archive_timeout runtime change

Hi

I use postgrsql 9.1.1 and I want to change archive_timeout on a running 
server. My default archive_timeout is 1800s, and nightly I want to 
change to 3600s or more, but I get the following error message:

set archive_timeout='3600';
ERROR:  parameter "archive_timeout" cannot be changed now

and

update pg_settings set setting='900' where name='archive_timeout';
ERROR:  parameter "archive_timeout" cannot be changed now

I know is possible to change this parameter with pg_ctl reload, but I 
think if is possible without changing the conf file.

Thanks

Levi