How Much Data Do You Need?
Credit: Photograph by Justin Steele

2. You're a Data Junkie Who Wants to Dial Up His Fitness
It turns out there's a subcategory of activity trackers, including the BodyMedia Armband and the Basis health and heart-rate monitor, that pack in a bunch of extra sensors to measure steps, calories, and sleep with far greater accuracy than simpler devices. I picked up a Basis and quickly got interested in its hyper-precise sleep readout, showing exactly how little mind-restoring REM sleep I was getting every night. After a month testing the watch, I also discovered that my temperature was low, as well as my calorie-burn tally, during my longer runs, which indicated I wasn't pushing myself as hard as I thought I was. You can also use these souped-up monitors to measure the effects of overtraining: for example, checking your resting heart rate when you wake up. "There is almost no better way to measure fatigue," says Steven Devor, a professor of exercise physiology at Ohio State University. "If you're typically at 60 beats per minute, and you see it's 66, you know you need to take a rest day."

I was impressed to hear about the results Sami Inkinen, a 38-year-old triathlete and co-founder of the online real estate company Trulia, saw from a body monitor called Beddit, a European device slated to hit U.S. markets soon. Persistent tiredness and lackluster race results suggested that he wasn't bouncing back quickly enough from workouts. The Beddit, a thin strip that slips underneath a sheet, measured each night's mind-restoring REM sleep, body-restoring deep sleep, how often he tossed and turned, and his resting heart rate.

It also charted Iniken's heart-rate variability, a widely-accepted marker for physiological stress, noting when a steady pulse indicated he was relaxed and when rapid fluctuations showed he was strained.

The Beddit's sensor (which uses your smartphone's microphone) detected sound levels in Inkinen's bedroom, too. "It helped me see that I was waking up in the middle of the night, every night, around the time sound spikes from street noise," Inkinen says. He adapted first by closing windows, then by tinkering with other factors known to affect sleep – cutting back on late-afternoon coffee and after-dinner booze, and establishing a highly regular sleep schedule.

The effect data tracking has had on Inkinen's performance has been dramatic: He became overall amateur champion at Ironman 70.3 Hawaii. He's told me since that he's made micro-monitoring his sleep a permanent part of his life. "It lets me see exactly how stressed my body is every morning, so now if it seems like things are falling apart, I might make that day's workout easier, or cancel a dinner or a meeting and make sure I get to bed earlier."

Your Tracker: Basis health and heart-rate monitor

The Basis is a multisensor that gauges, minute by minute, everything from skin temperature spikes to how hydrated you are. Best, it provides sleep metrics – such as hours spent in REM and how often you wake up – of near-clinical accuracy, according to experts.