Caffeine can give us a boost, but too much can mess with our sleep and make us feel jittery. So how do we know what’s the right amount?

Generally, government and health groups recommend that healthy adults consume no more than400 milligrams of caffeine a day. That comes out to about four, 8-ounce cups of coffee, says Jennifer Temple, a professor of exercise and nutrition sciences at University at Buffalo School of Public Health and Health Professions.

(No, that 20-ounce Starbucks Venti doesn’t count as one cup of coffee.)

And believe it or not, we are doing pretty well on this target. The average American adult consumes about 200 milligrams of caffeine a day and in Europe, it is 270 milligrams, according to a 2017 review study.

But not everyone is optimising their caffeine intake to maximise how it can help them—by sharpening concentration for work or giving them a boost before a run—without hurting their sleep or overall health.

Here’s how to think strategically about getting the most out of your daily dose.

Optimising the boost

Caffeine can help you focus and keep you alert.

About 100 to 150 milligrams—or one to 1.5 cups of coffee—is a ballpark amount that will deliver a boost, says Astrid Nehlig, an emeritus research director at the French National Institute of Health and Medical Research, who has studied caffeine’s impact on brain activity, though it varies from person to person.

The effects generally kick in about five minutes after consumption and increase to become optimal for between roughly 15 and 120 minutes, Nehlig says.

Caffeine has been linked to physical benefits, too. People walked more on days they drank coffee than on days they didn’t, according to a 2023 study of 100 people in the New England Journal of Medicine. Participants took an average of 1,000 more steps on days when they drank caffeinated coffee than when they didn’t.

Other studies have suggested that caffeine can sometimes help us work out harder, such as when we have it before high-endurance exercise like long runs or swims, or sports that require a sustained effort, like soccer, Nehlig says.

The same boost hasn’t been found with shorter efforts, such as a sprint. Caffeine doesn’t act directly on muscles but rather reduces your rate of perceived exertion and the time it takes you to feel exhausted.

The downsides

Caffeine’s main negative for your health is that it can disrupt your sleep.

The NEJM study that found that people walk more on days when they drink caffeine also found a downside. On days when study participants could drink as much caffeinated coffee as they wanted, they slept on average 30 minutes less than on days they didn’t drink any.

The impact on sleep varies greatly depending on how fast you metabolize caffeine, says Gregory Marcus, a cardiologist and professor of medicine at University of California, San Francisco and first author of the NEJM study.

On average, it takes about 4.5 hours for half of the caffeine consumed to pass through your system. However, genetic differences make some people metabolise it slowly or quickly, doctors and researchers say. The population is roughly split between fast and slow metabolisers.

The sleeping and walking study tested whether participants were slow or fast metabolisers of coffee. Those that were slow metabolisers slept nearly an hour less on the nights they drank caffeinated coffee, while the fast metabolisers didn’t experience any impact on sleep.

Where to get your caffeine

The best source of caffeine is unsweetened coffee or tea, says Dr. Frank Hu, professor of nutrition and epidemiology at Harvard T.H. Chan School of Public Health. These drinks have other beneficial ingredients, such as polyphenols, which have antioxidant effects which reduce inflammation.

The caffeine content in coffee and tea can vary, but soda can’t have more than 71 milligrams per 12 ounces, per Food and Drug Administration regulations.

Adults get most of their caffeine from coffee, but the market for energy drinks is growing. Pay extra attention to the caffeine in these drinks, because some contain very high levels.

Getting caffeine from soda or energy or sports drinks makes it more likely you are also getting a high dose of sugar and empty calories, says Hu.

Extra caution

Kids under 12 should avoid caffeine, while 12- to 18-year-olds should have no more than 100 milligrams a day, according to the American Academy of Pediatrics. Pregnant women are advised to have no more than 200 milligrams of caffeine a day.

People with chronic conditions like high blood pressure, diabetes or heart disease might want to be more cautious about their caffeine consumption, Hu adds. The NEJM study found that on the days when participants consumed caffeine, they had more abnormal heart rhythms in the lower chamber of the heart, which is associated with a greater risk of developing heart failure.

