A heat wave has scorched the Northeast, South, and Midwest this week, and those dangerously high temperatures can make it hard to sleep.

Studies show that extreme heat can affect both how much you sleep and how good that sleep is, said Chad Milando, a research scientist at the Center for Climate and Health at the Boston University School of Public Health. He and other experts said the people who are most vulnerable to poor sleep during a heat wave are low-income families who don’t have air-conditioning in their homes, as well as older adults or people with underlying health conditions that make them more susceptible to heat-related illness.

That’s why when temperatures rise, it’s essential to have a plan to keep cool when you sleep.

How Heat Affects Sleep

The body’s core temperature naturally drops during sleep, but hot environments can prevent the body from properly cooling. Studies also suggest that lower ambient temperatures signal to your body that it’s time to rest.

If the temperature in your bedroom is too high, it may be difficult to fall asleep, and you may wake up more frequently throughout the night, said Dr. Michael Irwin, a professor of psychiatry and biobehavioral sciences at the University of California, Los Angeles. You may also get less restorative sleep, he added.

To help your body regulate its temperature, your bedroom should ideally be between 65 and 68 degrees Fahrenheit, said Rebecca Robbins, an assistant professor of medicine in the sleep medicine division at Harvard Medical School. If your room is far hotter, you may wake up throughout the night, particularly during the stages of sleep when it’s not possible for the body to regulate its own temperature, she said.

“When exposed to extremes, we’re likely to wake ourselves up in order to shiver or wake ourselves up in order to sweat,” Dr. Robbins said.

Set Yourself Up for a Good Night’s Rest

There are steps you can take to sleep better during a heat wave, beyond just turning on air-conditioning, experts said.

“Sleeping well at night starts with staying hydrated and cool during the day,” Dr. Milando said. Drinking plenty of water when it is hot outside ensures that your body has enough fluid to cool down. When you’re dehydrated, you sweat less, and it becomes easier to overheat.

You can also keep your home cooler by closing your blinds or curtains to filter out direct sunlight, experts said. Keeping air circulating in your bedroom can also help. If you don’t have an air-conditioner, install a fan in an open window, which will help bring in the cooler air from outside, Dr. Robbins said.

To lower your body temperature before bed, place a damp rag on your forehead, Dr. Irwin said. “The moisture in that rag is going to evaporate across the night,” he said. But avoid ice packs, experts said, since placing them on your skin for too long can damage skin or cause frostbite.

Dr. Robbins recommended sleeping under a thin top sheet, which can promote airflow and make it easier to stick out your limbs out from under the sheet when you feel too warm. Pajamas should also be thin and loose to avoid trapping in heat. But when it’s extremely hot, “it might be a good time to try your birthday suit,” Dr. Robbins said.

If you are struggling to sleep peacefully during a heat wave, resist the urge to toss and turn in bed — it’ll only make you hotter, Dr. Robbins said.

“Try not to kick yourself for being awake, which we can all do,” she said. “Maybe get up, use the bathroom, try to keep the lights low, and then come back to your bedroom when you’re tired and get into bed when you are ready to sleep.”

The U.S. surgeon general, Dr. Vivek Murthy, announced on Monday that he would push for a warning label on social media sites that says the platforms can harm the mental health of young people.

When she heard the news, Jean Twenge, a research psychologist who has spent years warning about the risks of social media, had one thought: “Finally.”

Public health experts, educators and policymakers across the country have become increasingly concerned about the potential effects of social media on the mental health of teenagers, who spend an average of 4.8 hours a day on platforms like TikTok and Instagram.

The end of the school year can leave teens with extra free time to spend mindlessly scrolling on their phones. But the summer can also serve as a useful opportunity to reset family social media rules and take advantage of in-person hangouts and hands-on activities, said Dr. Marc Potenza, a professor of psychiatry at the Yale School of Medicine.

Here’s how to do a social media reset with tweens and teens this summer.

Get phones out of the bedroom.

This is Dr. Twenge’s top piece of advice to parents, she said. Have adolescents put devices elsewhere overnight, such as a communal charging station. Studies have found that screens can significantly disrupt sleep, and teens and tweens need eight to 10 hours every night.

