You can use the AT command (or a good scheduler such as OpalisRobot) to schedule a batch file that contains a net start and/or net stop.
Here is an example of scheduling RAS to stop at 23:00 and start up at 04:00
AT \\ServerName 23:00 /interactive /every:M,T,W,Th,F,S,Su cmd.exe /c "Drive:\Directory\RASSS.bat"
where RASSS contains:
net stop RemoteAccess
net start RemoteAccess
Note 1: RemoteAccess is the actual service name. To determine the service name, scan the registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
Note 2: Sleep is a Resource Kit utility. If you don't have it and can't afford it, schedule the net start as a separate job.