One day i charged my iphone till 90%to92% then in didn't use my phone and went to sleep after 10 hours I checked my phone and the battery is showing only 20% charge is left. In the meantime i didn't used my phone so is there a battery issue or some thing else???
Apple has a battery replacement program for iPhone 5. You can check eligibility on this Apple page
Apple has determined that a very small percentage of iPhone 5 devices may suddenly experience shorter battery life or need to be charged more frequently. The affected iPhone 5 devices were sold between September 2012 and January 2013 and fall within a limited serial number range. (...)
The program covers affected iPhone 5 batteries for two years after the first retail sale of the unit or until March 1, 2015, whichever provides longer coverage.
First, check to see what application is consuming battery power. To check this, open Settings>General>Usage. Under Battery, touch 'Battery Usage'. After some loading time, you will see a list of Applications that have used Battery power in the last 24 hrs and last 7 days. If there is an application using a significant percentage of power that you have not used, you may wish to kill that app or restart your iPhone.
Specifically to the iPhone 5, see @Tetsujin response on extended warranty from Apple.