Relaxation app Calm raised said Wednesday it has raised $88 million in its second funding round, valuing it at $1 billion.

TPG Growth led the Series B round, which included investments from Creative Artists Agency as well as existing investors Insight Venture Partners and Ashton Kutcher's Sound Ventures. Calm has now raised $116 million. The company will use the money to expand internationally and invest in more premium content.

Calm quadrupled its revenue in 2018, putting the company at a $150 million annual revenue run rate. The app says it has more than 40 million downloads worldwide and "well over" 1 million paying subscribers. Some of Calm's content is free, while some of it is available only to subscribers.

"Mental health was something people didn't really talk about until recently. Something is changing really dramatically. There's a societal shift of people taking care of their minds and being more interested in sleep and self care," co-founder and co-CEO Michael Acton Smith said in an interview.

Calm started in 2012 as a meditation app and has grown into a guide for sleep and relaxation. It and fellow meditation app Headspace have harnessed consumers' growing interest in and willingness to invest in their mental health and wellness.