The most likely cause of your blackouts is a condition called postural hypotension (also known as orthostatic hypotension). When you move from lying down to standing up, your blood pressure drops very low, probably because your nervous system is a bit slow in telling your heart and blood vessels to work harder to counter the effects of gravity.

When this happens, not enough blood reaches your brain, causing you to feel faint and collapse. This is usually harmless, though if you fall there's the risk that you may hurt yourself; older people sometimes break their bones this way.

In the rest of Europe, low blood pressure is considered a medical problem no matter what age the person is; it's often linked to a variety of symptoms and is frequently treated with drugs. In the UK, however, we tend to think generally it's healthy to have a low blood pressure, so it's rarely treated, especially if the person is young and otherwise fit.

But low blood pressure in older people, especially if it's a postural drop and might cause them to fall and break a bone, is usually treated.

You should definitely see your doctor about your blackouts because very occasionally there are other causes such as abnormal heart rhythms or epilepsy, although these are rare in young people.

It's important your doctor checks your blood pressure when you're lying down and standing to make sure of the diagnosis, and then decides whether treatment would be appropriate for you (for example, if there is a real risk that you could fall and be injured).

Beating postural hypotension

• Get up very slowly: go from lying down to sitting to standing.

• Make sure you have a healthy evening meal with plenty of carbohydrates to provide sustained energy release throughout the night. Waking up with a low blood-sugar level can aggravate hypotension and may be the actual cause of your blackouts.

• Keep well hydrated by drinking at least 1.5 to 2 litres of water a day. Dehydration can aggravate low blood pressure.

• Older people with postural hypotension are often advised to wear support stockings to ensure their blood circulates rather than pooling in the veins of the legs.

This article was last medically reviewed by Dr Trisha Macnair in March 2008