we know and love.
"I'M SCREAMING #DreamWithHarry", one person tweeted, while another wrote: "I'M GONNA SLEEP LIKE A BABY FROM NOW OMG".
So did 90k+ people, who liked Calm's tweet in under 24 hours. I'm so happy to be collaborating with Calm at a time when the world needs all the healing it can get.
Teasing the news over the weekend with a watermelon emoji, a book, a bed and the caption "This Wednesday", the brand revealed that Harry would be narrating the new section titled "Dream With Me". A free seven-day trial is also available.