Alternatively, press command + option + eject, or select sleep from the drop down menu. That would be the closest you could get to "hibernation" of the MBA... assuming hibernation is different than sleep.
FYI: In windows, usually sleep is power is shut off to the hard drive, monitor, cpu, but the ram stays powered to retain the data.
Hibernate, saves whats in RAM to the hard disk, then shuts it all down. When you re-awaken your computer, the information saved to the hard disk is then transferred back to the ram and the computer resumes (albeit much slower) just like if it was asleep. In hibernate you use even less power.
Sleep; from what i understand, on closing the lid the mac will write the ram to the hdd (hence the hard disk continues to spin for abt 20s after closing - light continues to glow), then go to sleep. Once there's low batt the com hibernates - that's when it takes much longer to start up.
I'm using the first gen mba, if i leave it overnight, sleeping averages appx 1.1% batt life per hour, so its quite okay i guess.