And people who get migraine headaches should try to drink no more than 100 milligrams of caffeine a day, the equivalent of a mug of coffee, says Dr. Amaal Starling, a headache specialist and neurologist at Mayo Clinic in Scottsdale, Ariz. She advises her patients who have daily or severe headaches not to drink any caffeinated beverages or switch to decaffeinated coffee.

WSJ’s Life & Work team presents Life Math, a series on how to optimise your time in 2024. Today: The best way to spend your time on the weekend.

Saturday and Sunday are often the most anticipated days of the week, yet optimising them remains an elusive goal for many of us.

Squandered weekends leave us feeling less happy and less motivated at work, research suggests. Those who put planning and intention into their weekends emerge on Monday feeling satisfied, accomplished and more productive throughout the workweek, says Elizabeth Grace Saunders, a time-management coach in Farmington Hills, Mich.

If we don’t plan our time well, we can end up marching through our obligations, or wasting time, without ever focusing on what we really want to do.

How to plan the perfect weekend? Behavioural researchers and time-management coaches suggest breaking it into six components: sleep, hobbies, socialising, exercise, work and chores, and unscheduled time.

Using recommendations from experts and federal guidelines, we came up with this equation. It’s important to remember these numbers aren’t hard and fast—stay flexible and make the math work for your life.

The perfect weekend equation:

Sleep (7 to 9 hours x 2 + ≤ 20 to 60 minutes napping) + Hobbies (~ 2 hours) + Socialising (0 to 2 events) + Exercise (≥ 45 minutes) + Work (≤ 2 hours) + Unplanned time (~ 3 to 4 hours) = A Great Weekend

Here’s how to incorporate those elements to build your best days off.

Sleep

This part of the “perfect weekend” equation is the most rigid.

Despite the tendency many of us have to take advantage of the time off by staying up and sleeping in later, we should try to keep our sleep schedules as consistent as possible to avoid social jet lag, sleep researchers say.

Sleep researchers generally permit one hour of wiggle room—so if you typically go to sleep at 11 p.m., try not to stay up past midnight. If your weekday alarm goes off at 7 a.m., rise and shine by 8 a.m. on the weekends. Finding yourself sleepy later in the day? Take a 10- to 30-minute nap in the early afternoon.

Most importantly: Make sure you’re getting the recommended seven to nine hours of sleep each night, even on weekends. If you’re among the roughly one-third of Americans who don’t get that recommended sleep during the week, you may be able to “catch up” by sleeping a few extra hours on the weekend, says David Reichenberger, who studies the links between sleep and health at Pennsylvania State University.

But don’t count on catching up forever. A recent study Reichenberger co-wrote found that among a small group of people who slept five hours a night during the week, their cardiovascular health measures worsened and didn’t return to baseline even after they were allowed to catch up on sleep over the weekend.

Hobbies

Having a hobby, or an activity we engage in during our time off for pleasure, has been linked to fewer symptoms of depression and higher levels of happiness, life satisfaction and even reduced risk of cardiovascular disease and cognitive decline.

Saunders, the time-management coach in Michigan, generally recommends people set aside roughly two hours for hobbies on the weekend.

Don’t worry if you’re not, say, a dedicated baker, painter or pianist. Hobbies can encompass much more than we might typically consider, says Daisy Fancourt, a professor of psychobiology and epidemiology at University College London who researches the link between social and behavioural factors and health.

Something as simple as reading a book or cooking a tasty meal can serve the same purpose: to give us a sense of happiness, meaning and control in our lives outside of work.

Unplanned time

Scheduling unstructured time may sound silly. But failing to block out free time can leave us filling it with whatever’s right in front of us, like working or mindlessly scrolling, says Laura Vanderkam, an author and time-management expert based outside Philadelphia.

If you can, leave unplanned a chunky part of your Saturday or Sunday, roughly three to four hours, says Saunders. “If you make your weekend as packed and as busy as your weekday is, you will not come out of the weekend feeling refreshed,” she says.

This time is a good opportunity to let our brains enter so-called “default mode,” where our thinking extends beyond the here and now, allowing us to reflect and find meaning and purpose.

“It’s really important that all of us have dedicated, protected time in our lives to just be here now,” says Mary Helen Immordino-Yang, a developmental psychologist and neuroscientist at the University of Southern California.

Socialising

Robust social relationships are powerfully linked to physical and mental health and longevity benefits, and the weekend is a natural time to take advantage of them.

