A shocking number of American kids are sad, suicidal and stuck on small screens sucking away their zest for life.

Why it matters: This is the indisputable and alarming trend among American children, based on the latest polling and deep research by an NYU professor in a book out next week.

The big picture: The pandemic is often cited as a driver of the teen mental health crisis, but it was brewing long before then. A growing body of research links the acceleration of the crisis to one of this century’s biggest events: the arrival of the smartphone.

• “Smartphones and social media fundamentally changed the way teens spend their time outside of school,” says Jean Twenge, a psychologist and author of the book “Generations.”
• “You take a generation of young people, they’re spending a lot more times in their rooms, alone, not sleeping, not hanging out with their friends in person. That’s a pretty bad formula for mental health.”

By the numbers: While the teen mental health crisis was slowly developing in the early 2000s, it rapidly worsened in the mid-2010s — which was also when teens’ smartphone and social media usage spiked, Jonathan Haidt, the NYU professor, notes in his forthcoming book "The Anxious Generation."

• Rates of depression and anxiety among U.S. adolescents were "fairly stable in the 2000s" but "rose by more than 50% in many studies from 2010 to 2019," Haidt writes in The Atlantic.
• The suicide rate for kids between the ages of 10 and 14 tripled between 2007 and 2021, according to the CDC.
• The share of U.S. high school girls who seriously considered attempting suicide jumped from 19% in 2011 to 30% in 2021, per the CDC. The share of boys who considered suicide rose from 13% to 14%.
• Just 1 in 3 12-to 17-year-olds say things are going well for children and teens today, per a recent Common Sense Media survey.

Zoom in: In study after study, teens say social media is making them stressed and depressed, but the time they're spending online keeps rising.

• In the early 2000s, middle and high school kids saw friends in person about 3 times a week. Now, that's closer to 1.5, according to data from the University of Michigan's Monitoring the Future project.
• At the same time, screen time has skyrocketed. Teens spend an average of 4.8 hours on social media apps like TikTok, Instagram and Snapchat every day, according to Gallup. Among teen girls, that ticks up to 5.3 hours.
• Teenagers are inundated with notifications, with one study estimating they get 237 pings a day, Haidt notes.

What to watch: TikTok's fate in the U.S. is currently being debated in Congress. If the app disappears from phones — though that's unlikely to happen any time soon — that would force a massive shift in how American teens spend their time.

• “It’s of course possible that people will replace TikTok time with YouTube time or Instagram time,” says Twenge. “However, TikTok’s algorithm is particularly effective at getting you to spend more time on it.”