Macbook goes in sleep mode while iTunes is still playing

Hi all

My macbook pro is going in sleep mode while iTunes is still playing. It started to be very annoying, and can't find a way to stop this behaviour.

I have my Energy Saver settings at 3 hrs, but doesn't seem to count. As it always goes to sleep after 30 mins even when iTunes is playing songs.

Do you guys came across similar issues? If so, did you manage to find a solution / workaround to stop it?

Thanks

Rob