Social activities often require more advance planning than other parts of the weekend equation, so set aside time during the week to text or email friends and family about getting together, says Saunders, the time-management coach.

People typically spend twice as much time—nearly an hour—socialising on weekend days as on weekdays, according to the U.S. Bureau of Labor Statistics’s latest data on time use.

The amount of time you should spend socialising on the weekend depends on how energised or drained that togetherness makes you feel, she says. Introverts typically benefit from one social event every weekend or every other weekend, she says, whereas two social events per weekend is a sweet spot for extroverts.

If you have kids and most of your socialising naturally revolves around them, try to set some adults-only social time, too, says Vanderkam. You may find it easier to relax without your kids running around, and it can be easier to have uninterrupted grown-up conversations.

Work and chores

Pick a couple of small, achievable projects to see through to the finish line rather than trying to take on five things at once, says Vanderkam. You probably can’t clean out the entire garage, sort through your kid’s closet, vacuum out the car, wash all the laundry and grocery shop in one weekend.

Professional work, too, is sometimes inevitable on weekends. Avoid it if you can, but if a little work will help you feel less anxious, set some boundaries, behavioural researchers say. Stick to a clear time frame and goal, such as finalising one section of a report within a two-hour window.

Physical activity

Federal guidelines recommend at least 150 minutes of moderate-intensity aerobic exercise weekly, plus strengthening activities twice a week. If you’re spreading that out across the week, you may only need to set aside about 45 minutes for Saturday and Sunday.

But there’s good news for people who like to cram most of their exercise into the weekend. People who condensed their workouts into one or two days experienced health benefits similar to those who spread them out, a 2022 JAMA Internal Medicine study found.

The flexibility of the weekend allows for longer, varied workouts that can overlap with “hobbies” and “social” categories, says Heather Milton, a clinical exercise physiologist and supervisor of the NYU Langone Sports Performance Center. Try to incorporate both elements of aerobic and strength training, as well as some flexibility, she recommends.

It can help to plan an exercise block for the same time each weekend—such as a weekly Saturday morning yoga class or Sunday morning jog. Don’t have the time? Just try to move. Ideally, every 30 minutes or so, says Milton.

“Weekends are great for relaxation, but try not to Netflix and chill for 12 hours of the day without getting off the couch,” she says.

Chinese women have had it. Their response to Beijing’s demands for more children? No.

Fed up with government harassment and wary of the sacrifices of child-rearing, many young women are putting themselves ahead of what Beijing and their families want. Their refusal has set off a crisis for the Communist Party, which desperately needs more babies to rejuvenate China’s aging population.

With the number of babies in free fall—fewer than 10 million were born in 2022, compared with around 16 million in 2012—China is headed toward a demographic collapse. China’s population, now around 1.4 billion, is likely to drop to just around half a billion by 2100, according to some projections. Women are taking the blame.

In October, Chinese Leader Xi Jinping urged the state-backed All-China Women’s Federation to “prevent and resolve risks in the women’s field,” according to an official account of the speech.

“It’s clear that he was not talking about risks faced by women but considering women as a major threat to social stability,” said Clyde Yicheng Wang, an assistant professor of politics at Washington and Lee University who studies Chinese government propaganda.

The State Council, China’s top government body, didn’t respond to questions about Beijing’s population policies.

Party lectures on “family values” are having little effect, even in rural parts of China.

Outside a mall in Quanjiao, a county in Anhui province, He Yanjing, a mother of two, said she has gotten several calls from community officials to encourage her to have a third child. She has no such plans. The preschool her son attended cut the number of classrooms in half because there aren’t enough children to fill them, she said.

Her friend, Feng Chenchen, the mother of a 3-year-old girl, said relatives are pressuring her to have more children, hoping she has a baby boy.

“Having had one child, I think I’ve done my duty,” Feng said. A second child, she said, would be too expensive. She said she tells relatives, “I can have another kid as long as you give me 300,000 yuan,” around $41,000.

Many young people in China, disheartened by a weak economy and high unemployment, seek alternatives to their parents’ lives. Many women view the prescribed formula of marriage and children as a raw deal.

