We've all been there: your mind goes blank in an exam, when you walk into a room for something, or when you have to speak off the cuff.

It's annoying and stressful, but it's usually normal—so what's really going on, and when should you see a doctor?

Mind blanking, which happens about 15% of the time, is not the same as mind wandering. It means either you can't pull out a memory you need, or you zone out completely with no thoughts at all.

People with anxiety, ADHD, or little sleep get it more often, and it's also common in some dementia types.

The brain's executive attention network, which helps us focus, store memories and get them back later, is the key to understanding this issue.

A brain chemical called noradrenaline keeps this network working by controlling how alert we are.

This network can be easily disrupted: tiredness makes parts of the brain "sleep" even when you're awake; stress makes noradrenaline spike, making you focus only on dangers, not daily memories; multi-tasking also stops your brain from storing memories well.

Mind blanking is fine most of the time, but you need to see a doctor if it happens a lot, starts suddenly, or messes up your daily life. Your doctor may ask about your health history, check your thinking skills, or send you for a brain scan or further tests.