When the pandemic upended our lives, it gave researchers a rare chance to learn more about who we are and how we live. The simultaneous changes endured by the entire world created experiments that could never have happened otherwise. What happens when sports teams play in empty stadiums? When people see their doctors online? When the government sends people money? When women stop wearing high heels? When children stop going to school?

Research was challenging in such an extraordinary period: It’s impossible to know whether changes were caused by the specific thing scientists were studying, or by some other aspect of the pandemic — or whether we could expect the same result in normal times.

Perhaps one of the most important takeaways from the pandemic was that science is a process. Just as our understandings about masks or vaccines changed as the pandemic went on, these lessons might also change with time. For now, here are 15 things we learned.

1. Flu season doesn’t have to be so bad.

Flu virtually disappeared during the pandemic. The precautions people took to prevent the spread of Covid also played a role in preventing other respiratory viruses, experts found. Slowing their spread doesn’t necessarily require extreme measures, like stay-at-home orders, the studies showed. Simple behaviors — masking, hand-washing and avoiding social gatherings or workplaces when sick — help keep people healthy. Even those precautions haven’t stuck, though: This year, flu is surging.

2. Home-field advantage got less mysterious.

When sports teams started playing in empty stadiums, researchers could more rigorously study why players seem to do better at home. A variety of studies found that, yes, the fans made a difference: Home teams played worse without them around. They were less likely to win at home and had poorer performances — and the effect was smaller for teams that had frequently played in front of smaller crowds before the pandemic. But there was also evidence that it wasn’t just about fans. When the N.B.A. restarted play, the top 22 teams isolated in Orlando, Fla., allowing researchers to study the effects of jet lag. Rebounding, shooting accuracy and wins were all higher among players who didn’t have to travel across time zones.

3. Teenagers need to sleep in, but schools won’t let them.

Most teenagers were sleep-deprived before the pandemic — they don’t naturally tend to feel tired until around 11 p.m. and need around 10 hours of sleep a night. But when schools closed, teenagers around the world started sleeping according to their natural rhythms. They went to bed later (by about two hours, one study found) and slept longer. They woke up naturally, without an alarm or a parent, which doctors say is the sign of sufficient sleep. Teenagers lost these gains when schools reopened at their usual early start times. When high schools start later, other research has shown, it’s associated with improved concentration, behavior, attendance, learning and mental health.

4. High heels aren’t just uncomfortable — they’re dangerous.

Starting in March 2020, the number of women showing up at emergency rooms with injuries they said were from wearing high heels, like fractures or sprains, declined sharply. In 2020, there were 6,300 hospital visits for high heel injuries, down from 16,000 during each of the four years prior, according to data analyzed by Philip Cohen, a sociologist at the University of Maryland. Now he’s looking into whether injuries have increased since people have begun socializing and working in offices again, or whether the pandemic has hastened the trend toward flats and sneakers.

5. Patients don’t always need to see a doctor in person, if at all.

Telehealth, once uncommon, accounted for half of medical visits early in the pandemic, found a study of two billion medical claims in the United States. Mostly, patients and doctors were satisfied with seeing one another online. Telehealth lowered health care costs, and was especially useful for treating chronic illnesses and for psychotherapy. And in some cases, the pandemic revealed, people don’t need to see a doctor at all. The number of people showing up with mild appendicitis decreased, while the number with complicated appendicitis didn’t change, which researchers said suggested that some patients who would typically have had surgery recovered on their own.

6. Women are better patients than men.

During the pandemic, women were more likely than men to wear masks, get vaccinated and follow other public health guidance. This was true in many countries. When men and women lived together, the men were a little more likely to follow health rules, but still less likely than the women. One group of researchers studied professional tennis players at the U.S. Open in 2020. The women were more likely than the men to skip the event because of safety concerns. This aligns with gender differences in health overall, researchers said — women are more likely to seek preventive care, visit doctors and follow health recommendations. It’s probably one reason women tend to live longer.

