Hayes was found in his cell Monday at Northern Correctional Institution, the Hartford Courant reported. He was described as unresponsive.
A state official told the newspaper that Hayes may have tried to kill himself with a drug overdose, saving up medication.
Hayes and Joshua Komisarjevsky were sentenced to death for killing Jacqueline Hawke-Petit and her daugthers, Michaela and Hayley, in their home in Cheshire in 2007. Dr. William Petit survived with severe injuries.
Thomas Ullmann, the public defender who represented Hayes at his trial, said his client is suicidal.
"The conditions of his confinement are driving him crazy," Ullmann said. "They are so oppressive. They are driving him into madness. I heard just last week that he looks in terrible shape."
Hayes has filed a federal civil rights complaint, saying he is being harassed and his mail tampered with. In a 2012 interview, he said he had tried to arrange to be served oysters because he is severely allergic to them.