Starting Remote iTunes Airplay in Sleepmode starts playing and stops after one Minute

Hi,

I have a whole apple configuration:

* Wireless Network provided by Time Capsule

* A Macbook Pro

* Airport Express to Airplay Music in rooms

* iPhone5 with Apple Remote to connect.

When my MBP sleeps and is connected to a power plug, I can start apple remote on my iphone and connect to itunes on my MBP.
(However this won't work when the MBP is not connected to a powerplug). My MBP is connected wireless to the network.

Now: When I press play on the remote app, I can hear music from an internet radio station.It plays about a minute and then stops playing. The connection to remote is interrupted an comes back immediatly. I have to press play again, because I can see it's stoppted.

Any ideas?

regards,

Jascha