Why do we spend 10 per cent of our waking hours with our eyes closed - blinking far more often than is necessary to keep our eyeballs lubricated? Scientists have pried open the answer to this mystery, finding that the brain uses that tiny moment of shut-eye to power down. The mental break can last anywhere from a split second to a few seconds before attention is fully restored, researchers from Japan's Osaka University found. During that time, scans that track the ebb and flow of blood within the brain revealed that regions associated with paying close attention momentarily go offline. And in the brief break in attention, brain regions collectively identified as the "default mode network" power up. Discovered less than a decade ago, the default mode network is the brain's "idle" setting. In times when our attention is not required by a cognitive task such as reading or speaking, this far-flung group of brain regions comes alive, and our thoughts wander freely. Most of us take between 15 and 20 such moments of downtime per minute, and scientists have observed that most blinking takes place near or at the point of an "implicit stop". While reading or listening to another person, that generally comes at the end of a sentence; while watching a movie, we're most likely to blink when an actor turns to leave the scene or when the camera shifts to follow the dialogue.