Some people with atrial fibrillation, particularly older people, do not have any symptoms.
The abnormality in heart rhythm is often only discovered during routine tests or investigations for another condition.
Typically, a cardioversion (where the heart is given a controlled electric shock to restore normal rhythm) is carried out.
At this point, many people feel much better and realise they had not been feeling normal.
People often attribute tiredness and feeling lethargic to ageing, but once normal rhythm is restored, they realise these symptoms were caused by atrial fibrillation.
The most obvious symptom of atrial fibrillation is heart palpitations – where the heart feels like it's pounding, fluttering or beating irregularly, often for a few seconds or possibly a few minutes.
As well as an irregular heartbeat, your heart may also beat very fast (often considerably higher than 100 beats per minute).
You can work out your heart rate by checking the pulse in your neck or wrist.
Other symptoms you may experience if you have atrial fibrillation include:
The way the heart beats in atrial fibrillation reduces the heart's performance and efficiency.
You should see your GP immediately if you notice a sudden change in your heartbeat and experience chest pain.
An electrocardiogram (ECG) can be used to confirm a diagnosis of atrial fibrillation. An ECG is a test that records the rhythm and electrical activity of your heart.
Read more about how atrial fibrillation is diagnosed.