The nightlife in Ios is unique to any other place in Europe, because the bars and clubs are so different, and have their own personality. There is something for everyone. The village of Ios is pumping from 11 pm until 6-7 am every night throughout summer. That is 8 hours of solid partying, if you can handle it.
All venues are very close, and walking distance to each other. The entrance is either free, or not more than 5-6 EUR (including a drink). This makes it very easy for everyone to experience loads of different places every night.