Soretna,
From what I understand from the Puranas, The Sun doesn't experience these negative periods such as the Kaliyuga. It is always illuminated and the demons and reptoids cannot handle such light and radiations.
Due to the proximity, I think that human society is always regenerated and fostered; from The Sun, and from the higher, celestial planetary systems.
In The Bhagavad GIta, the Krishna Avatar tells that his incarnations are cyclical. In the Puranas, the Kalki Avatar is decribed, that He comes at the end of the Kaliyuga and eliminates the demons from the inner surface, and re-populates the outer surface because the outer surface becomes beautiful again.
So I think that there are always humans and always demons, just as there is always good and bad. But the demons live below wherein they escape from radiations which are harmful to them, and don't influence the surface much until the Kaliyuga comes around the next time.
The Bhagavad Gita War was when the demons started their influence on the surface again, anticipating the Kaliyuga. They manipulated the end of the Vedic warrior communities, corrupted them, and then started the manipulation of the surface on a broader scale.
Dean