job doesn't fire on schedule time

Hi,
I'm new in *PostgreSQL *and I have to schedule a SQL job that fires 
every day at 00:00:00.
I created the job with its schedule and step (a simple select of a 
function that checks a field value of a table and send a e-mail).
The problem is that the job is fired every  10 sec (the default polling 
time) and not at the schedule time.
Can you help me?
Thanks.

Teresa Fazio