A study by the University of Toronto Scarborough suggests that mental alertness can significantly boost daily performance. The researchers found that clear and efficient thinking is equivalent to a productivity gain of about 40 minutes. The study, published in Science Advances, followed participants over a 12-week period to better understand why people sometimes struggle to carry out their plans. The results suggest that daily fluctuations in mental alertness are a key factor. On days when participants felt more mentally alert, they were more likely to set goals and achieve them, whether it was tackling schoolwork or handling everyday tasks like preparing dinner.

What Mental Sharpness Really Means

“Some days everything just runs like clockwork; other days you feel like you’re fighting through fog,” says Cendri Hutcherson, associate professor at the Department of Psychology at the University of Toronto Scarborough and lead author of the study. “We wanted to understand why that is and how much these mental ups and downs actually matter.” Mental sharpness refers to how clear, focused, and efficient a person’s thinking is at a given moment. When it’s high, people can concentrate more easily, make decisions faster, and consistently see tasks through to completion. When it’s low, even simple activities can feel difficult.

Instead of comparing different people, the research team followed the same individuals over an extended period. This allowed them to observe how changes in a single person influenced their daily success or difficulties. The participants, all of whom were students, took short daily tests that measured how quickly and accurately they could think. They also reported on their goals, productivity, mood, sleep, and workload. This detailed approach helped the researchers link mental sharpness directly to real-world outcomes, rather than relying on general averages.

The results showed a clear pattern. On days when the students were mentally sharper than usual, they achieved more of their goals and often set higher goals for themselves, particularly for academic tasks. On days when their mental performance declined, they found it even harder to complete routine tasks. These effects were consistent regardless of personality traits. Traits such as perseverance or self-control influenced overall performance but did not prevent people from having less productive days. “Everyone has good days and bad days,” says Hutcherson. “What we’re capturing is what distinguishes those good days from the bad ones.”

The Impact of Mental Alertness in Everyday Life

One of the most striking findings was how much mental alertness matters in practice. By analyzing cognitive performance over several hours of work, the researchers estimated that a deviation from the usual performance level can affect productivity by about 30 to 40 minutes on a single day. The difference between the best and worst days could amount to about 80 working minutes in total.

What Influences Mental Alertness on a Daily Basis

The study also sheds light on what causes these daily fluctuations. Mental performance is not fixed; it fluctuates depending on short-term factors. Students tended to perform better when they had more sleep than usual and worked earlier in the day, while mental performance gradually declined over the course of the day. Motivation and concentration also boosted mental performance, while depressive moods were associated with lower levels.

Workload had mixed effects. Longer working hours on a single day were associated with higher mental performance, suggesting that people can cope with immediate demands. However, prolonged periods of overload had the opposite effect: they lowered mental performance and made it harder to stay productive. “That’s the trade-off,” says Hutcherson. “You can work hard for a day or two and handle it just fine. But if you grind away for too long without breaks, you’ll pay the price later.”

Why Does this Affect the Execution of Plans?

Executing plans requires several cognitive steps:

• Actively keeping goals in mind
• Setting priorities
• Controlling impulses (“I’m not going to do X right now; instead, I’ll stick with task Y”)
• Starting tasks, even when they’re unpleasant

All of this falls under what are known as executive functions. These are particularly susceptible to fluctuations. On “bad” days, it’s not that the willpower is gone—it’s simply that mental control functions less effectively. The 30–40-minute difference in productivity—how does that happen? It’s not a single major interruption, but rather the sum of many small effects:

• It takes longer to get started
• You get distracted more often
• You switch between tasks more frequently (inefficient multitasking)
• You make poorer decisions (e.g., choosing easy tasks instead of important ones)

These micro-losses add up over the course of the day. That’s why researchers estimate a difference of about 30–40 minutes compared to “normal” levels—and up to ~80 minutes between very good and very bad days. The study shows differences not only between people but also within an individual. So you’re not “fundamentally unproductive,” but rather have better and worse days—and the differences can be surprisingly large.

Practical implication:

This puts the classic advice “You just need to be more disciplined” into perspective. Instead, it makes more sense to:

• Schedule important tasks during periods of high alertness (e.g., in the morning, if that applies to you)
• Consciously choose simpler tasks on bad days
• Use systems (checklists, routines) that require less mental effort

Although the study focused on students, the findings are likely applicable more broadly. The results point to practical ways people can improve their chances of having more productive days. “Our data highlights three things you can do to maximize mental performance: get enough sleep, avoid burnout over extended periods, and find ways to circumvent depressive traps,” said Hutcherson. She also emphasized how important it is to be patient with yourself when you’re not at your best. “Sometimes it’s just not your day, and that’s okay. Maybe this is the day to cut yourself a little slack.”