It happens just as you’ve dozed off to sleep.

As you being to drift into dreamland, suddenly you are wrenched back to Earth by a sudden jolt.

It’s as if you’ve fallen off the kerb or tripped – and afterwards you feel wholly awake.

This is known as a ‘hypnic jerk’ and is actually an evolutionary adaptation that protected us from prey, says Jason Ellis, Professor of Sleep Science at Northumbria University.

Waking up one last time before we fall asleep allowed us to check for anything nearby that might eat us.

Or – if nodding off in a tree – it let us ensure our chosen bed for the night was actually safe.

But if this phenomenon is keeping you awake, there are things you can do to lower the chances of one happening.