Self knowledge through numbers is the motto of the "quantified self" movement. Calories consumed, energy expended, work done, places visited or how you feel. By recording the data of your daily life online, the life-loggers claim, you get to know who you really are.

So far this type of self-tracking is the obsession of a geeky minority. But through our smartphones and social networking sites more and more of us being drawn into this world by stealth. Frances Stonor Saunders asks what it means for our ideas about privacy and sense of self.

Producer: Fiona Leach.