Popularized by technology writers Gary Wolf and Kevin Kelly (Kelly also co-founded Wired magazine), the quantified self is a broad term used to encapsulate all of the different ways in which people are using increasingly effective and available technology to track their daily lives. Whether it's to simply achieve a certain goal or reach some sort of Buddha-like higher level of what Wolf calls "self-knowledge through numbers," there's no shortage of tools out there to track everything from your exercise and sleeping habits to heart rate, glucose levels and blood-oxygenation sensors [sources: Antephase, Wolf].
Like many movements before it, the idea of the quantified self came about at the intersection of a recognized need and the technological advances necessary to meet it. Humans have long been into measuring stuff, but until fairly recently gathering data was a protracted process that required manually entering information into spreadsheets and other software and then using it to build individual charts and graphs. It was also susceptible to human error and subjectivity, dependent on both the data gatherer's focused efforts and variation of observation and gathering cycles to paint a complete picture [sources: Wolf, Wolf].
Then came automated technology, smarter and smaller sensors, cloud computing and social media. Small, battery-powered sensors can now be used to collect data, which Web-based apps and sync-able devices compile and store automatically and present in readily understandable packages [source: Wolf].
Take FitBit, for example. These paper clip-sized activity tracking devices continuously sync to smart devices and computers to give a wide range of fitness-related info, including the number of steps a user takes in a day and the distance the user travels by foot, as well as calories burned, total activity time, weight and body fat changes. It also allows users to log the calories and food they intake each day. An accompanying app presents this data in easy to read charts and graphs and awards "badges" for achieving personal goals. Similar sensor-based devices can be used to track length and types of sleep [sources: FitBit, Wolf].
But the quantified self isn't limited to physical fitness goals. Self-knowledge seekers are using data gathering tools to add endless variables to their logs, from focusing ability and mood to caffeine consumption and menstrual cycles.