Automatic update of packages using cron-apt
cron-apt Contains a tool that is run by a cron job at regular intervals. By default it just updates the package list and download new packages without installing. You can instruct it to run anything that you can do with apt-get (or aptitude).
It can optionally sends mail to the system administrator on errors, log to syslog or a separate log file.
Observe that this tool may be a security risk, so you should not set it to do more than necessary. Automatic upgrade of all packages is NOT recommended unless you are in full control of the package repository.
Install cron-apt in debian
#apt-get install cron-apt