“Parents are allowed a little bit of what I like to call ‘digital hypocrisy,’” Dr. Twenge said. “But this is one everyone should follow.”

Focus on in-person activities.

You can help your children scale back on social media any time of year, but summer provides unique opportunities for unsupervised, in-person play, said Lenore Skenazy, the president of Let Grow, a nonprofit that promotes childhood independence and resilience.

“We all remember our summers at the beach or going to the pool or the sprinklers,” she said. These experiences “aren’t just precious memories,” she said, “they’re developmentally rich.”

Making time for unstructured play allows children to solve problems, resolve boredom on their own, make friends and figure out what interests them most, according to Ms. Skenazy.

This might mean sending your children to summer programs during the week that offer plenty of phone-free time. On weekends, you could build in time for your kids to get together with friends while leaving devices at home.

You may also want to set a specific time of day that is always “outdoor time,” Ms. Skenazy said. This can help avoid “begging and negotiating” to stay on devices longer.

Be upfront about your rules.

When it comes to limiting kids’ social media use, “be very overt” about your rules, said Dr. Robert E. Lovern, a psychiatrist at the Walter Reed National Military Medical Center who has helped teenagers and their families reduce screen time. Tell your children exactly how much screen time they have each day and put time limits on apps. Parents should try to follow these rules, too, he said.

Adhering to those limits can be tricky during the school year, when many children rely on school-issued laptops for homework. Dr. Twenge, who has three teenagers, said her daughters’ school laptops had YouTube available.

“It drives me bonkers,” she said. “And I cannot hover over them all the time. So that’s why I think the summer is such a great opportunity for a reset.”

There are not clear public health guidelines about how much social media time is too much, said Dr. Jessi Gold, a psychiatrist and chief wellness officer with the University of Tennessee system. It may help to look for signs of problematic use, such as children becoming extremely irritable when they stop using a device, or their social media use interfering with other activities.

Explore boredom.

Kids often turn to social media when they’re bored to get a “dopamine hit,” Dr. Lovern said. Eventually, that can lead to difficulty coping with boredom, which can make children easily frustrated.

Kids often say they feel bored when they are in fact lonely or want attention, so it can help to ask if they are looking for comfort or companionship, experts say. The organization Children and Screens advises parents to allow their child to come up with their own screen-free solutions to boredom.

Help teens tune into how social media makes them feel.

Dr. Gold said that there were many factors besides screen time driving the youth mental health crisis, and that she didn’t believe that restricting access to social media altogether was the answer.

She wants to see parents and caregivers help young people develop the skills they need to use social media mindfully and in moderation. For example, she suggested working with kids to identify how certain content is making them feel.

“You could say: ‘Hey, let’s do an experiment. For the next week, instead of just mindlessly scrolling TikTok, pay attention to how you feel,’” she said. Is your teen clenching her jaw or fist after scrolling, or feeling sad or uncomfortable?

Encourage your child to jot down those feelings in the moment, she said, and find time to talk about them later.

The bird flu outbreak in dairy cattle has so far spilled over to just three farmworkers in the United States, as far as public health authorities know. All of them have had mostly mild symptoms.

But that does not guarantee that the virus, called H5N1, will remain benign if it begins to spread among people. Accumulating evidence from the animal world and data from other parts of the globe, in fact, suggest the opposite.

Some dairy cows never recovered from H5N1, and died or were slaughtered because of it. Infected terns seemed disoriented and unable to fly. Elephant seal pups had trouble breathing and developed tremors after catching the virus. Infected cats went blind, walking in circles; two-thirds of them died.

“I definitely don’t think there is room for complacency here,” said Anice Lowen, a virologist at Emory University.

“H5N1 is a highly pathogenic type of influenza virus, and we need to have a high degree of concern around it if it’s spilling over into humans,” she said.

In ferrets experimentally inoculated with the virus through their eyes — the presumed route of infection in the U.S. farmworkers — the virus rapidly spread to their airways, lungs, stomach and brain, according to a report published on Wednesday.