7. Not even being stuck at home makes men do more housework.

During lockdowns, there was a lot more domestic labor to do. More dishes piled up, with more needy children underfoot. But even when men worked from home, women still handled more of the work. Eight in 10 mothers said they managed remote schooling (fathers overestimated their contribution). That could be a reason mothers’ antidepressant use increased when schools were closed, but not fathers’. Mothers were also more likely than men to cut back at work — though they returned as soon as they could. Only couples who really wanted egalitarian relationships, researchers wrote, could overcome “the stickiness of gender inequality in household work.”

8. Alcohol restrictions can save lives.

Many places had curfews or bans on selling alcohol during lockdowns — and it appeared to have saved lives. In South Africa, hospital admissions to trauma units and deaths declined. In Southern India, traumatic brain injuries decreased. In other parts of the world, however, alcohol use increased significantly — and, along with it, domestic violence and other problems.

9. Office workers don’t need to be chained to their desks.

Even without in-person meetings, work travel and days spent in cubicles, business continued on. The lesson, said Nick Bloom, a Stanford economist: “Work from home works.” Researchers are still studying how remote work affects productivity, collaboration and creativity. But some version of it seems here to stay: Just over a quarter of paid work days are now worked from home, compared with about 7 percent prepandemic. Remote work has downsides — for innovation, mentorship and service jobs in downtowns. But it also has benefits that workers aren’t eager to give up, like no commutes, more focused work time and making it easier for parents to juggle child care. As a result, it also improves retention.

10. Computers are no replacement for classrooms.

Five years later, the data is clear: When it came to learning, remote school wasn’t enough. Across the country, in rich and poor districts, and among students of different races, test scores in reading and math fell. Many students still haven’t caught up. There was learning loss even in countries with much shorter school closures than the United States. Other factors hampered students’ learning, including poverty and stress, but the importance of attending school in person is clear: The sooner children returned to classrooms, even part-time, the better they did.

11. There’s a simple way to bring children out of poverty.

The monthly checks that the U.S. government sent most parents during the pandemic were enormously successful in bringing children out of poverty, a variety of research has found. Families used the money to pay for food, child care, health care and housing. The benefits weren’t just financial — the checks improved parents’ mental health and family well-being. In 2022, when the checks ended, child poverty doubled. The expanded child credit was part of a rapid $5 trillion expansion of the social safety net.

12. Premature births might be prevented by taking care of moms.

The first reports came from Denmark and Ireland in 2020: The number of babies born premature or at a very low birth weight plummeted early in the pandemic. Soon it became clear that this trend was global: One study estimated that worldwide, 50,000 premature births — a leading cause of infant mortality — had been prevented in just the first month of the pandemic. Researchers aren’t sure exactly why, but a leading theory is that staying home benefited pregnant women — they could rest more, and were exposed to fewer stressors, pollutants and viruses. Perhaps giving pregnant women a break would make them, and their babies, healthier.

13. Dolphins talk more when people aren’t around.

When humans were less active — what scientists call the anthropause — animals began breeding more and traveling farther. Dolphins whistled longer, birds changed their songs, sea turtles laid more eggs. But the anthropause also revealed the ways in which animals have adapted to people, and humans’ disappearance disturbed delicate balances. In some places, predators or invasive species arrived. Urban wildlife that had become accustomed to coexisting with humans, like crows or raccoons, retreated. It revealed the ways in which humans both threaten and protect the natural world, scientists said.

14. Trees and plants make people happier.

Unable to spend time in indoor public spaces, people flocked to natural areas when they could, and were better off for it. A study in Hong Kong compared people who lived near urban green spaces with those who didn’t, and found that parks provided physical activity and a refuge. A study in nine countries found that access to nature — even a balcony or garden at home — buffered the stress of lockdowns and improved people’s moods. And a study in Taiwan analyzed the “window/wall ratio” in people’s quarantine rooms and found that more windows, especially if people could see vegetation, made them happier.

15. There’s no substitute for human contact.

Across the globe, when people didn’t see other people, their mental health — as measured by loneliness, depression and anxiety — got worse. Social media was not a substitute, and often made mental health deteriorate. The pandemic made clear that socializing is particularly important for two age groups, researchers said: young adults and older adults. The older group had better mental health, as well as cardiovascular and cognitive health, when they had structured socializing, like activities at community centers or weekly visits or phone calls.