You can function just fine on 6 hours of sleep a night...or so you think. Here's how to tell if you're secretly sleep-deprived. By Corrie Pikul 1. You’re clicking your pen and tapping your feet. What's happening: When you move your muscles, you cue your brain to stay alert, says Hans P.A. Van Dongen, PhD, assistant director of the Sleep and Performance Research Center at Washington State University.