Other studies have found similar patterns in mice fed contaminated milk. The findings suggest that entry through the eyes or digestive system ultimately may not make the virus any less a threat.

H5N1 has shown itself to be promiscuous, rapidly gaining new hosts — wild birds and poultry, mice and bears, cats and sea lions. Since its discovery in 1996 in Hong Kong, it has also infected nearly 900 people.

An older version of the virus circulating in Asia has killed about half of those infected.

Of the 15 people known to have been stricken with the version that is now circulating in cattle, one in China died and another was hospitalized. Two patients in Chile and Ecuador had severe symptoms. Four Americans — one last year and the three infected with the latest outbreak — have fared better.

Crucially, no forms of the bird flu virus seem to have spread efficiently from person to person. That is no guarantee that H5N1 will not acquire that ability, said Yoshihiro Kawaoka, a virologist and bird flu expert at the University of Wisconsin-Madison.

“I think the virus is clearly changing its property, because we never saw outbreaks in cows,” Dr. Kawaoka said. Conjunctivitis, also known as pink eye and the primary symptom in two of the three farmworkers, is not typical of H5N1 infection. The appearance of the virus in mammary glands — in cattle and even in non lactating mice — was also unexpected.

The worry now is that as H5N1 continues to infect mammals and evolve, it may pick up the mutations needed to spread efficiently among people, setting off another pandemic.

The incubation period for flu is two to four days, and a human-to-human version could spread far before cases were detected, said Erin Sorrell, a virologist and a senior scholar at the Johns Hopkins Center for Health Security.

“If it goes into the general public, it’s too late,” she said. “We’ve missed the boat.”

Flu is typically most severe in older adults and children under 5. (An outbreak of swine flu in 2009 was not as devastating as feared, yet it killed nearly 1,300 children.) The severity of illness also depends on how much virus the infected patients are exposed to and for how long, as well as the route of entry and their genetic background and general health.

Infected people generally have fever and respiratory symptoms; some cases advance quickly to pneumonia or death. If the bird flu virus were to adapt to people, the world would need billions of doses of vaccines and antivirals to stave off these outcomes.

The federal stockpile holds four types of flu antivirals, but the drugs must be taken within 48 hours of symptom onset to be effective. One recent review found too little evidence to gauge the effectiveness of three of the four drugs, including the commonly used oseltamivir, sold as Tamiflu.

Some new versions of H5N1 have mutations that make the virus resistant to oseltamivir and to the other two drugs, but those changes, fortunately, have not been widely transmitted in animal populations. No mutations have been observed against the fourth drug, baloxavir.

But there are only a few hundred thousand doses of that drug in the stockpile, according to David Boucher, the infectious disease director of the federal Administration for Strategic Preparedness and Response.

Vaccines are a better bet to stem a pandemic, but enough doses are not likely to be available for many months, at the least. Even if global production of seasonal flu vaccines were entirely shifted to vaccines against H5N1, the number of doses manufactured would be enough for fewer than two billion people, assuming two doses were needed for each person.

In the United States, the national stockpile holds hundreds of thousands of vaccine doses that could be rolled out to those at risk, including children. Companies contracting with the government could make more than 100 million doses in the first 130 days, Dr. Boucher said.

Officials recently announced that they had taken steps to ready 4.8 million doses that could be bottled without disrupting seasonal flu vaccine production.

But most of these plans will help only if the virus cooperates.

Since H5N1’s first appearance, it has branched into many forms, and scientists have created a library of 40 so-called candidate vaccine viruses to match. Having them ready to go saves crucial time, because creating a new candidate can take three months, said Todd Davis, a virologist at the Centers for Disease Control and Prevention.

So far, he said, the virus has changed only minimally, especially the part of the virus that binds to human cells, called hemagglutinin or HA.

If the virus were to spread among people, it would first have to change significantly, some experts noted. “If this virus jumps into humans, you can bet that the HA is going to change, because right now the HA of this virus does not bind very effectively to human cells,” said Scott Hensley, an immunologist at the University of Pennsylvania.

Traditional flu vaccines are made by growing candidate viruses in eggs or in mammalian cells, both of which are fraught with potential problems: The virus may not grow quickly enough, or it may mutate too much as it grows.

