Moore, in an interview with Jane magazine, said she felt "really low, really sad" for no apparent reason, People reported.
"I'm a very positive person, and I've always been glass-half-full. So it was like someone flipped a switch in me," Moore said in the interview.
Moore said her breakup last year with "Scrubs" star Zach Braff -- who also admitted to suffering from mild depression -- did not trigger her depression, but may have aggravated it. She did not say what may have caused it.
Professionally, Moore said she has been working on new songs, spending time at the remote Allaire Studios in Woodstock, N.Y.
"Writing has been really therapeutic," she told Jane. "These little nuggets that have come up over the past eight months have made me look at things in a different way."