You have to know about the hidden option of your iPhone seen in the Clock App.
1) Tap the Clock icon on the Home Screen.

2)Then Click on the Timer icon on the Right side lower border of the page opened.

3) Then set the count down time which you want.

4)Then click on the “When the Timer Ends” button to get the settings which we want like the “Sleep iPod” option.

5)Then activate it by Clicking on the “Set” button.

6) Then you will go back to the timer page, where you have to click on the “Start” button to activate the Sleep option.

So now i think you can enjoy the option which is unknown to many.