I spent the morning struggling to rework a chapter of a book I’m writing. (I had all the pieces, but I couldn’t seem to make it flow.) By the time I figured it out, I was mentally shot.

Fortunately, my lack of willingness to expend any more mental energy didn’t seem like it would be a problem since I planned to lay hardwood floors that afternoon. Four or five hours of relatively mindless effort? Sounded great to me.

Except it wasn’t. Just a couple hours in, I was physically shot. I forced myself to keep going — if I have a superpower, it’s the ability to perform endless amounts of mindless menial labor — but staying the course was really tough.

All I had done was sit and write all morning. Why would I already be so physically tired? Didn’t seem to make sense.

Except it did. In a study published in the Journal of Applied Physiology, participants were split into two groups. One group spent 90 minutes working on a difficult computer task that required significant cognitive effort. The other group spent the time watching a 90-minute documentary about cars and trains, a task that required zero mental effort.

Afterwards, both groups were hooked up to cardio monitors, put on exercise bikes, and told to pedal as fast as they could while the resistance steadily increased. Participants were asked at regular intervals during the test to rate how hard they were working.

Then, during another session, the groups were flipped: those who watched the (boring) documentary performed the complex computer task, while the other group watched the movie. Then they repeated the cardio test.

The result? When participants spent 90 minutes on the difficult cognitive task, they quit much sooner than when they did not. They also rated their perceived effort as much higher during the test, even though their actual effort was the same. (Pedaling at 60 percent of max felt like 80 percent.)

Even though the “pre-effort” was all mental, physical performance was dramatically impacted.

Intuitively, that makes sense. When you’re physically tired, it’s hard to stay the mental course. (As General Patton said, “Fatigue makes cowards of us all.”)

But it’s clearly also hard to stay the physical course when you’re mentally tired.

Or when you haven’t gotten enough sleep.

You probably know research shows people who sleep for five to six hours are 19 percent less productive than people who regularly sleep for seven to eight hours per night, and that hardcore sleep warriors who average less than five hours a night are nearly 30 percent less productive.

You probably also know research shows only getting six hours of sleep makes any task that requires focus, deep thinking, or problem-solving a lot harder. (In fact, where attention and reaction time are concerned, sleeping only six hours is like drinking a couple of beers, and sleeping only four hours is like drinking five beers.)

And you probably know research shows sleep deprivation makes completing any activity that requires multiple steps (basically any meaningful activity) much more difficult.

That’s why most people try, when they need to be at their best, to get a good night’s sleep. To be properly hydrated. To eat a healthy breakfast. Maybe even to gain the mood-boosting affect that results from 20 minutes of moderate cardio exercise.

But what you do (or more to the point, don’t do) before you need to be at your best — especially if that task requires not just mental acuity, but also a healthy dose of willpower and perseverance to see the challenge through — matters just as much.

If you’re need to lead a problem-solving meeting, one where you need to be enthusiastic and engaging — where you need to be “on” — don’t tackle a difficult mental task in the hour or so before.
If you need to lead an afternoon training session — and be “on” throughout — don’t spend the hour before trying to solve a cash flow shortage.
If you need to spend the afternoon loading a truck with product so it gets delivered on time, don’t spend the hour or so before trying to solve a complex workflow problem. 

Achieving any challenging long-term goal requires perseverance. Determination. The ability to stay the course.

Mental fatigue will increase your perception of effort, and increase your perception of how hard it is to keep going.

Which means you’ll be much more likely to quit, or at the very least, not perform at your best.

Even though that might be the last thing you want to do.