Disable iPod at certain time
You can do it with Activator. After installing it in the Cydia you can assign Actions to the Events. For example you can assign this workflow:

Anywhere &gt; Build &gt; Scheduled &gt; Sun,Mon,Tue,Wed,Thu,Fri,Sat 12:00 AM &gt; Save &gt; Done


Now select:

12:00 AM &gt; Lock Device


Although this workflow just locked your device and does not restrict the usage after the 12:00AM, it acts like an alarm.

The other tweaks that can do the restriction are: AppCap, Parental Controls.