If any of your sims bars get low, simply click on self and the option to cure will be displayed, select cure. As has been said already, it may fail and you will need to do it again.
This, for me, is the most valuable reward object. Once you have it the sim need never, sleep, eat, bath etc. again if you don't want to. However, all these activities have their own benefits, so I would not suggest using it exclusively to maintain your sim. It's great though if used correctly. Some suggestions: Restore your sim's sleep bar, then send them to bed. They will sleep for just a few seconds and get up with the fully rested moodlet. If you do this (using a tent of course) in a tomb when they have the wish to sleep in a tomb, you get the additional benefit of HP for no effort. Have the sim write or work from home 'til stressed then use the cure and their happiness will be fully restored. Remove all negative moodlets immediately they appear; you can exercise for hours this way for example.
It's an hour before work and your sim is tired, but, not exhausted. This is the one time you might consider zap. With luck you will reduce the energy bar to zero on first attempt and can then use cure to fully restore it. Make sure you do this in plenty of time though, or you could have them rushing off to work in dire condition.
It also cures the Mummy's Curse immediately.
