During this time he took part in excavations in Southern Italy and Greece and participated in field trips to Athens and Olympia.
From 1976 to 1979 Sinn was Referent at the German Archaeological Institute at Athens, where he participated in the excavations of the Heraion of Samos among other things.
In 1989, Sinn received his habilitation from the University of Bonn for his work, Die griechischen Heiligtümer als Stätten der Hikesie (Greek Sanctuaries as Places of Hikesia (Prayer)).
This involved control of the university's antiquities collection in the Martin von Wagner Museum.
Sinn's chief research interests are the functions, organisation and topography of Greek sanctuaries, especially Olympia, Greek architectural sculpture, minor arts, vase painting, the history of ancient sports and ancient architecture.