Apple: Computer Sleep is Missing on Yosemite

Apple have removed the Computer slider from the Energy Saver settings. This is deliberate.

The decision is related to Apple's introduction of improved hardware, Power Nap support, and the engineers' belief that OS X is best placed to manage the computer's power state.

However, if you really would like to change the Computer Sleep, you may use the pmset command as follows:
<< For battery >>
pmset -b sleep 10

<< For AC >>
pmset -a sleep 10

* Note: 10 is in minute.