"pg_ctl: cannot be run as root"

Hi there,

I installed postgres without creating another postres user/admin than
myself, as there is no other user on my machine and it's only for
testing purposes.

But strange enough, I only can start the postgres server from time to
time, normally when I have freshly restarted the machine. Now for
example, I did some changes to the postgresql.conf file, and when
trying to restart the postgres server, I get this:

gridmac7:~ schwarzer$ sudo /opt/local/lib/postgresql82/bin/pg_ctl -D
Documents/data_postgres -l logfile start
pg_ctl: cannot be run as root
Please log in (using, e.g., "su") as the (unprivileged) user that will
own the server process.

What is this? Thanks for any advice,

Stefan

PS: Running Leopard on MacPro.