What's your work limit?

You're late to work (again), behind on a project, or can't remember the action points from the last meeting. If you're one of the roughly 10 million U.S. adults with attention deficit hyperactivity disorder (ADHD), it can be a constant challenge to stay on task. Dr. Anthony Rostain, professor of psychiatry and pediatrics at the University of Pennsylvania School of Medicine, in Philadelphia, says you can get distracted by external stimuli like noise or internal stimuli like daydreaming. These different distractions require different coping strategies, he explains. From our friends at Health.com, here's how to pinpoint your weaknesses and 10 strategies for getting the job done.More from Health.com: 5 reasons you can't concentrate

(MoneyWatch) COMMENTARY Last week, I had to put in some long hours on Monday and Tuesday. I was still at the computer until close to midnight Monday, and some projects on Tuesday kept me busy into the evening as well.

But then a funny thing happened. I couldn't seem to focus on Wednesday. I spent a big chunk of the afternoon puttering around, and wandering out of my home office, vaguely hoping some emergency required my presence (it didn't). Thursday wasn't much better. Friday I quit early.

After studying hundreds of time logs over the years, I've started to suspect that most people have a work limit. Each week, we have a certain number of productive hours that we can invest in any given project. Past that, we reach a point of diminishing returns. This number is different for everyone, but for me it's about 45 hours. Having put in 28 of those on Monday and Tuesday, my brain was not prepared to log more than a 6-hour workday on any day the rest of the week. That's not to say I couldn't be at my desk longer than that. If I'd been at an office where my presence was expected, I probably would have sat there. But I would have been surfing the web pointlessly and checking e-mail every 5 minutes. That's just wasted time. Better to go for a walk, read a book, or go home.

If you want to figure out your work limit, you can try logging your time, and seeing when in a week you stop wanting to start new projects, and start getting more interested in Pinterest. A particularly compelling project can up your work limit temporarily. But when you finish, you'll probably feel a bit of malaise. Diminishing returns are hard to fight.

Once you know your number, you can try pacing your workweeks with it in mind. If you stay late one night, consciously give yourself a structured break the next day. Slip out. Go somewhere new. Or come to work later.

And if you're a manager? Don't push your employees past their work limits either. You may do fine the first or second late night, but past that you won't get anything out of pushing those boundaries, other than people puttering around until the dinner you ordered arrives. You'll save money by sending you team home.

What's your work limit? When do you experience a point of diminishing returns?