As soon as I was medically cleared to exercise (after chemo and before rads), I started a light walking program. Only 15 minutes at a time and pretty slow. Then I started going and down the stairs in my townhouse several times a day, which I'd avoided. The more I exercised, the less tired I was, AND I slept better.
Be sure you are getting enough protein. Ask your doc if you can take supplements, especially B complex.