Why do I wake up feeling unrested?

The rude arithmetic of sleep: eight hours in bed minus an hour awake at 2am, twenty minutes to fall asleep, and fragmentation you never consciously noticed equals considerably less sleep than the clock suggests. Sleep efficiency — the share of time in bed actually spent asleep — is the metric most people have never heard of and most need. This guided journey covers the gap, the fragmenters, and the counterfeiter that passes itself off as sleep's friend.