Even one of Hollywood's sweethearts can't stay upbeat all the time. Jennifer Aniston recently revealed during The Dr. Oz Show that she became quite grumpy when she stopped exercising in order to get into character for the upcoming Golden Globe-nominated drama Cake.

"It's interesting when you stop exercising. It really was interesting how my serotonin levels went down. My stamina was shot. I was cranky. I was irritable," Aniston, 45, said on a pre-taped episode. "I'm usually really not any of those things. I found myself short. I was hungry like crazy."

As previously reported, the Friends alum often went without makeup and stopped working out for two months to play Claire Simmons, a woman who suffers chronic pain after a car accident. But for Aniston, not looking like her red carpet self wasn't the downfall of not hitting the gym.

"You realize exercise is just so important to our soul," she explained to Dr. Oz. "Not just being able to fit into skinny jeans, to your state of mind, your soul, all of that. I surrendered into it. I actually didn't fight it. I didn't think oh this is horrible. It was such a part of what I enjoyed about mining this character."

One thing that has become awful (off set!) is her lack of sleep. Aniston came prepared to The Dr. Oz Show with a list of questions about her night habits, even asking if she was "going to die young" because of it.

"I have so many questions for you by the way. I have a hard time sleeping. Sleep is a tough one for me. My brain, the committee in my head, if I wake up at three in the morning I just start having conversations and they won't shut up and then I can't get back to sleep," Justin Theroux's fiancee said. "I do have three animals and a human being in the bed, so there are all these distractions. I got an app on my phone that allows you to sort of track your sleep rhythms. I don't really quite ever get into the deep, deep, deep sleep."

To find out Dr. Oz's response, watch the video above now!

Aniston's appearance on The Dr. Oz Show airs on Thursday, Dec. 18.