As per Bhagwat Gita :
यथा प्रकाशयत्येकः कृत्स्नं लोकमिमं रविः।
क्षेत्रं क्षेत्री तथा कृत्स्नं प्रकाशयति भारत।। (Bhagwat Gita: Chapter Thirteen verse 34)
“yatha prakasayaty ekah
krtsnam lokam imam ravih
kshetram ksetri tatha krtsnam
prakasayati bharata” (Bhagwat Gita: Chapter Thirteen verse 34)
“Sri Krishna said: O Arjuna, as the sun alone illuminates all this universe, so does the living entity, one within the body, illuminate the entire body by consciousness.”
Just like the ‘one Sun’ illumines ‘all this world’ by his radiance, so the ‘Soul within our body’ illumines this entire Body. We just forget & think that the food that we eat actually does all the work & illumines this body but actually its the Soul energy in the body which processes this food & actually does all the work & illumines this body. Lord Krishna gives the analogy of the sun which illumines the whole universe as in the same way that God, the one Supreme Soul illuminates all the individual Souls and each individual Soul illuminates the entire physical body.
Soul sits in the heart of an individual being & from there Soul illumines our whole body. Just like a single sun is able to illumine the whole universe; in the same way from within the heart, the Soul is able to illuminate the entire physical body by the medium of consciousness. Soul, though residing in the heart by its quality of consciousness illuminates the physical body as light illuminates a room. Above verse of the Bhagwat Gita illustrates that the Sun serves to highlight two aspects of the Soul, viz that, like the sun, the Soul is one in whole body and that It remains unaffected.