30 minutes. Some studies show that sleeping this long may cause sleep inertia, a hangover-like groggy feeling that lasts for up to 30 minutes after waking up, before the nap's restorative benefits become apparent.
60 minutes. This nape is best for improvements in remembering facts, faces, and names. It includes slow-wave sleep, the deepest time. The downside: some grogginess upon waking up.
90 minutes. This is a full cycle of sleep, meaning the lighter and deeper stages, including REM (rapid eye movement) sleep, typically likened to the dreaming stage. This leads to improved emotional and procedural memory (i.e. riding a bike, playing the piano) and creativity. A nap of this length typically avoids sleep inertia, making it easier to wake up.