Too much exercise can be bad for your mental health, according to staff at one specialist hospital.
Working out is usually a good thing
They warn some people can do themselves serious damage through developing a compulsion to work-out to excess.
Berkshire's Huntercombe Hospital treats people whose addiction to exercise is as strong as any associated with drink or drugs.
However, experts stress it is vital that people do take regular exercise to keep fit and healthy.
Out of control
Laura Kibble, 22, is an exercise obsessive who used to get up hours before her three-year-old child every day to spend time working out on her trampoline.
Her obsession got so serious that she was referred for treatment at the Huntercombe.
"When Luke was about eight months old I had a lot of time on my hands, so I got a bit depressed and I found that getting out and about with the wee boy and his buggy would lift me up a bit.
"It just spiralled from there really. I found that exercise would lift my mood, and make me feel better about my body. What started out as a little bit of healthy exercise soon became out of control."
Laura bought herself an exercise DVD, which she planned to use twice a week for an hour.
"I found myself doing it first thing in the morning before the little one got up, I would put him to his bed for an afternoon nap and I would do an hour then, and at night I would do an hour as well. This was in between the time I spent out walking. It became a bit of an obsession."
Laura said her exercise obsession was linked to the fact that she was struggling with an dieting disorder.
"It was like a routine - I just had to do it."
Laura has now managed to conquer her problems. She still exercises, but only in healthy moderation.
"I still get the urge, but I don't want to go back to that. You are tired constantly, and you have not got a life."
Diane Whiteoak, manager of the Huntercombe Hospital, said the amount of exercise a person needed varied between individuals.
However, the hospital had treated people who were so driven to take exercise they were on the point of collapse.
People who have an eating disorder are particularly vulnerable. Often they will exercise in secret.
Ms Whiteoak said people who were worried that they were becoming addicted to exercise should ask themselves whether it was starting to take over their lives.
For instance, did they force themselves to exercise when feeling unwell, or did they prefer to exercise rather than being with friends?
"The main difference between healthy exercise habits and people who are exercise dependent is how activity fits into their life.
"If they put workouts ahead of friends, homework and other responsibilities they could be developing a dependence on exercise."
Ms Whiteoak also warned that excessive exercise can damage tendons, ligaments, bones, cartilage and joints.
"When minor injuries aren't allowed to heal they often result in long-term damage, and instead of building muscle too much exercise actually destroys muscle mass."
She said once a person's sense of self-worth was bound up with how much exercise they took, they should seek help, possibly from a parent, teacher, or GP.
"If you have got to the point where you are not enjoying exercise but you feel obligated to do it, or you feel anxious, or guilty if you miss even one work-out then it is beginning to affect your life."
Steve Walker, a personal trainer and keep-fit fanatic, said he regularly saw people overdoing it at the gym.
"You will see them almost sweating their health away. After about three to four hours five days a week there is nothing left of them. They are constantly getting colds, they are always ill, they have never got any energy, they are constantly lethargic.
"Physically and mentally they feel about as low as they can get, so they go and do more and more to try to solve the problem, much like a drug addict who wants a bigger and bigger fix."