Pokémon Sleep records and rewards your sleep with Pokémon that you’d otherwise have to spend many waking hours catching. Developed by Japanese games studio Select Button Inc. and published by The Pokemon Co., Pokémon Sleep was the most downloaded game during the week starting July 16, according to mobile data analytics firm data.ai. The game celebrated hitting 10 million downloads Friday by offering some in-game items for free.

Users play by sleeping with their smartphones close to their heads, and the game keeps track of vibrations caught by the phones’ sensors to estimate sleep quality. The better your sleep metrics, the more creatures you collect.

You can also speed things up in the free-to-play game by buying in-app items to befriend more Pokémon, or by paying for a premium subscription that comes with bonus sleep points.

“I really make an effort to go to sleep at my set bedtime, which I never did before,” said Tomoki Toma, a 23-year-old from just outside Tokyo who’s used Pokémon Sleep faithfully for more than four weeks.

I got Psyduck!!! 😍 (sorry for the lack of audio I got too excited and forgot to record it) pic.twitter.com/ENsrvVhex6

— Pokémon Sleep OUT NOW! (@PokemonSleep) July 21, 2023

Pokémon Sleep enters an arena crowded by health-oriented apps such as Calm, BetterSleep and Sleep Cycle. Pokemon, a joint venture owned by Nintendo Co., Creatures Inc. and Game Freak Inc., said it spent over four years on the game, trying out different game designs in an effort to add “an element of fun.”

But interest is waning as more users voice boredom about a game whose outcome is determined while the player is asleep. What’s more, users can’t transfer creatures they find in Pokémon Sleep to other games. Searches for the title have sunk to about half their peak worldwide, according to Google Trends.

“It’s a lot of fun collecting different Pokémon, but the game does get a little repetitive after a while,” said 22-year-old Tokyo resident Reina Watanabe, who started playing three weeks ago.

There’s also plenty of ways to cheat: you can tell the game you’re going to bed when you’re actually curling up with a book, leave the phone on your bed while you get coffee, or just manually enter false sleep data.

Whether Pokémon Sleep can keep users checking in every night depends both on the draw of the characters as well as the perceived health benefits. Game play is limited, but there’s a long list of idle games that remain popular for years, running in the background on phones everywhere, such as AFK Arena, AdVenture Capitalist, Neko Atsume: Kitty Collector and Egg Inc. 

Pokémon Go broke records and celebrated its seven-year anniversary last month in part because the augmented reality game designed by Niantic Inc. reinvented itself through updates. That title’s success contrasts with Niantic’s struggles on some other games, which prompted the San Francisco-based company to close its Los Angeles studio and cut 25% of its staff.

Pokemon is exploring ways to link its newest game to more devices in the future, according to the company’s Pokémon Sleep producer Kaname Kosugi.

“Pokémon Sleep is fun, but without the quintessential Pokémon battles, it just doesn’t feel like you’re playing a game,” said Masamitsu Takahashi, 30, a longtime Pokémon fan from Gifu Prefecture in central Japan. “But I’ll wait to see if any major overhauls are in the works before I delete the app.”