It’s always such a relief to climb into bed at the end of a long day.

But the exact reason why humans need sleep is still a mystery, according to new research.

For a long time it has been thought the brain uses sleep to flush out toxins and get rid of damaging molecules.

However a new study now suggests the opposite may be true – and experts say being active might be more beneficial for ‘cleaning’ the brain.

A team from Imperial College London’s UK Dementia Research…