Molly Chen, 28 years old, said the demands of caring for ageing relatives and her job as an exhibition designer in Shenzhen leave no room for kids or a husband. All she wants to do in her free moments is read or scroll through pet videos.

Chen followed the story of Su Min, a retiree who video-blogged about her solo road trip around China to escape a bad marriage. Chen said that the story, as well as online videos that women post about their lives, have deepened her impression that many men choose wives mostly as caretakers—for children, husbands and both sets of ageing parents.

She lamented that she doesn’t have time even for a pet. “I can’t afford to take care of anything else outside of my parents and work,” Chen said.

Shrinkage

When Beijing said it would abolish its 35-year-old one-child policy in 2015, officials expected a baby boom. Instead, they got a baby bust.

New maternity wards were built only to close a few years later. Sales of baby-care products, including formula and diapers, have dropped. Businesses that focused on babies now target seniors.

New preschools built to make child-rearing more affordable struggle to fill classrooms and many have closed. In 2022, the number of preschools in China fell 2%, the first decline in 15 years.

Demographers and researchers predict that data will show Chinese births dipping below 9 million in 2023. The United Nations forecasts 23 million births in India, which in 2023 passed China as the world’s most populous country. The U.S. will have around 3.7 million babies born in 2023, the U.N. estimated.

The one-child policy brought much of China’s demographic gloom: There are fewer young people than in the past, including millions fewer women of childbearing age every year. Those women are increasingly reluctant to marry and have children, accelerating the population decline.

In China, 6.8 million couples registered marriages in 2022, compared with 13 million in 2013. The country’s total fertility rate in 2022—the average number of babies a woman has in her lifetime—is approaching one birth per woman, or 1.09. In 2020, it was 1.30, well below the 2.1 needed to keep a population stable.

The campaign for a “birth-friendly culture” has taken on the tone of an urgent national mission, with government-organised matchmaking events and a program encouraging military families to have more babies.

“Soldiers win battles. When it comes to giving birth to second or third children and implementing the national fertility policy, we are also taking the lead and charging to the front,” Zeng Jian, a top obstetrician-gynaecologist at a military hospital in Tianjin, told state media in 2022.

In August, residents of the western city of Xi’an said they received an automated greeting from a government number during the Qixi Festival, the Chinese equivalent of Valentine’s Day: “Wishing you sweet love and marriage at an appropriate age. Let’s extend the Chinese bloodline.”

The message drew a backlash on social media. “My mother-in-law doesn’t even push me to have a second child,” one person wrote. “I guess next, arranged marriages will come back,” another commented.

Beijing leans more to encouragement than the kind of coercion that marked the one-child policy. Local governments offer cash incentives for couples having a second or third child. A county in Zhejiang province gives a $137 cash bonus to every couple getting married before age 25.

In 2021, the city of Luanzhou asked unmarried people to sign up for a government-sponsored dating initiative that uses big data to find matches citywide. A district in the city of Handan provides a one-stop wedding-planning service.

Hide and seek

The shift means some women have gone from trying to dodge punishment for having too many children to being hounded to have more.

A decade ago, a woman surnamed Zhang was in a cat-and-mouse game with authorities after she decided to have a second child. She asked that her first name not be used.

While pregnant, she left her job to stay out of public view, fearful officials would pressure her to have an abortion, she said. After giving birth, in 2014, she stayed with relatives for a year. When she returned home, local family-planning officials fined her and her husband around $10,000. She said she was forced to have an intrauterine device implanted to prevent pregnancy. Authorities required her to have it checked every three months.

Months later, the Chinese government announced the one-child policy would be scrapped. For a while, authorities still demanded Zhang have her IUD checked.

She now gets text messages from officials encouraging her to have more children. She deletes them in anger. “I wish they would stop tossing us around,” she said, “and leave us ordinary people alone.”

There has been a tightening of licenses for clinics offering medical procedures to block pregnancies. In 1991, the height of the one-child policy, 6 million tubal ligations and 2 million vasectomies were performed. In 2020, there were 190,000 tubal ligations and 2,600 vasectomies.

On social media, people complain that getting a vasectomy appointment is as difficult as winning the lottery.

Officials have also tried to dial back abortions, a key tool for officials during the one-child policy. They have fallen by more than a third—from more than 14 million in 1991 to just under 9 million in 2020. China has since stopped releasing data on vasectomies, tubal ligations and abortions.

