GEOFF BENNETT: Doctors recommend that teenagers get eight to 10 hours of sleep every night, but many are sleeping far less than that, and nearly one in four also suffers from insomnia.

William Brangham reports from California on why it's so hard for so many teens to sleep, and how it's taking a toll on their mental health.

It's part of our ongoing series Early Warnings: America's Youth Mental Health Crisis.

WILLIAM BRANGHAM: It'll be another long night for 15 year-old Keiko Rakin, as she prepares for another day of high school in Alhambra, California.

KEIKO RAKIN, High School Student: I have homework to due every night.

I usually have two tests a week.

I'm in sports and, that's every day after school for two hours.

I'm in five clubs.

And... WILLIAM BRANGHAM: Five clubs?

KEIKO RAKIN: Five, and I have leadership positions in all of them.

And I can get overwhelmed.

You know, I can cry.

I have a hard time breathing.

And it's just me thinking, I have so much to do and I just don't have the time to do it.

WILLIAM BRANGHAM: The academic pressure, the college pressure, the sense that she's not doing enough, it's turned Keiko into a night owl.

Most nights, this high-school sophomore says she goes to bed around 1:00 a.m. and only sleeps five or six hours.

Does that feel like enough sleep for you?

KEIKO RAKIN: No, because it's really hard for me to get up.

And the whole day, I'm constantly yawning.

And I feel like I can fall asleep in class.

WILLIAM BRANGHAM: Keiko is not alone.

According to the CDC, more than 70 percent of American teenagers aren't getting enough sleep.

LISA DAMOUR, Author, "The Emotional Lives of Teenagers": There's no question in my mind that teenagers' sleep is less than it's ever been and probably worse than it's ever been.

WILLIAM BRANGHAM: Lisa Damour is a clinical psychologist in Shaker Heights, Ohio and the author of "The Emotional Lives of Teenagers."

While there are multiple factors causing 40 percent of high-school students to report having persistent feelings of sadness and hopelessness, Damour says sleep is a major culprit.

LISA DAMOUR: When teenagers are not getting enough sleep, they are grumpier.

They have a harder time focusing.

They have a harder time remembering things.

They're more likely to have accidents.

They like themselves less.

They like other people less.

The bottom line on this is that if we could bottle what sleep does for teenagers and truly for all of us, this would be the most valuable drug on the market.

WILLIAM BRANGHAM: So just imagine devising this experiment.

Researchers make their subjects wake up hours before their normal wakeup time.

Then you force them to perform complex mental tasks for five days straight.

That's basically describing the average teenager's school week.

Before the pandemic hit, the average public high school start time across the nation was 8:00 a.m. Last year, California became the first state in the nation to mandate that classes begin no earlier than 8:30.

Well, that's a welcome change for many, it hasn't changed much for Gabby Wong.

She's a junior at Mark Keppel High in California's San Gabriel Valley.

GABBY WONG, High School Student: Sleeping is just so -- it's a victorious feeling, almost, because it's like, I finally get to have enough sleep.

WILLIAM BRANGHAM: As co-captain of the school's debate team... GABBY WONG: You can have an adequate amount of speech time, and so that the judges see that you're up here.

WILLIAM BRANGHAM: ... one of her many extracurriculars, her school day starts at 7:30 a.m. GABBY WONG: You know, I get nightmares so frequently, like, anxiety-riddled nightmares.

WILLIAM BRANGHAM: It's the beginning of a grueling schedule that often keeps her up past midnight, something she admits has taken a toll.

GABBY WONG: I have suffered with mental health issues since I was 11.

But every night before I go to sleep, I just stare at the ceiling and I think, what have I not done?

What assignment have I not finished?

What extracurricular activities are coming up?

And you're thinking, I am not having an easy time with this.

Why am I the only one struggling with this?

But everybody is struggling with it.

WILLIAM BRANGHAM: Gabby told us her sleep is often fluctuating.

She sleeps between three and seven hours a night during the week, but up to 12 hours a night on the weekend.

GABBY WONG: I still wake up feeling tired every day, no matter what.

WILLIAM BRANGHAM: Are your parents on you about this?

GABBY WONG: Yes.

