Why does battery die during sleep mode?
What happened could’ve been because of different reasons. Certain old Macbook Pros had troubles with sleeping (hence why zneak wants to know about the model).
However I believe that your computer woke up (wake on lan? hung process?) during the night and it burned through your battery. What are your Energy Settings?
Also, get some sort of CPU meter like MenuMeters or iStats Menu (€) and make sure your CPU/Cores are not 100%, that indicates a process that’s using all your CPU power (and a lot of energy!)