Pressurised populace

Wang Feng, a sociology professor at the University of California, Irvine, said there have been two conflicting shifts in Chinese society: a rising awareness of women’s rights and increasingly patriarchal policies.

For the first time in a quarter-century, no women are among the top two dozen officials on the Politburo. Since Xi took power in 2012, China has fallen 38 places in the World Economic Forum’s Global Gender Gap Report to No. 107 in the 2023 ranking of 146 nations.

In the Mao era, the party promised to end Confucian traditions that discriminated against women. Xi has instead stressed Confucian values, including the filial duty to have children. Families also pressure women into traditional roles.

Sophy Ouyang, 40, has known since middle school she didn’t want to marry and have children. Ouyang studied computer science, one of the few women in her village to pursue advanced schooling, and works as a software engineer in Canada.

Ouyang said that throughout her 20s, her family leaned on her to marry. Her mother said that if she had known Ouyang wouldn’t have children, she would have stopped her from getting a higher education.

Ouyang cut off contact with her family more than a decade ago. She has blocked her parents, aunts and uncles on social-media apps. “If I’m a bit more gentle with them,” she said, “they will take advantage.”

The Chinese government, which sees feminism as a nefarious ideology backed by foreign forces, has detained women’s-rights activists and erased their social-media accounts in a yearslong crackdown.

Even so, women have become more vocal online about their experiences relating to relationships, families and work. Their posts show a personal form of feminism that is harder for authorities to police.

Simona Dai, 31, started a podcast entitled “Oh! Mama” about birth and marriage after she learned that her mother had an abortion when she was eight-and-a-half months pregnant with a girl in the early 1990s.

Dai got married when she was 26 and said she had to endure her husband’s chauvinistic views, especially during the pandemic, when they argued about household chores. She became adamant about not having children, despite pressure from the couple’s families.

She has since applied to end her marriage. “If I didn’t divorce, I might have to have a baby,” she said.

A national debate over the treatment of women erupted in early 2022, when the video of a woman—a mother of eight, kept in a shed with a chain around her neck—sparked a social-media storm. The woman’s plight resonated with Chinese women who saw a connection to their own roles.

In recent years, Beijing has raised its guard against similar instances of social-media outrage.

A woman who worked at a branch of the All-China Women’s Federation in Guangzhou from 2020 to 2021 said the branch focused on preventing gender-related topics from going viral. She said it paid more to a tech company to police social-media comments than its budget for women’s advocacy.

During training, she said, employees were warned of serious repercussions if women’s issues in Guangzhou drew unwanted social-media attention. The women’s federation didn’t respond to requests for comment.

China’s cyberspace watchdog, which polices material seen as harmful to Chinese internet users, said in December that it was targeting content “spreading wrong views on marriage.”

Some women who decided years ago against marriage and children consider themselves lucky.

Ouyang, the software engineer in Canada, said, “I feel like I’ve completely dodged a bullet.”

—Jonathan Cheng and Grace Zhu contributed to this article.

It is barely past New Year’s Day. If you’ve taken the day off, congratulations: You have aced your first test of vacation-day math.

We get only so many days of paid time off a year. And that is if you’re lucky—one in five U.S. private-sector workers gets no PTO, according to the Bureau of Labor Statistics. Without a strategy, you can have a generous number of vacation days and still feel like you never truly got away from it all.

Think of the times you took a mini-break from work only to feel stressed before and afterward. The average American worker with five years at the company gets 15 paid days of vacation in a year, BLS data show. That leaves little room for bad planning if you want a serious break or two, plus some long weekends and the occasional personal day.

Maxim No. 1: A vacation day equals more than a day of vacation when you attach it to a public holiday or two. Taking the day after New Year’s this year snags you a four-day weekend at the start of 2024. Timed just right between federal holidays and weekends, 15 days of PTO can turn into nearly 50 days of extended break time this year. (That is, if your co-workers don’t beat you to claiming those dates.)

Another guiding principle—Fridays off are overrated, and not just because they are peak traveling days. For a long weekend or a random personal day, there is evidence to suggest a Monday and Wednesday can be more satisfying. But more on that later.

