The app, Calm, has become the first company focused on mindfulness and meditation to receive unicorn status following its latest funding round.

Created by British entrepreneurs Alex Tew and Moshi Monsters founder, Michael Acton-Smith, Calm raised $88 million in its Series B investment round with a valuation of $1 billion. The latest investment takes Calm's total raised to $116m.

Launched in 2012, the app has received over 40 million downloads and now has over one million paying subscribers, which it says led to revenue quadrupling in 2018 and the company turning a profit for the first time. And now, it is on course to reach revenues of $150m.

At the start of 2018, Calm raised $27m with a valuation of $250m, meaning its valuation has also quadrupled in a little over a year.

The app offers a range of mindfulness and meditation exercises, as well as 'Sleep Stories', capitalising on the growing trend of focusing on one's own mental wellbeing. However, Michael Acton-Smith has previously stressed that Calm is not a treatment tool for medical mental health problems.

Following the unicorn valuation, Acton-Smith said: "We started as a meditation app, but have grown far beyond that. Our vision is to build one of the most valuable and meaningful brands of the 21st century. Health and wellness is a $4 trillion industry and we believe there is a big opportunity to build the leading company in this fast-growing and important space."

Calm said it will use the bulk of its investment for international growth and its content.