With mine, when I drain it completely, it just shuts down, i.e. it doesnt go to sleep mode as there is no pulsating light on the front right corner. Once I start charging, however, and I can again switch it on, it resumes where I left off, so the computer has saved the contents of the memory onto the hard disk. On my SR MBP it was different, it would actually go to sleep mode when the batteries were almost completely drained.