If you want this to be automatically done, even if you sometimes switch off your machine, I suggest that you install anacron
Code:
apt-get install anacron
Otherwise if your machine is switched off during the daily scheduled update, it will not be done.
Anacron takes care of this and will remember old scheduled task that haven't been done (you will get some cpu load a few minutes after switching on your machine because old task will launch but this should really be done)