-Sue Shellenbarger
A friend you admire just asked you to head a fundraiser. You have a full-time job, and this sounds like a lot of work. But look at her: a working mom who is active in the school PTA, just ran her first marathon, does Suzuki violin with her daughter and even carries on an active social life. Why can’t you do as much as she can? And just how do you tell when you have taken on enough?

The sweet spot is different for everyone. Unfortunately, most people don’t know when they have reached their limits and need to stop saying yes to new activities. Many take their cues from others, such as a boss who never says no to new demands or a colleague who juggles career, family and board duties.

Learning to notice and heed early warning signs of overload takes practice and planning, say people who found their limits the hard way. Six years ago, Michael Mawdsley kept piling it on until he was “crazy-busy,” working 80-hour weeks, helping raise his two small children and serving on the board of a nonprofit organ-donation group.

The Bend, Ore., executive for a nonprofit group says he competed to be seen as the hardest worker. “I liked my boss to know that I sent him the earliest email of the day, as well as the latest email of the day,” he says. Brushing aside insomnia and an upset stomach, he became terse and impatient with employees. When his boss warned him that he could lose his job if he didn’t improve his people skills, he finally realized he was “over the line,” he says.

Working with a coach, he delegated more administrative work and trimmed his weekly work hours to 50. He took more time to listen to his employees and be home with his wife and children. Now, he says, “I’m more aware when I take on too much.” When he notices early signs of overload, including fatigue, shallow breathing or tension in his shoulders, he tells himself, “here’s the reset point,” and schedules more sleep or recreation. He has since been promoted twice.

A person’s capacity to manage a lot of tasks and activities is shaped by genetics and early experience, says Amit Sood, a professor of medicine at Mayo Clinic College of Medicine, Rochester, Minn. Some people inherit a tendency to be more reactive to stress and overload. Those who receive plenty of early nurturing and warmth, and who experience just enough early adversity to instill resilience but not too much, are often better able to cope, says Dr. Sood, author of “The Mayo Clinic Guide to Stress-Free Living.”

Personality matters, too. Altruism is a trait that helps people find meaning in their tasks and activities, enabling them to do more without harmful effects, Dr. Sood says. By contrast, people with neuroticism, who are more prone to experiencing negative emotions such as anxiety or anger, tend to have more strain when managing multiple roles, says a 2012 study in the Journal of Vocational Behavior.

Cali Yost estimates that 15% of workers at the hundreds of companies where she has done training sessions manage long to-do lists effortlessly. These high achievers, whom she calls “the naturals,” usually keep a single calendar for all their work, family, personal and community activities, to help them see the big picture and avoid forgetting things. Most are “master collaborators, communicators and coordinators,” asking for and giving help from others, says Ms. Yost, a consultant and trainer on work-life flexibility for the past 20 years and author of “Tweak It,” a recent book on the topic.

But few people have a good sense of their capacity for handling multiple tasks and activities. When overload hits, the first thing most people notice is that “the balls start to drop. You miss a pickup with your kid, or you miss a client call,” says Ms. Yost.

Other early symptoms of overload can include insomnia, minor illnesses, rising blood pressure or frequent aches and pains. Emotional signals include irritability and anxiety. Leadership coach Janet Harvey calls it “the frayed-at-the-edges” syndrome—being curt with colleagues, finishing people’s sentences for them or obsessing about what you haven’t gotten done. Relationships and social life tend to deteriorate. Some people lose their ability to say no or to negotiate when making commitments, says Ms. Harvey, chief executive of InviteChange, Edmonds, Wash.

Overload takes a cognitive toll too, hampering working memory and concentration. It can make it hard to detach from work and relax, starting a downward spiral of fatigue and procrastination, says a recent study in the International Journal of Stress Management. In the study, 547 working adults responded to written surveys over two months on such topics as how often their jobs required them to work very fast and how easy it was for them to relax after work.

Marguerite Samms became overwhelmed in 2007 with working long hours while raising her four children, then ages 5 to 11. She put in as many as 12 hours a day while trying to get her children to violin and fencing lessons, swim-team practices and doctor appointments. Anxious and exhausted, “I could no longer make decisions that were smart,” she says. “Little things started feeling really overwhelming.” During a routine meeting with her boss, she found herself at a loss for words. “All I could say was, ‘I resign,’ ” she says. Her boss agreed to give her two weeks off and reduced her work hours, says Ms. Samms, a Tacoma, Wash., health-care executive.

She has since trained herself to notice signs of overload, including a tendency to become resentful when she’s too busy or stressed. She has gathered a network of friends and colleagues who help her track how she’s allocating time and energy.

In a seeming paradox, some people learn to stay in their sweet spot by adding something to their to-do list: an enjoyable activity. That can energize them and lend the perspective needed to drop activities that aren’t fun, says Pat Mathews, a Chambersburg, Pa., leadership coach. An information-technology manager she counseled was considering quitting his job because he felt so drained. Instead, he devoted more time to a singing group, which he enjoyed, while dropping a stressful position on his condo-association board. The changes re-energized him, letting him spend more time with his family and be more patient with his employees.

Amy Ruppert, chief learning officer in Chicago for Coaching Out of the Box, a training firm, advised one client to spend 15 more minutes in the shower every morning. “That was all she could handle in the beginning,” she says. Ms. Ruppert advises clients to ask themselves a question before agreeing to any new task: “If I say yes to this, what am I saying no to?”

Rachel Davidson draws the line at losing sleep. She needs seven hours a night, says Ms. Davidson of New York City, a foundation director and former state-court judge. Over the years, she has learned to turn down requests to serve on committees and do volunteer work. When tapped to take on something new, she asks herself: “This is another two hours out of my sleep. Is it worth it?”

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