Personally I don't feel there is a need for a nightly scan when you have a slew od resident scanners trying to stop the malware getting on your system in the first place.
However there is no scheduled scan function in avast, this is one of the restrictions in the free version, unlike avg free which restricts the protection levels/functions. avast's Home version has virtually all the features of the Pro version, but requires interactive input and no scheduler.
These automated options are only available in the Pro version. This is a limitation of the Home (free) version that it has interactive input requirement, there have to be differences in the Home/Pro version and this is one of them, the programmers have to eat
Check this work around out -
HOWTO: Scheduled Automatic Scans avast! Home Version!