Schnecker,
The intent of the library was to provide a simple interface to the radio. If you dive into the library source code, you will find that putting the radio to sleep is handled for you in the GDO0 ISR of the A110x2500Radio class - there is no need to call sleep(). Also, the interface correctly handles waking up the radio whenever a receiverOn() or transmit() operation occurs - there is no need to call wakeup().
I hope this clarifies things for you. Feel free to contribute or modify your local version of the library to extend its features (but do share!).
-Brandon