A person with mental illness will do better in life if they have activity in their routine rather than inactivity. Being engaged in life is essential to recover functioning and to have quality of life in spite of these illnesses.
I believe that medication and inactivity when put together may cause brain atrophy. The "use it or lose it" truism is especially applicable to people who take antipsychotic medication. While on antipsychotic medication, a great deal more effort is needed to become mobilized. This can be fairly uncomfortable, but is necessary if a person doesn't want to continually lose capabilities.
Mental exercise should include both physical and mental activities, since, in order to use your body, you must use your brain. An example of the physical might be checking the oil on your car, cooking an omelet, or playing a game of ping-pong. An example of mental activity might be balancing your checkbook, reading a novel, or attentively watching news on television or on the internet.
A psychotic or manic episode is quite a shock to the brain. When in the recovery phase and given medication, a person may be set up for brain atrophy. It is like having a sprained ankle, and then being put in a cast. While the splint allows for the tissues to mend, it also prevents movement, and this can cause weakness. Medication can be seen as such a splint, however, for the brain. Once past severe symptoms, it is important for someone with either bipolar or schizophrenia to make an effort. However, this does not come easily.
I have seen mentally ill people who have become resigned to their seeming fate. They do not make an effort, they seem like immobilized blobs, and they function at an impaired level. The mental health treatment system assumes that mentally ill people will be this way.
I know of a middle-aged woman who is not willing to walk a short distance to accomplish simple errands, who rarely gets up out of her chair, and who does not clean her apartment, nor properly take care of her cat. At one point, she fell and was wedged between pieces of furniture in her bedroom. She couldn't free herself, was trapped for three days, and was lucky that her neighbor, who was concerned, knocked down or opened the door. This person didn't learn from her folly, and to this day continues to be immobile.
When computers became prevalent, and when I acquired a PC, it was very good for my mental condition. E-mail communication allows me to circumvent much of my shyness and to be in contact with people. When dealing with people through the internet, outward appearance is usually irrelevant, and this can be a good thing. In the process of solving computer problems that will inevitably, sooner or later occur, it gives the brain a workout.
When I was briefly self-employed at helping people with their PC problems, it allowed me to run a zero overhead company in which I got out-and-about doing meaningful work. Many years before that, in the 1980's and early 1990's, I did repair of home electronics. This was good for me because it required a lot of focus.
Relationships are also a source of stimulation, and are invaluable for a person's development.
If someone has an episode of severe mental illness and is subsequently medicated, it is important to remain active, and to do so even if this is uncomfortable.