Which historical event marks the beginning of the middle byzantine period?

Empress Theodora’s reinstatement of the venerations of icons was the historical even that marks the start of the middle byzantine period.  Empress Theodora was known as the most influential and most powerful among the Byzantine empresses and is known in the modern times as the saint of Eastern Orthodox Church.  After the success of the Nika revolt, which was a way of Theodora to prove herself to be a worthy leader,  she and Justinian,  her husband rebuilt and reformed Constantinople and made it as the most splendid city the world has seen for centuries. The term “byzantine” even refers to the art and architecture of Constantinople and its surrounding reigns.

this is a good question! i'll do some research and come back when i have the answer!