iPhone: Naps can improve your productivity, but you need to hit the sweet spot of length to get any benefit from them. Power Nap HQ is an app that helps you with just that.
Power Nap HQ works similar to most sleep-tracking apps. You turn on the app, set your optimal nap time, set it on your bed, then get to napping. The app monitors your sleep and tries to wake you at the best time. In my experience, it did a pretty good job, and I didn't feel the usual grogginess of a mid-afternoon nap. The app then logs all that data and creates a nap graph to track what works best for you.
Power Nap HQ ($1.29) [iTunes App Store]