In order to get the “power” effect, you should only be napping for around 20 to 30 minutes. In order to be productive after your nap, you need to make sure that your sleep cycle isn’t making its way to the REM stage, one of the deepest , according to Dr. Dasgupta. If you go into the deeper stages of sleep that typically occur after 30 minute or so of being asleep, you can wake up feeling more tired and groggy than you did before the nap. This is called sleep inertia . The thing is, when we’re sleeping, our brain's inching its way through a multi-stage pattern that generally lasts 90 to 120 minutes . Some of the sleep stages in the cycle are lighter, while some are deeper. It generally takes between 70 and 120 minutes to achieve REM, which is associated with dreaming, Dr. Dasgupta says. “At nighttime, we want sleeping to be consolidated,” he says. “We want to get to deep stages of sleep at night — but, when we nap, we want to stay in the lighter stages of sleep.” These are referred to in the medical community as N1 and N2