In 2009, the candidate virus grew well in eggs but evolved into a poor match for wild H5N1 virus, introducing long delays in distribution to the public. “By the time the vaccine stocks were made and distributed, the initial wave of pandemic had already subsided,” Dr. Hensley said.

CSL Sequiris, a leading manufacturer of seasonal flu vaccines, has a cell-based H5N1 vaccine that is already approved by the Food and Drug Administration.

In the event of a pandemic, once CSL receives a candidate vaccine virus that matches the circulating virus, it could provide 150 million doses for Americans within six months, said Marc Lacey, an executive director at the company. (The firm also has contracts with 19 other countries.)

But 150 million doses would protect only about one in five Americans. Federal officials also are exploring mRNA bird flu vaccines, which could be made very quickly, as the Covid pandemic illustrated, to protect both cows and people. Dr. Hensley’s team is testing an mRNA vaccine in cows.

Officials have hesitated to deploy vaccines for cows because of trade concerns, experts said: Some countries bar imports of products from vaccinated birds and animals.

But immunizing cows would curb the risk to farm workers, and to other cows, and limit the opportunities for the virus to keep spreading and evolving, experts said.

So far, federal officials have also been reluctant to vaccinate farm workers, saying that the risk is still low.

The real danger, Dr. Lowen of Emory said, is if a farmworker becomes infected with both H5N1 and a seasonal flu virus. Flu viruses are adept at swapping genes, so a co-infection would give H5N1 opportunity to gain genes that enable it to spread among people as efficiently as seasonal flu does.

The possibility underscores the importance of vaccinating farmworkers, Dr. Lowen said: “Anything we can do to limit seasonal infection in people that are occupationally exposed to H5N1 could really reduce risk.”

Space changes you, even during short trips off the planet.

Four people who spent three days off Earth in September 2021 experienced physical and mental changes that included modest declines in cognitive tests, stressed immune systems and genetic changes within their cells, scientists report in a package of papers published on Tuesday in the journal Nature and several other related journals.

Almost all of what changed in the astronauts returned to normal after they splashed down on Earth. None of the alterations appeared to pose a showstopping caution for future space travelers. But the results also highlighted how little medical researchers know.

Christopher Mason, a professor of genomics, physiology and biophysics at Weill Cornell Medicine in New York City and one of the leaders of the research, called the collection of papers and data “the most in-depth examination we’ve ever had of a crew” as he spoke during a news conference on Monday.

The four astronauts traveled on a mission, known as the Inspiration4, which was the first trip to orbit where not one of the crew members was a professional astronaut. Jared Isaacman, a billionaire entrepreneur, led the mission. Instead of bringing friends along, he recruited three travelers who represented a wider swath of society: Hayley Arceneaux, a physician assistant who survived cancer during her childhood; Sian Proctor, a community college professor who teaches geoscience; and Christopher Sembroski, an engineer.

The Inspiration4 crew members consented to participating in medical experiments — collecting samples of blood, urine, feces and saliva during their flight — and to allowing the data to be cataloged in an online archive known as the Space Omics and Medical Atlas, or SOMA, which is publicly available.

Although the data is anonymous, that does not provide much privacy because there were only four crew members on Inspiration4. “You could probably figure out who is who, actually,” Dr. Proctor said in an interview.

But she added, “I just feel that there’s more good than harm that comes from me being able to share my information and for science to progress and learn.”

SOMA also includes data from other people who have flown on private space missions, as well as Japanese astronauts who have flown to the International Space Station, and a study that compared the health of Scott Kelly, a NASA astronaut who lived on the International Space Station for 340 days in 2015 and 2016, with his twin brother, Mark, a retired astronaut who is now a senator representing Arizona.

With more private citizens buying trips to space, the hope is that SOMA will quickly fill up with more information about a wider range of people than the older white men who were selected to be astronauts in the early decades of the space age. That could lead to treatments tailored to individual astronauts to combat the effects of spaceflight.

The wealth of information has also allowed scientists to compare short-term effects with what happens during longer missions.

