You've probably experienced it—your alarm is set for 6:30 a.m., yet somehow your eyes snap open a few minutes before it goes off. There's no sound, no external cue, just the body somehow knowing it's ...

If you're a very heavy sleeper, you already know a regular alarm clock usually doesn't get the job done, and phone alerts or easygoing sunrise alarms don't rouse those who are deep in an REM cycle, ...