If you like reading about productivity, then you’ve likely heard the now-common advice to do the hard thing first thing. Brian Tracy of Eat that Frog! fame is one of the biggest proponents of this strategy, and his teachings have done much to convince folks that doing hard things in the morning is essential.

Got a book proposal to write? Do it first thing. Got a mission critical presentation to finish? Do it in the morning before anything else.

That said, although I hear folks say this a lot, most folks don’t actually remember why it’s the case. If you, like me, need a reminder, listen up. It turns out, it’s not about forcing yourself to be a morning person (as many people think), or only about the fact that finishing one task helps you feel more productive and encourages you to keep going (true, but not the only reason).

Instead, the reason that doing the hard thing first thing is a good idea is all warding off the energy-zapping power of decision fatigue.

Yup. Listen up.

The reason you need to do your big job in the morning is that you will always have the most energy before you get decision fatigue. Decision fatigue is the thing where you spend energy not knowing the right choice to make.

Ever feel tired after shopping? Or after searching for the perfect flight for an hour? That’s flagging energy because you’ve taxed your mind with decisions. Email is great for doing this, and so one of the great reasons to not get super engaged in email first thing is to keep your mind energetic for your big task. Ultimately, doing your focused work before you get into your email and start messing with a long list of requests from others, is essential.

Try it one day this week and see if it works for you. Commit yourself to not doing an hour of emailing before writing that memo, and see if you feel more energized and do better work than doing the reverse.

If you have tried this already, have you had good results?