The 'Dallas Buyers Club' star believes swimming competitively and ballet dancing as a youngster has helped her keep in shape as an adult as it inspired her to stay active and maintain her fitness.
She told Hello! magazine: "I've always been active. When I was growing up I was a ballet dancer and I also swam competitively, so, although I didn't realise it at the time, I was setting myself up for a lifetime of activity."
Jennifer - who has three children, Violet, eight, Samuel, two, and Seraphina, five, with her actor husband Ben Affleck - is also grateful to her personal trainer, Valerie Waters, for helping her stay in shape.
She added that the pair enjoy chatting together in between exercising and often get up to watch the sunrise prior to their gruelling work-outs.
She said: "I have a wonderful personal trainer called Valerie Waters - she's my sunrise sister because we watch the sun rise together and then she puts me through a workout.
"She's wonderful because I always feel I've had a good workout with her but we also have a good chat along the way."