Sun in sixth house is in the house of his enemy Saturn. But Sun is primarily considered a malefic planet. Sixth house is a malefic house. So two negatives make for a positive. A more popular term for malefic houses is dushamsthana. As long as Sun is strong native does well in his life. Native is mostly healthy, victorious over his enemies and confident in face of adversities and troubles. 

      According to B. V. Raman,  Sun in sixth house makes a native a very good politician, famous and successful. But if Sun is afflicted then it may not bode well for his health. Long and troublesome illness are also possible according to B. V. Raman. A fortified Sun could give good administrative abilities, some enemies but wealth and success in all endeavors. Affliction with Saturn could give heart-trouble and chest pain. Having said that Sun in sixth house is in the house of debt, diseases and service. Sun is confident, enterprising, cheerful and ambitious. Natives fail to empathize with those who work under them, are debt or disease ridden. Such qualities make them unpopular with those working under them and those who are suffering. Although overall health is fine, there could be some adverse results to health if there is affliction due to gochara transit of planets. 

     Sun in sixth house is not bad overall. If there are health troubles then natives are advised to pray to Sun and offer water in a copper pot. Keep in mind that water offered is collected in the tulsi plant, neem plant or peepal plant. Be sure to eat their leaves regularly to avoid health concerns. It is not superstition but ayurveda at work in this remedy. Also chant the Gayatri Mantra and speak Surya Mantra regularly. 

References:

Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.