President Nicos Anastasiades suffered a nosebleed and underwent nasal cauterization at the hospital, where he opted to stay overnight "for clearly precautionary reasons," said government spokesman Nicos Christodoulides.
Anastasiades, 68, was in "good health," and the decision to remain hospitalized was his, not his doctors, said Christodoulides.
Doctors advised Anastasiades to rest for two days.
He had traveled to Brussels to attend the two-day EU summit, which began today. But he will be represented there by Greek Prime Minister Antonis Samaras, said Christodoulides.