For example, I recently downloaded "Sleep Cycle" and it's really changed how I sleep and how I wake up. The idea is that everyone runs through three stages of non-REM sleep and REM sleep every 90 minutes. Ideally, your body best wakes up when it's in the lightest stage of sleep which is in Non-REM. So, you set the time you want to wake up, you put your phone under the sheet on your bed (it needs to be face down and connected to the charger), and you go to sleep. If I want to be awake by 5:45am, that is when I set the alarm for and it says it will wake me between 5:15am and 5:45am based upon where I am in the sleep cycle. It has a quiet bell tone that gradually gets louder. You can set the alarm tone yourself to something of your choosing. Then in the morning when you wake up, you see a graph of when you were awake, when you were in a light sleep, and when you were in a deep sleep. AND, it tells me the quality of my sleep based upon the data. It used to be that I would just wake up when the alarm sounded by it had nothing to do with how ready my body was to wake up. So even on nights when I get 7 or less hours of sleep, I still feel rested and awake in the morning and throughout the day.
Another handy, dandy app I use that is for women is iCyclus. I can track my monthly cycle there. So when I find myself extra tetchy, craving chocolate, wanting to sleep more, and feeling bleh, I look at the app and I say, "oh yeah, that's it." It's not for those using NFP, I believe there are fancier apps for that.
Anyone else have handy apps? Apps that keep you organized? Apps that you can't live without?