In the Renaissance and the Enlightenment there was a widespread view in Europe that the period between the end of the Western Empire (AD 476) and the Renaissance had been a time when mankind had 'hibernated' intellectually and spiritually - a time of domination by the Catholic Church, a long period of darkness and obscurantism.
There was also a widespread feeling that in the Renaissance mankind again took control of its destiny.
It was on this basis that west Europeans divided history into Ancient, Medieval and Modern (with various subdivisions). It long been customary to subdivide Modern History into Early Modern (up to the French Revolution, 1789) and the rest of the Modern period.
There are problems with these divisions. Firstly, the division is geared to western Europe. The division between Ancient and Medieval doesn't really apply to Byzantium, let alone China, for example. Secondly, many take the view that the 'barbarity' and 'obscurantism' of the Middle Ages is grossly exaggerated in this view.