Mental fatigue: when your mind stops deciding.

It's six in the evening. A quote sits on your desk, waiting for approval. You read it, reopen it, then leave it for tomorrow. At home you're asked what you want for dinner and you say, 'You choose.'

Your body is fine. You could walk for an hour. It's your head that's shut up shop. I see it often in the business owners and managers I work with, and nearly all react the same way: they grit their teeth and push harder. That's what makes it worse.

How to spot it

Mental fatigue doesn't make you nod off. It shortens your fuse.

• Decisions weigh on you. You put off the small ones and rush the big ones to get them out of the way.
• You lose the thread. You open a tab and can't remember what you were looking for.
• You snap. A simple question from a colleague irritates you, and home is worse.
• You reach for a pick-me-up. Another coffee, something sweet, your phone in hand for no reason.

What drains your mind

It's nearly always lots of small things adding up in one day. Hardly any reflect how good you are at your job.

Doing three things at once. Your mind works on one thing at a time. When you think you're doing three, you're jumping between them. Do that for years and your attention can suffer, along with your working memory, which holds the pieces of a line of thought while you follow it.

Notifications. Each one is a small stimulus that breaks your attention and sends you hunting for novelty again. You get dozens a day, and your mind never stays on one thought long enough to finish it.

Your plate. An all-sugar breakfast sends your blood sugar up and down fast, and the 11am dip in concentration often starts there. Before you blame your willpower, look at what was in your bowl.

How to keep a clear head

The decisions stay the same. You can change the order, and how you tackle them.

The hard stuff, late morning. That's when many people think most clearly. Put the decision that matters there, and leave emails and signatures for the afternoon. And each morning, write down the day's three priorities. Three, in writing, so you stop re-deciding them every hour.

Single-task blocks. Sixty to ninety minutes on one task, phone face down, inbox closed, notifications cut to the bone. Then a proper break.

Breaks that are actually breaks. A few minutes on your feet by the window: light and movement reset your attention better than a screen. Scrolling social media just gives your head more work.

Breathe before you decide. Two breaths in through your nose, the second short, then a long breath out through your mouth. One to three times. When the out-breath is longer than the in-breath, your body eases off and you decide in less of a rush.

The right fuel. In a comparison of two groups, people who had a sweet breakfast were hungry within two hours; with a savoury one, after five. So in the morning, eggs, Greek yoghurt and nuts; at lunch, vegetables and protein before carbs. Last coffee straight after lunch.

Pick the easiest habit and start tomorrow. Your head makes every decision that matters, at work and at home. It deserves the same care you give your diary.