Realistically, then, how can you figure out how many hours you should be working? After all, it is unlikely that your to-do list is ever going to be finished. What are the signs that you’re working too many hours?

1. DOING TOO MUCH “FAKE WORK”

If you’re engaged in knowledge work, then you have to sustain your concentration on tasks in order to be effective. If a particular project calls for a long workday, then you can dig deep into your reserves and push through to get it done. When you do that, though, you’re often borrowing resilience from subsequent workdays.

Whenever you have been working too many hours, you will find an uptick in what I affectionately call “fake work.” That’s when you sit at your desk and do things that look like work but aren’t. You shuffle a few papers around. You check some websites. (Reading this article might be your version of fake work . . .) You may pick up your phone and get sucked into social media.

2. FEELING UNCREATIVE

Much of your work probably involves solving new problems. It is easy to do something when the answer is already known. But often you have to find a way to do something that hasn’t been done quite that way before. You’re being paid to do something that is at least a little creative.

When you work too many hours, you are likely to find it difficult to generate really creative solutions. As you get mentally tired you may find it hard to hold all the elements of a problem in mind. Your thoughts will tend to skip from one thing to another. As a result, you may either get stuck when solving a problem, or fall back on a tried-and-true solution. Either way, this is a good sign that you ought to leave work and do something else.

3. FAILING THE READING TEST

Concentrating is tiring. Even if you’re getting enough sleep (though there is a good chance that you’re not), your work will drain you physically. There is an easy way to determine whether you need a break—and it takes only 10 minutes.

Grab a book or article that explains a complex concept that you need to understand for work. (In general, you should have a stack of things you want to learn about nearby, because expanding your range of knowledge is crucial for effective thinking.) Read for 10 minutes.

If you can stay awake and retain what you read after 10 minutes, you’re doing fine. If you find yourself falling asleep or find your mind wandering just a few minutes into reading, then it is time to give yourself a break. Cut back on the hours you’re working so that you can pass this reading test most of the times that you take it.