By Linda A. Hill and Kent Lineback, contributors

FORTUNE – For most managers, hardly a day goes by without confronting the challenge of employee motivation. Are you doing enough to engage the people who you work with? Are you doing too much or just not the right things?

An article in the September 30 issue of Science describes the efforts of two sociologists at the University of Vermont who tried to better understand the rise and fall of people’s spirits. They studied 2.4 million people’s moods by analyzing the words they used in over 500 million tweets originating in 84 English-speaking countries over two years (February 2008 through January 2010).

What they found was a daily cycle of positive and negative feelings that seemed to apply consistently across cultures, geographies, and time zones. Around the world, people’s positive moods peaked in the morning (6-9 a.m.), dropped through the day until reaching a trough by mid/late-afternoon, began to pick up in late afternoon, and peaked again in the evening. The glib conclusion is that work is the culprit that destroys people’s good moods, but the cycle holds on days off too, so what’s going on is much more complicated.

The study confirmed a long-recognized phenomenon that has huge implications for managers but is rarely reflected in how bosses try to energize their staff.

What social scientists call positive affect (good feelings) and negative affect are not related to each other. As the authors of the study say, the two “vary independently and are not opposite ends of a single dimension. NA (negative affect) is neither the mirror image of PA, nor do the two measures move consistently in parallel.”

In the work world, we talk not of positive or negative “affect” but of motivation, commitment, and engagement — the willingness of people to expend the extra effort that extraordinary performance usually requires. Whatever it’s called, the phenomenon is the same: “motivated” and “unmotivated” or “engaged” and “unengaged” are two different things. The factors that drive one are different from the factors that drive the other.

A good way to understand the difference between such causes and drivers is to think of your health. If you’ve ever been seriously ill, you understand how depressing that can be. But recall what happened when eventually you recovered. You were energized by good health — for a time. But soon enough you came to expect it. Health became just normal, the way life should be. It didn’t energize you any more. So health can be a disincentive when you lose it, but good health doesn’t motivate you to do great things.

The implication for you as a boss is that if you want motivated (or committed or engaged) people, you must take two kinds of actions. You must remove restraints and replace them with drivers of motivation.

Removing the things that inhibit motivation will lead, at most, to a neutral place. To paraphrase the authors of the Twitter study, motivation and a lack of motivation (or engagement and disengagement) aren’t opposite ends of the same spectrum. They are two spectra that have to be managed separately.

If you’re like most managers we’ve met, you tend to focus on the causes of low motivation, such as poor pay and benefits, lousy work conditions, demeaning or obstructive policies and rules, status, or bad relationships with co-workers — the kinds of things that people will name if you ask them what’s wrong. You do have to deal with these things, but that’s not enough. If you want high performance, you must go further and also offer compelling reasons for people to commit themselves wholeheartedly.

What are those things? This is hardly a complete list, but it’s a good start. How many of them have you put in place?

Compelling purpose. People want to feel like they are a part of something important. Do you talk about the purpose of your work — not just what you do but why you do it? Do you talk about those outside your group who benefit from what you do?

Challenging goals and plans. Besides feeling like they are a part of something important, people often want to strive toward something important. They need challenging goals based on their purpose and plans that show how they can achieve those goals.

Clear roles and responsibilities. People often need to know what they’re responsible for individually and what others expect of them. They want to see the link between their work and the team’s overall purpose because that’s how they will feel both valued and valuable. With clarity about what they do, and clear feedback about how they’re doing, they can take control of their own work, and you won’t need to supervise their activities, which can be a powerful source of discouragement.

In simple terms, if you want superior performance, you must both remove obstacles and put in place reasons to work hard. Pay attention to both. They’re not the same.

Frederick Herzberg was the great expounder of this important idea in the latter part of the 20th century. For a summary of his thinking, see One More Time: How Do You Motivate Employees?

Linda A. Hill, a professor at Harvard Business School, and Kent Lineback, a writer with 30 years of management experience, are co-authors of Being the Boss: The 3 Imperatives for Becoming a Great Leader.