How sensitive are you to caffeine? You might be more or less sensitive than you think, and the iPhone-only app called UP Coffee (free) by Jawbone could help you figure that out. You use UP Coffee to manually log your caffeine intake, and the app then assesses how different amounts of caffeine affect your sleep. You need a Jawbone UP 24 or original Jawbone UP for the sleep-measuring part. But once your connect the UP Coffee app to the fitness-tracking Up by Jawbone app, the rest is automatic. I tested the app on an iPhone 5 with a new UP24 for about two weeks.

While your sleep data from the Up by Jawbone app pours into UP Coffee, the reverse is true, too. When the two apps are syncing to one another, you can see on the main dashboard of the fitness-tracking app a summary of how many cups of coffee (or the equivalent in caffeine milligrams) you had that day, right alongside how many steps you took and how much you slept. The Up Coffee app, meanwhile, only looks at caffeine consumption and sleep data, ignoring how much you exercised that day and other stats.

In configuring the app, you enter your height, weight, age, sex, and perceived sensitivity to caffeine. When you log caffeine intake, the home screen shows a beaker filling with brown bubbles, representing how much you consumed. During the day, those brown bubbles disappear, representing your body metabolizing the caffeine. The app has quick entry options for logging an 8-oz cup of coffee, as well as specialty drinks from Starbucks. Tea, energy drinks, chocolate, and even other-the-counter medicines that contain caffeine are options, too.

Another screen in the app shows a timeline, with black bars spiking high onto the graph when you drank coffee, and slowly petering out until a "sleep ready" point.

I never thought I was sensitive to caffeine—I once fell asleep in a movie theater after chugging a cappuccino. But the app showed a clear correlation. On days when I had more caffeine, I slept less at night. The one night I conked out hard and stayed that way for more than eight hours (that's championship-level sleeping for me), I just happened to have had very little coffee that same day. Another night when I slept more than seven hours, I also drank a minimal amount of coffee. Huh. Maybe caffeine does have an effect on me?

But this data can be misleading. It doesn't take into consideration other factors. For example, the app didn't assess how much activity I got. On a day when I drank only one cup of coffee and slept seven and a half hours that night, I also walked 14 miles, much of it uphill on a hike. Of course I was tired, and maybe that's also part of the reason I slept so well. I also wonder if it's possible that there are days when I'm more active because I had a solid jolt of caffeine? I want the app to show me more correlations that include activity in the data for just that reason.

The UP Coffee app can't tell you much of anything until you use it for at least seven days, and certainly its ability to draw correlations will improve the more you use it. But without including activity in the assessments, the app is always going to fall a little short with the conclusions it draws.  Even though the UP Coffee iPhone app by Jawbone is extremely limited in what it does, it can help you monitor your caffeine consumption and help you keep an eye on whether it's affecting your sleep, to the extent that the correlations are valid. I do recommend anyone with a Jawbone UP or UP24 give UP Coffee a try for a few weeks. At the very least, it'll force you to look at how much caffeine you're consuming.