According to the National Sleep Foundation, a short nap of 20 to 30 minutes can improve alertness and performance without creating feelings of grogginess or interfering with nighttime sleep. A NASA study that examined sleepy pilots and astronauts found that those who napped for 40 minutes improved their performance by 34 percent. While napping can be beneficial, napping can have a negative impact as well. For example, those who nap too late in the day may struggle to get a decent night's rest later that night, when the length and quality of sleep may not be as long or as strong as it would had they not napped so late in the day. Men and women who find themselves suddenly need naps despite no obvious cause of fatigue should consult their physicians, as this might be indicative of a sleep disorder or another medical condition.