sluggish for a while after you wake up. Naps are recommended for people who have accumulated a sleep deficit, for people who are sleepy, and for people who suffer from certain sleep disorders, such as narcolepsy . But naps are not recommended for people with insomnia, because they can wind up "borrowing" from their next night's sleep. Research has shown that in extreme cases, such as when people have to drive alone and cannot get several hours of sleep in a row, taking 10-to-20-minute naps at regular intervals over the course of the day can partly meet their need for sleep and enable them to perform at minimally acceptable levels for a few days. But their performance under these conditions will always be far poorer than if they were completely rested. People who are performing monotous tasks such as driving a vehicle for hundreds of kilometres may experience "micro-naps" that last a few seconds. The frequency of these micro-naps is proportional to the individual's sleep deficit, but they can also occur in people who have gotten enough sleep, especially during cyclical variations in alertness , such as those that occur about once every 90 minutes.