Singer Alanis Morissette has opened up on how Christmas carols change her mood.

"I have a seasonal affect thing, so as soon as it gets dark at five pm my mood (changes)," she said during a chat show, reports AceShowbiz.

Once the holiday season begins, she even finds some of the festive tunes "devastating".

"The chord changes in some of the Christmas songs are really actually really devastating," she said.

"I notice it in my kids' faces. I'll start playing Christmas songs and something happens (to me). My mum used to have to pull over to the side of the road when so many of the Christmas carols were being played because I would get really emotional -- like existential ache," she added.