Many thousands of people are thought to be prone to faints - a condition called vasovagal syncope.

BBC: Feeling faint? Cross your legs

Some variation is normal, but if it gets too long it can upset the timing of heartbeat with potentially dire consequences - dizziness, faints and, rarely, sudden death.

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While sufferers can keep faints at bay by avoiding dangerous movements - such as standing up too quickly - and consuming enough salt and fluid, some doctors recommend exercises that patients can carry out if they feel a faint coming on.

BBC: Feeling faint? Cross your legs