A weeklong vacation vs. long weekends

First, some science: To really recharge, you need at least one weeklong vacation, bracketed between two weekends, research suggests.

In one study of more than 50 people who took vacations for an average two weeks, participants’ well-being levels didn’t peak until their eighth day off. A 2023 study of more than 300 vacationers found people who took between eight and 14 days off reported greater and longer positive effects once they returned to work, such as better sleep, than those who took shorter breaks.

One to two weeks off, in fact, appeared to have longer-lasting benefits than lengthier vacations. After a while, “you creep back to old habits,” says Ty Ferguson, a research associate at the University of South Australia in Adelaide who co-wrote the study. His own recent getaway—several days down the coast—went bust when his three children, ages five and under, came down with a bug. Then it was time to return.

“I should take more of my own advice,” he says.

One reason taking a week-plus vacation is important is that is enough time to actually reduce workloads. Network-equipment giant Cisco recently conducted a deep data dive on employees’ work habits and well-being, examining more than three years’ of metrics such as virtual meetings, badge-ins, PTO and engagement surveys. When workers took a day or two off, the number of meetings they had in the month didn’t change much—they just packed in more work before and after their time off.

Meeting loads dropped sharply for workers who took at least five consecutive days off. The fewer the meetings, the greater tendency to report healthier routines and better stress-coping abilities, Cisco found.

“I always believed in the long weekend because it can be so hard to take a week off,” says Cisco’s chief people officer, Kelly Jones. “I was wrong.”

Maximising public holidays

To get the most out of your finite days off, consider Gail Martino’s PTO hack for 2024. “I’m a leisure laggard,” says the senior project manager in New Haven, Conn., of her habit of waiting to take vacation time until things get slower. (Hint: That is never.) Then there is a scramble to use it or lose it toward the end of the year, with the days she does take off feeling not terribly satisfying.

“You wonder, why am I so tired?” she says.

In recent years, she’s become a bird watcher and wants to take a couple of birding trips along the Eastern Seaboard in 2024. “I spend a week in the woods, among trees and nature, and that is an incredible break,” she says. “Now I want to chart out the entire year.”

Scanning the 2024 calendar, she devised a spreadsheet of dates bridging public holidays and weekends with a theoretical 15 vacation days and four personal days. (Working at Unilever for 18 years, she got about a week more PTO than that in 2023.) The result was 50 days of extended breaks, including 9-day stretches in July and over Christmas:

A little tweaking can wring nearly the same number of extended break days with just 15 vacation days and no personal days—that is, if you get a full slate of federal holidays off and don’t have to trade off with co-workers:

The case for Mondays and Wednesdays off

Want to take a three-day weekend that isn’t attached to a federal holiday? Take Monday off instead of Friday, suggests Jim Burch, a 38-year-old software engineer in Phoenix and an avid hiker. Taking Fridays off often results in cramming five days of work into four, he points out.

“I’d get so stressed out on the Thursday before,” says Burch, who at his current job, has more autonomy over his schedule than in earlier jobs.

Delaying gratification until Monday means your co-workers have no choice but to start the workweek without you. Back Tuesday, you can quickly catch up on whatever emails or developments you missed, he says.

Then there is the unexpected pleasure of a Wednesday off. “It is like a midweek weekend,” says Rachel Blenkhorn, a social-media production manager for a real-estate investment trust who lives in Warren, Mich. It is long enough to relax or take care of appointments yet short enough to get back in the work groove on Thursday, she says.

There is science as to why, says Dawna Ballard, a professor at the University of Texas at Austin and an expert in chronemics, the study of time as it relates to communication.

“Everyone has a different chronotype,” or their own biologically driven pace, she says. A break after two days’ work gives you a second chance in the week to return to your internal rhythm. Psychologically, it also creates a bit of “slack” in the workweek, alleviating the stress that comes from feeling like there is too little time to get everything done.

However you plot your vacation days in 2024, don’t leave any on the table. They aren’t just good for you, there is evidence they are good for your career.

An Ernst & Young study of its employees showed every extra 10 hours of vacation was linked to an 8% improvement in year-end performance reviews. Another study found people who took more than 10 vacation days a year were more likely to get a raise or bonus than those who took fewer days.

Now that is a formula anyone can get behind.