The thinking goes that tackling the most challenging, unpleasant task gets it out of the way first thing in the morning, and that the joy of crossing it off your to-do list before lunchtime will motivate you to sail through your workday. In reality, how many times have you valiantly tried diving into the most difficult thing on your agenda, only to push it aside by 10:00 a.m., all in a huff, in order to go answer emails?

“Eating the frog” is bad for your brain

It’s no wonder why. There’s a lot of evidence that mood affects our ability to work effectively. The psychologist Alice Isen spent her career exploring the relationship between mood and thought, and found (not so surprisingly) that having a positive mood makes us more productive and creative. That means that you should think intentionally about how you manage your mood during the workday–but particularly in the morning.

Related: This well-known productivity advice is actually pretty bad

More recent research suggests that fast thinking elevates your mood, while slow thinking depresses it. It’s intuitive enough: Completing goals makes you feel good, while struggling with them feels bad. If the “frog” refers to a goal you’ve been procrastinating on, it makes sense that you’d want to knock it out early in order to attain that mood-boost. But your likelihood of actually doing that quickly is pretty slim if it’s a hard, unpleasant, froggy task or problem.

Indeed, many of us start our days with less challenging tasks that depress our moods in much the same way. When I give seminars on workplace behavior, I often ask how many people start their day by checking their email. Almost all the hands in the room go up. Email does have one characteristic that’s associated with positive mood: It’s often possible to answer a few emails and clear them from your inbox, which gives you the chance to complete some goals. And that can feel good.

But generally speaking, email creates just as many chances to have your mood crushed first thing in the morning. You often get reminders of tasks you haven’t finished or requests you need to answer. You may get questions about tasks you thought were already finished. Or puzzling emails with problems you can’t solve right away. All these elements can make you feel worse. And that will affect your attitude–and, subsequently, your productivity, for the rest of the day.