Why does battery die during sleep mode? I'm running MacOSX 10.5.8 on a MacBook Pro.

I was working on my computer last night. I finished work, closed the lid and expected the computer to go to sleep. I assume this is the Apple Safe Sleep mode. My battery was about 70% charged.

Tonight, I open up my laptop and discover that the computer refuses to start and battery has been completely discharged. When I plug the computer in again, all information from my previous session was lost. It's as if the computer shut down completely.

What's happening here? Shouldn't the computer go to sleep and save all information to disk? When I wake the computer up from sleep mode, shouldn't it restore all information from my previous session?

This only happens about 5% of the time that I close the lid. Usually, this works as I expected.

Update to answer @zneak's question:

This is the hardware model, according to About this Mac: More Info...: Hardware.

Model Name:       MacBook Pro
Model Identifier:   MacBookPro3,1
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!)