Achieving any challenging long-term goal requires perseverance. Determination. Staying the course.

Obvious? Absolutely. But no less true. 

So what makes people stop? And more to the point, why are you sometimes able to tap into considerable stores of determination and willpower… while at other times, you’re not?

That’s a question researchers at the University of Bologna set out to answer. Participants were split into two groups. People in one group spent ninety minutes working on a difficult computer task that required significant cognitive effort. Those in the other group spent the time watching a documentary about cars and trains, a task that required no mental effort (other than possibly trying to stay awake.)

Then both groups were hooked up to cardio monitors, put on exercise bikes, and told to pedal as fast as they could until unable to continue as 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, with those who watched the (boring) documentary performing the complex computer task while the other group watched the movie.

What happened? People who first spent ninety minutes on the difficult cognitive task quit much sooner than when they did not. They also rated their perceived effort as much higher during the test, even though their bodies were performing at the same level.

Bottom line: Even though the pre-effort, and subsequent effect, was all mental, their physical performance was still dramatically impacted. Intuitively, that makes sense. Determination, willpower, and perseverance are mental attributes, not physical. Granted, when you’re physically tired it’s hard to stay the mental course. 

But it’s also hard to stay the physical — and determination and perseverance — course when you are mentally tired.

You probably know that 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.

Or that 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, only sleeping six hours is like drinking a couple of beers, and only sleeping four hours is like drinking five beers.

Or that 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 gained from twenty 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 need to run a marathon brainstorming meeting, don’t engage in a difficult mental task in the hour or so before. If you need to run a lengthy training session, don’t spend the hour before trying to analyze your way out of a cash flow crisis. If you need to spend the afternoon filling a truck with product so it gets delivered on time, don’t spend the hour or so before trying to debug your fulfillment software. 

Mental fatigue increases your perception of effort, which increases your perception of how hard it is to keep going.

Which means you’ll be much more likely to quit.