'What's eight times eight?' I asked Isabelle, my 9-year-old daughter who was practicing her times tables. Her eyes glazed over.

We were on a plane and had about two hours to kill. I figured running through the twos to the twelves would take about 20 minutes.

But we were 40 minutes into the task and still at it.

The first 10 minutes had gone smoothly; with quick, accurate replies she was well on her way to finishing in our allotted time.

But as we continued, each question took longer to answer. It wasn't that the questions were getting harder -- I had mixed up the tables, interspersing easy questions with harder ones -- so what was it?

'What are you thinking about?' I asked her.

'Fairy dust,' she said bashfully.

It was only once we completed the tables -- an hour after we started -- that I realized my mistake: I had pushed her past her point of task fatigue.

This is not just a 9-year-old issue.

We all reach task fatigue. It's that point of diminishing returns when the effort we're putting into staying on task seems greater than the effort we're putting into moving forward in the task. It's that point when our productivity plummets, when our eyes glaze over and our mind wanders. Not that it's always bad to let the mind wander, it's just bad if you're trying to focus.

If we're really driven, we try to push through. We snap our heads back toward our computerscreen and tell ourselves to pull it together and keep working.

Which, it turns out, is a huge productivity-sapping mistake.

When Isabelle finally finished her times tables, she took out her book and started reading - with total focus. So it wasn't that her focus was depleted, it was just that her focus for the math task was depleted and as a result, it took her a painful, frustrating hour to complete it.

See, task fatigue is not the same as focus fatigue. We still have plenty of focus left to use, just not on the same task that exhausted us. The solution? Diversify your focus. Give up the illusion of singular focus and choose multiple areas on which to focus.

This sounds counter-intuitive; we always hear that if you really want to get something done, you should spend all your time on it.

If you're out of a job, you're more likely to find one if you spend all your time looking for one, right?

Wrong!

How long can you spend on job search sites before fatigue sets in? For most of us, not very long.

And here's what happens when we try: We unconsciously and insidiously distract ourselves with activities of little or no productive value. If you're Isabelle, you think about fairy dust.

To guard against this, identify five broad areas of focus that add value to your life. That way, when your focus on one task starts to slip, you've got four other valuable and productive areas waiting to take its place. You won't waste time spinning your wheels or deciding what to do next.

So how does it work in practice? Let's say you're looking for a job. Obviously, you'd put that at the top of your focus list, but you might also want to focus on spending time with family; learning Chinese; working on your blog and growing your personal network.

To be clear, I'm not suggesting you multitask. Multitasking is even more of a drain on focus than focus. I'm suggesting you switch-task - move from one task to another once you start to lose concentration.

Certainly there are some important areas of focus that you will enjoy more than others. Isabelle enjoys reading more than math, and can maintain her focus longer in that area. But even with areas you find enjoyable, task fatigue sets in. And then, it will be time to switch again.

'Daddy,' Isabelle said to me after reading for some time, 'let's do some math facts before we land.'

'Eight times eight?' I asked.

'Sixty-four!' She replied, without any hesitation at all.