It is very ofter for a every normal person to run scheduled jobs.
This could be backups, some processing that takes a lot of time etc.
Here is how to run a cron job every 6 hours
#min hour day/month month day/week
0 */6 * * * /home/user/script.sh
Have you tried to use "*" instead of 0 for the minutes ? ;) :D
I did and received a lot of emails ... it was working.