Macbook or macbook pro knows the following sleep states:
- sleep:machine will go to sleep only (saves state in RAM only, battery keeps RAM contents)
- sleep & hibernate: machine sleeps and hibernates. (default)
- hibernate only:machine will go to hibernate only. (saves state on disk, battery will not be used)
Just sleep means that the notebook will go to sleep fast, but you loose the ability to change the battery as the battery is needed to keep the contents of the memory (RAM).
Just sleep and hibernate will wake the computer fast, but sleeping will take ages as the contents of the memory are saved to disk before entering the sleep.
SmartSleep.prefPane let’s you select each select sleep state. Additionally the new SmartSleep state lets your notebook just sleep while the battery has a high level. If the battery level drops below a certain point ( default is less then 20% or 20 minutes ) it will switch to sleep and hibernate. So you have the best of both worlds.