He was arrested June 2 when he was leading a demonstration against poor power scenario and had locked electricity board officials. He was produced in court June 3 and sent to judicial custody after he refused to furnish a bail bond.
According to jail sources, Sinha was reading a newspaper sitting on a chair that broke, leading to his fall.
The Bharatiya Janata Party (BJP) leader sustained minor injuries on his head. Later, jail doctors treated him.
Thursday, central BJP leaders like Rajiv Pratap Rudy and Shahnawaz Hussain had met Sinha in jail.
Jharkhand BJP president Ravindra Kumar Rai and Leader of Opposition Arjun Munda along with others have decided to intensify the agitation against poor power scenario in the state from June 16.