During Mr. Kelly’s year in space, age markers in his DNA known as telomeres grew longer — suggesting, surprisingly, that he had become biologically younger. But the telomeres mostly returned to their earlier size after he returned to Earth, although some ended up even shorter than before he had left. Scientists interpreted that as a sign of accelerated aging.

The telomeres of all four of the Inspiration4 astronauts also lengthened and then shortened, indicating that the changes occur in all astronauts and that they occur quickly.

“A remarkable finding in a number of ways,” said Susan Bailey, a professor of radiation cancer biology and oncology at Colorado State University who led the telomere research.

Cells use RNA, a single-stranded string of nucleic acids that translates blueprints encoded in DNA into the production of proteins. Dr. Bailey said that RNA corresponding to the telomeres had also changed in the astronauts and that similar changes had been observed in people climbing Mount Everest.

“Which is a strange connection,” she said.

That suggests that the cause of the growing and shrinking telomeres is not weightlessness but rather the bombardment of radiation that people experience at high altitudes and in space.

That was not the only effect of spaceflight.

Afshin Beheshti of the Blue Marble Space Institute of Science and NASA’s Ames Research Center in California, pointed to molecular changes in the astronauts’ kidneys that might portend the formation of kidney stones. That would not be a problem during a three-day space trip but could become a medical crisis during a longer mission.

“Halfway to Mars, how are you going to treat that?” Dr. Beheshti said.

But now that the possibility is known, researchers could study how to prevent the kidney stones or develop better methods to treat them.

The astronauts took several tests on iPads to measure their cognitive performance in space. One test evaluated what is known as psychomotor vigilance, a measure of the ability to focus on a task and maintain attention. The astronaut stared at a box on the screen. A stopwatch then suddenly popped up within the box, counting the time until a button was pressed.

If the response was too slow, longer than 355 milliseconds, that was regarded as a lapse of attention. On average, performance in space declined compared with when the Inspiration4 astronauts took the same test on the ground. Other tests indicate deficits in visual search and working memory.

“Our cognition performance was unaffected in space, but our speed response was slower,” Ms. Arcenaux said in an email. “That surprised me.”

But Dr. Proctor said that might not have been a true difference in their ability to perform tasks in space, just that they may been distracted. “It’s not because you don’t have the ability to do the test better,” she said. “It’s just because you look up for a minute, and there’s the Earth out the window, and you’re like, ‘Whoa.’”

One of the advantages of gathering all of the data is to look for connections between the changes, something that was difficult for scientists to do with earlier, narrower data sets. “When you look at it as a whole, you start seeing the puzzle pieces together,” Dr. Beheshti said.

That could point to a common cause, “and then the countermeasures are easily more targetable,” he said.

Since they returned to Earth, life for some of the Inspiration4 astronauts has in many ways returned to the way it was before they went to space. Ms. Arcenaux is back to working 12-hour shifts as a physician assistant at St. Jude Children’s Research Hospital in Memphis. Dr. Proctor is still a community college professor. Mr. Sembroski, who lives near Seattle, now works as an engineer at Blue Origin, the rocket company owned by Jeff Bezos.

But Dr. Proctor is now also a science envoy for the United States Department of State. This week, she is visiting Peru and Chile, telling of her experiences at schools and universities. “I now have also this kind of global platform where I can go and do things like inspire and help prepare the next generation,” she said.

Ms. Arcenaux said that she remembered looking down at Earth from the cupola window of the SpaceX Crew Dragon spacecraft on the second day of her journey.

“I feel so connected with my fellow earthlings,” she said. “We are all one on this beautiful planet.”

As for Mr. Isaacman, he’s not done with space. He and three other nonprofessional astronauts will embark on a mission called Polaris Dawn, which may launch next month. During that flight, again in a SpaceX Crew Dragon capsule, Mr. Isaacman and another crew member are planning to attempt the first private spacewalk.

A new study, published Wednesday, found that girls are starting their menstrual cycles earlier now than in previous decades. The trend is more pronounced for girls from minority groups and those from lower-income backgrounds.

