The Sun has to pass through the underworld during the night to reach the Eastern horizon by dawn.
Words describing his real name are mostly related to the word SUN in most of the Indo-European languages.
Vedic: स्वर्ग (Svarga, Swarga, Svarga Loka)
Slavic: Svarog Svarožič Swarożyc Zuarasici
East Slavic: Даждбог Dazhbog Dabog Dadźbóg
East Slavic: Khors Hors
Old Prussian: Suaixtix
Hittite: Sun goddess of Arinna