Sleep your Mac differently depending on the battery level.
Your 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)
• hibernate only machine will go to hibernate only. (saves state on disk, battery will not be used)
• sleep & hibernate machine sleeps and hibernates. (default)

Just sleep means that the notebook will go to sleep fast, but you lose 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.