A source close to the couple insists there was no infidelity or other smoking gun behind their breakup, and says it was a mutual decision by two people who remain friends.
"It wasn't contentious," the source tells PEOPLE. "It had kind of just run its course … They just sort of looked at each other and said, 'We like each other and we make each other laugh, but this isn't for the long-term.' "
Hectic schedules may have also contributed to the split. In addition to starring in and producing Ghost Whisperer, Hewitt is preparing for an upcoming Lifetime movie, and getting ready for a tour to promote her first book, The Day I Shot Cupid, due out March 23. In the book, the star – quite fittingly – offers advice on relationships, love and weathering, among other things, breakups.
"She's working constantly, and Jamie's working on the show and touring [with his stand-up comedy act]," says the source. "It just isn't conducive to a relationship."
Still, the source says the former couple's on-set relationship remains drama free and professional.