My parents have cut -- my Wi-Fi is off by 12:00 a.m., because they're so concerned about me not sleeping.

But the thing is, that concern develops, and then it ends up with me being anxious about not finishing certain things by 12:00.

WILLIAM BRANGHAM: Every student we spoke with said this wasn't a result of parental pressure.

They said these intense schedules and expectations were just part of being a teenager today.

Another major impediment to teenage sleep, technology.

Roughly nine out of 10 teens say they have access to smartphones or laptop computers.

And nearly half say they are online almost constantly.

LISA DAMOUR: There are plenty of teenagers who are losing sleep because they have their technology in their rooms, because they are on their phones late at night, because social media is so hard to pull away from.

So there's definitely reason to think that using smartphones, and especially at night, has something to do with sleep loss in teenagers.

ADRIANA GALVAN, UCLA Center for the Developing Adolescent: The brain parts that are lit up, so to speak, are showing us variations among adolescents who get better or worse sleep.

WILLIAM BRANGHAM: At UCLA's Center for the Developing Adolescent, neuroscientist Adriana Galvan and developmental psychologist Andrew Fuligni are studying the links between sleep and teenage mental health.

ADRIANA GALVAN: It's a chicken-or-egg problem.

Is it that the mental health concerns or issues cause poor sleep, or is it the other way around?

And they're related.

It almost doesn't really matter.

But we know that people who suffer from, for example, anxiety or depression, which are the most common mental health challenges that adolescents may undergo, are associated with poorer sleep.

WILLIAM BRANGHAM: Fuligni and Galvan have found that students who perform well in school academically often sleep less, and are more likely to face mental health issues compared to their well-rested peers.

But, even more alarmingly, other research has found that students who sleep less than six hours a night are three times as likely to consider or attempt suicide compared to students who sleep eight hours.

This all comes at a time when teenagers experience a natural shift in their circadian rhythm that begins to push them to stay up later.

ANDREW FULIGNI, UCLA Center for the Developing Adolescent: It's not their fault.

It's not their choice.

ADRIANA GALVAN: Yes.

ANDREW FULIGNI: It's what the biology is telling them to do.

So they have to go to bed later.

And then we're actually many times asking them to go to school earlier and actually loading on the academic demands in the evening.

WILLIAM BRANGHAM: So what should parents do who are trying to help their teens get more sleep?

Experts say their behavior is just as important.

ANDREW FULIGNI: It's not necessarily you're going to be on them all the time and harassing them to go to bed, but have an agreed-upon pattern, and that -- and times that you're going to go to sleep.

That's reasonable for teenagers.

ADRIANA GALVAN: We're in a society where Americans do not prioritize sleep.

It's not just the adolescents who are doing this.

All of us stay up late to get a little bit more work done or to go work out or get up early to go workout.

All of that is passed on to what our kids see and model their behavior after.

GABBY WONG: You have any ideas for, like, what the middle event should be, like in between those two?

GEOFF BENNETT: Gabby told us she's committed to trying to get more sleep going forward.

She also co-founded a mental health nonprofit that aims to reach even younger students in the San Gabriel Valley.

GABBY WONG: We would talk about how depressed we were, and a lot of adults would think you're only in like, what, fourth grade, fifth grade?

But that was how we were feeling.

And nobody validated that, because they didn't expect it.

And so seeing that, I don't want someone in the shadows to be going through that without anybody, frankly, to step in and look at that and say, I see you.

What you're feeling is valid.

Just because you're young, it doesn't mean you can't feel that way.

WILLIAM BRANGHAM: For Keiko Rakin, she also hopes her school will create an anonymous tip line for students to talk to other students about their mental health challenges.

KEIKO RAKIN: Teenagers, as a whole, we have those points where we go into these dark places, that we can't eat, we can't sleep, we can't focus.

And I have been there a couple of times, but I think that just talking to someone, if it's alone -- like, you're alone that night, and you just really need someone, that just having that connection, whether it's done in person or not, could really help.

WILLIAM BRANGHAM: It would be much-needed help that hopefully leads to some much-needed sleep.

For the "PBS NewsHour," I'm William Brangham in Alhambra, California.