Have you ever felt exhausted before the week has even started?

Nothing particularly demanding has happened yet. You slept reasonably well. But on Sunday evening or Monday morning, you already feel heavy, slow and reluctant.

That’s a pattern many people recognise, especially during long stretches of stress. It makes more sense once you know that fatigue isn’t simply a sign that your energy has run out.

Your brain is constantly weighing up how hard things feel, how worthwhile the effort seems and, importantly, how long it expects that stress to last. Fatigue is partly a decision it makes about how to manage your resources.

If your brain expects a sprint, a demanding but time-limited effort, it allows you to push hard, because recovery is coming. If it expects a marathon, with responsibilities that keep coming, uncertainty that doesn’t resolve and no clear end in sight, it starts to hold back. Tiredness, lower motivation and a sense that everything takes more effort are ways of slowing you down to protect what you’ll need later.

This is why you can feel drained on an ordinary day, yet find energy quickly when something urgent or meaningful comes along. The fuel hasn’t changed; your brain’s prediction has.

Try this: for the next week, notice when tiredness shows up. Is it before a task or after it? Does it lift when something feels meaningful, and deepen when you’re facing something that feels endless?

Then look at one demand that feels ongoing and give it an end point. Break it into stages, decide when you’ll stop for the day, or plan a recovery point after it. Knowing that effort has a limit is part of what allows the brain to release energy for it.

Fatigue isn’t always a sign of doing too much. Sometimes it’s a sign of doing something that feels as though it will never stop.

If your tiredness is persistent, severe, unexplained or getting worse, it’s worth speaking to your GP, as fatigue has many possible causes.

For the fuller picture, including the role of cellular energy and four ways to work with fatigue, read Fatigue and the Nervous System.