[Vala] problem using sleep

[a a <valaprgrmr gmail com>]

Hello.
I'd like to sleep for X minutes and then run a command as root.
I've tried this with no success:
Process.spawn_command_line_async("sleep "+time_min+"m ; gksudo
'/etc/acpi/hibernate.sh'");
(in the terminal it works)

How should I do it?

Thanks :)