force os x serve to sleep

Hello,

im wanting to use an external drive (connected to mac mini) as a source for TM backups across the network.
think I've come to the conclusion no amount of 'file sharing' settings can make this happen,
so setting the mac min up as a 'server' is the way forward.

but i still want the mac mini to sleep,

is their a way of having the server run, but only when needed, or accessed by an external comp on the network.

ie - can the mac mini be woken, external drive used to back up for...

force os x serve to sleep