Mac OS sleep isn’t really sleep. It’s write out an image of memory, then sleep. Useful when you want to swap batteries without shutting down, but not much otherwise. I found out about SmartSleep a couple of weeks ago and it has one dead simple rule: just sleep if the battery is above a threshold and do sleep+hibernate otherwise. Now I’ll no longer have to stick my ear by the disk to listen for it spin down before throwing my portable in a bag :)