When evolution was determining when we’d go to bed and when we’d wake up, it didn’t count on us needing to work through the night at a hospital, or taking a 10-hour flight to the other side of the world, reports Metro.co.uk.

‘When we travel from one time zone to another, we lose time,’ she explained. ‘For example, if someone travels from the UK to India then they lose five hours – so when it is still 4am here, it’s 9am there. This makes us very tired, as we are still in the depths of night over here.

‘So when a person lands there, they will still be very tired as the internal body clock is still set to 4am. It can take 48 hours or more for the body clock to re-adjust to the time there, based on the light and darkness. This is what we call “jet lag”.’