Why is ninety minutes the magic number? Our mental energy tends to oscillate in ninety-minute waves. We sleep in ninety-minute cycles, moving between periods of light, deep, and REM sleep. Our energy continues to follow the same rhythm after we wake: we feel rested for around ninety minutes and then tired for a short period of time—around twenty to thirty minutes. A short break every ninety minutes or so takes advantage of these natural peaks and valleys in energy cycles.
