How does the Sleep mode work on this device? Is it active at all times? For example, if I set it to 30 minutes, after it automatically turns off, does it immediately 30 minutes again the next time I turn it on, and keep a running total even if I turn it off in between. It seems that my Express has sometimes auto shut off after only a few minutes playing, and it seems the only thing that could be causing that would be a cumulative play time toward the Sleep setting.