The sleep aids market is predicted by the market-research firm Persistence to be worth $81bn (£64bn) by next year.

Fitness tracking is one of the Apple Watch’s most popular features, and this fall it is introducing sleep tracking for the Watch. When it launches, it will roll out to tens of millions of users. (Apple will likely soon pass 100 million cumulative Watch sales, if it hasn’t already.) Oura, meanwhile, is approaching 200,000 rings sold.