It’s true that waking up before you go into deeper sleep (such as napping for only 20–30 minutes) or at the end of a sleep cycle (every 90–120 minutes) might help you avoid grogginess or “sleep inertia.” However, you can simply overcome it by allowing 15–30 minutes to fully awaken. It also can help to stand upright and spend time in light (ideally daylight). As long as you have enough time to fully overcome sleep inertia, you’ll probably find that the benefits of a little extra sleep are worth it. Trust your body. It has an amazing ability to recuperate during naps by quickly falling into whatever stage of sleep it needs most.
Signal, T. L., van den Berg, M. J., Mulrine, H. M., & Gander, P. H. (2012). Duration of sleep inertia after napping during simulated night work and in extended operations. Chronobiology International, 29(6), 769–779. doi:10.3109/07420528.2012.686547