I have a DG834G router, so I assume that the Web GUI will be similar
1. You need to identify the IP address of the PS3
2. Go to 'Content Filtering' then 'Schedule'
3. Set the schedule to 'Every Day' and set your 'Start Time' and 'End Time' that you want to block
4. Go to 'Content Filtering' then 'Firewall Rules'
5. Under 'Outbound Services' click 'Add'
6. Choose 'Any(ALL)' for the Service
7. Choose 'Block by schedule, otherwise allow' for the 'Action'
8. Choose 'Single Address' for the 'LAN Users'
9. Enter the IP address of the PS3 in to the 'Start' fields
10. 'WAN Users' can be left as 'Any'
11. 'Log' can be set to 'Match'
If at any time you want to permanently block the PS3's access to the Internet, at point 7 choose 'Block Always' for the 'Action'
I use the above for my son, to try to limit the time he plays online games on his XBOX360, when he should be doing his homework, or as in your case, sleeping
The above can be used to block any LAN IP address, but you cannot have more than one set schedule