The findings add to growing evidence suggesting that some girls around the world are hitting puberty earlier in life — a shift that researchers say is associated with negative health outcomes later in life but one that still can’t be fully explained. Several studies have consistently found that the decline in age seems to be steepest among racial minority groups, girls from lower socioeconomic groups and those who have higher B.M.I.s.

“In pediatric practice, there’s been kind of a trend toward just assuming that Black girls go through puberty earlier. But what’s going on and how? What are the health outcomes associated with that?” said Dr. Juliana Deardorff, head of the Maternal, Child and Adolescent Health program at University of California, Berkeley, who was not involved with the study but reviewed its findings. “We should be thinking about this, not just normalizing these disparities.”

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The Background

The American College of Obstetricians and Gynecologists and other medical groups consider the onset of menstruation (known as menarche) a sign of overall health, alongside other measures like cycle regularity, blood pressure, body temperature and heart rate. Researchers have linked early menarche and persistent cycle irregularity to an increased risk of conditions later in life, like diabetes and cardiovascular disease, as well as breast and endometrial cancer.

According to ACOG, typically girls are between 12 and 13 years old when they first menstruate and it takes up to three years for their cycles to hit a regular cadence.

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The Research

The new study draws on data from more than 71,000 women who used iPhones and agreed to share health information in an app. The women were born between 1950 and 2005, and of multiple ethnicities, including Black, Hispanic, Asian and white. The participants also reported on whether their socioeconomic status was low, medium or high.

Overall, the study found that while the median age at menarche hasn’t fallen too drastically, hovering around 12 years old, the share of women who started their menstrual cycles when they were younger than 11 years old increased significantly over time. Almost 16 percent of women born between 2000 and 2005 were between nine and 11 at menarche, compared to just over 8 percent of women who were born between 1950 and 1969. The study also found that more women were experiencing irregular cycles for three years or longer after menarche.

The trend toward earlier menarche age was more pronounced for Black, Hispanic and Asian women than for white women, as well as those with lower socioeconomic status. An estimated 46 percent of these declines could be explained by body mass index.

“One of the things that’s striking about this study is that, historically, everybody has attributed these changes in menstrual cycles to increases in body fat and B.M.I.,” Dr. Deardorff said. But this study underlines the fact that “even among people of healthy weight and potentially underweight, these trends are still occurring,” meaning that other factors might be at play.

Some studies have suggested that exposure to pollutants and certain chemicals can disrupt the endocrine system and the functioning of the menstrual cycle, though the evidence isn’t conclusive. Researchers have also linked early menarche to external stressors, like structural racism, abuse at a young age and financial insecurity, said Dr. Deardorff, who has co-authored several studies on how those kinds of stressors might affect puberty.

“It’s no longer controversial to say that stress can get under the skin to affect different processes,” she said, referring to the concept of weathering, an increasingly accepted theory that links exposure to chronic, prolonged stress to accelerated aging. And, Dr. Deardorff added, age of puberty or menarche should be seen “as a kind of bellwether” of the kinds of stresses that children might be facing.

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The Limitations

The study was based on self-reported data, relying on participants’ memories of their first period and how long it took their period to become regular. While age of menarche is a more salient milestone that women are likely to remember later in life, “time to regularity after the onset of menarche is really tough to report retrospectively,” Dr. Deardorff said.

The study population also consists only of women who use iPhones, said Dr. Shruthi Mahalingaiah, co-author of the paper and assistant professor of environmental, reproductive and women’s health at the Harvard T.H. Chan School of Public Health. That means that the study excludes a large chunk of the U.S. population, particularly those from lower socioeconomic status, who may not use the smartphones. It is also unclear if these trends in the United States could be generalized to other populations around the world, she said.

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The Takeaway

Race, socioeconomic status and B.M.I. on their own can’t fully explain the disparities in menarche timing, Dr. Deardorff said. Those three factors are inextricable from other variables that might also play a role, such as where people live, the chemical exposures they encounter and the availability of nutritious foods and access to health care. It’s really hard, then, “to tease apart what their cumulative or interactive effects might be.” Future studies, she added, should follow girls at different phases of their lives to better understand how multiple factors during puberty affect their health in adulthood.