Jon Hamm is ''much happier'' after receiving treatment for alcohol addiction.

The 'Mad Men' star, who recently checked out of the Silver Hill Hospital in New Canaan, Connecticut after spending 30 days at the rehab facility, reportedly ''seemed sad'' before he decided to seek help.

A source close to the 44-year-old actor told E! News: ''People around him have known for a while about his problem with alcohol. In the last few years, he has seemed sad. He's doing well now and appears to be much happier.''

John's longtime girlfriend Jennifer Westfeldt, 45, helps ''keep him in line'' and is believed to have urged him to seek treatment.

Another insider told PEOPLE.com: ''He needs [Jennifer]. He has to have that [stability] in his life.''

Jon previously admitted he battled depression after his father died when he was 20.

Speaking in 2010, he said: ''I was ... unmoored by that. I struggled with chronic depression. I was in bad shape.

''I did do therapy and antidepressants for a brief period, which helped me. Which is what therapy does: It gives you another perspective when you are so lost in your own spiral. It helps. And honestly? Antidepressants help!''