It started with mild anxiety.

Emily, who asked to be identified only by her first name because she was discussing her mental health, had just moved to New York City after graduate school, to start a marketing job at a big law firm.

She knew it was normal to feel a little on edge. But she wasn’t prepared for what came next: chronic insomnia.

Operating on only three or four hours of sleep, it didn’t take long for her anxiety to ramp up: At 25, she was “freaking nervous all the time. A wreck.”

When a lawyer at her firm yelled at her one day, she experienced the first of many panic attacks. At a doctor’s suggestion, she tried taking a sleeping pill, in the hopes that it might “reset” her sleep cycle and improve her mood. It didn’t work.

Americans are chronically sleep deprived: one-third of adults in the United States say they get less than 7 hours a night. Teenagers fare even worse: About 70 percent of high school students don’t get enough sleep on school nights.

And it is having a profound effect on mental health.

An analysis of 19 studies found that while sleep deprivation worsened a person’s ability to think clearly or perform certain tasks, it had a greater negative affect on mood. And when the National Sleep Foundation conducted a survey in 2022, half of those who said they slept less than 7 hours each weekday also reported having depressive symptoms. Some research even indicates that addressing insomnia may help prevent postpartum depression and anxiety.

Clearly, sleep is important. But despite the evidence, there continues to be a shortage of psychiatrists or other doctors trained in sleep medicine, leaving many to educate themselves.

So what happens to our mental health if we aren’t getting enough sleep, and what can be done about it?

How does poor sleep affect your mood?

When people have trouble sleeping, it changes how they experience stress and negative emotions, said Aric Prather, a sleep researcher at the University of California, San Francisco, who treats patients with insomnia. “And for some, this can have a feed-forward effect — feeling bad, ruminating, feeling stressed can bleed into our nights,” he said.

Carly Demler, 40, a stay-at-home mother in North Carolina, said she went to bed one night and never fell asleep. From that point onward, she would be up at least once a week until 3 or 4 a.m. It continued for more than a year.

She became irritable, less patient and far more anxious.

Hormone blood work and a sleep study in a university lab offered her no answers. Even after taking Ambien, she stayed up most of the night. “It was like my anxiety was a fire that somehow jumped the fence and somehow ended up expanding into my nights,” she said. “I just felt I had no control.”

In the end, it was cognitive behavioral therapy for insomnia, or C.B.T.-I, that brought Ms. Demler the most relief. Studies have found that C.B.T.-I. is more effective than sleep medications are over the long term: As many as 80 percent of the people who try it see improvements in their sleep.

Ms. Demler learned not to “lay in bed and freak out.” Instead, she gets up and reads so as not to associate her bedroom with anxiety, then returns to bed when she’s tired.

“The feeling of gratitude that I have every morning, when I wake up and feel well rested, I don’t think will ever go away,” she said. “That’s been an unexpected silver lining.”

Adults need between 7 and 9 hours of sleep a night, according to the Centers for Disease Control and Prevention. Teenagers and young children need even more.

It’s not just about quantity. The quality of your sleep is also important. If it takes more than 30 minutes to fall asleep, for example, or if you regularly wake up in the middle of the night, it is harder to feel rested, regardless of the number of hours you spend in bed.

But some people, “have a tendency to think they’re functioning well even if they’re sleepy during the day or having a harder time focusing,” said Lynn Bufka, a clinical psychologist and spokeswoman for the American Psychological Association.

Ask yourself how you feel during the day: Do you find that you’re more impatient or quick to anger? Are you having more negative thoughts or do you feel more anxious or depressed? Do you find it harder to cope with stress? Do you find it difficult to do your work efficiently?

If so, it’s time to take action.

How to stop the cycle.

We’ve all heard how important it is to practice good sleep hygiene, employing the daily habits that promote healthy sleep. And it’s important to speak with your doctor, in order to rule out any physical problems that need to be addressed, like a thyroid disorder or restless legs syndrome.

But this is only part of the solution.

Conditions like anxiety, post-traumatic stress disorder and bipolar disorder can make it harder to sleep, which can then exacerbate the symptoms of mental illness, which in turn makes it harder to sleep well.

“It becomes this very difficult to break cycle,” Dr. Bufka said.

Certain medications, including psychiatric drugs like antidepressants, can also cause insomnia. If a medication is to blame, talk to your doctor about switching to a different one, taking it earlier in the day or lowering the dose, said Dr. Ramaswamy Viswanathan, a professor of psychiatry and behavioral sciences at State University of New York Downstate Health Sciences University and the incoming president of the American Psychiatric Association.

The cycle can afflict those without mental health disorders too, when worries worsen sleep and a lack of sleep worsens mood.

Emily, who worked in the big law firm, would become so concerned about her inability to sleep that she didn’t even want to get into bed.

“You really start to believe ‘I’m never going to sleep,’” she said. “The adrenaline is running so high that you can’t possibly do it.”

Eventually she came across “Say Goodnight to Insomnia” by Gregg D. Jacobs. The book, which uses C.B.T.-I. techniques, helped Emily to reframe the way she thought about sleep. She began writing down her negative thoughts in a journal and then changing them to positive ones. For example: “What if I’m never able to fall asleep again?” would become “Your body is made to sleep. If you don’t get enough rest one night, you will eventually.” These exercises helped her stop catastrophizing.

Once she started sleeping again, she felt “way happier.”

Now, at 43, nearly 20 years after she moved to New York, she is still relying on the techniques she learned, and brings the book along whenever she travels. If she doesn’t sleep well away from home, “I catch up on sleep for a few days if necessary,” she said. “I’m way more relaxed about it.”