From my personal experience, i would define health from a biological standpoint. To me, health is the absence of disease or disease causing agents, such as bacteria and viruses. When a person is healthy, all of their anatomical structures and systems are working in sync to fend off disease. On the other hand, when a person is ill there is a concrete reason that they are ill, be it bacterial toxins, viral infection, chemical imbalance, or organ system malfunction. My opinions on health and illness stem from when I was about 5 or 6. I was in and out of hospitals or doctor’s offices regularly with all kinds of minor infections. In these cases, the doctors would always just prescribe some antibiotic which would get rid of some disease causing agent, which would in turn make me “healthy” again.
Insomnia, in my opinion, is an illness. It can be caused by hormone imbalance, misuse of drugs such as amphetamines, neurological disorders, as well as other things. Some of these causes can be treated simply by balancing out the chemicals in the body. Hormone treatments can be used to correct imbalance, and discontinuing the misuse of amphetamines will eventually remove them from the body.
Old age, on the other hand, should not be classified as an illness. Old age is not caused by some infectious agent, you can’t “catch” old age as you would another infection. Old age may lead to an increased number of infections by way of the wearing out of the immune system, but by itself, it is not an illness. If anything, old age should be seen as a sign of, more or less, lifelong healthiness. If a peson manages to live into their nineties or hundreds, much of it could be attributed to a strong immune system, or striving to avoid contracting an infectious disease.