Katz said the experience allowed him to finally "close the circle." The frustration he felt in the year following his father's death was gone.

Lucid dreams are generally understood to occur exclusively during REM, the final phase of the sleep cycle that is most closely related to wakefulness and the one generally associated with dreams. Research on the prevalence of lucid dreamers suggests that if you've never had a lucid dream, you may be in the minority.

In a 2004 study of psychology undergrads in Germany, 82 percent of participants reported having experienced a lucid dream at least once. In Japan in 2008, 47 percent of surveyed undergrads had experienced a lucid dream, with 19 percent of participants lucid dreaming frequently (at least once a month). A recent Brazilian study found that 77 percent of those surveyed had experienced a lucid dream.

Another German study attempted to understand lucid dreaming as it relates to age and surveyed students between the ages of 6 and 19. They found that by age 19, more than 50 percent of the sample had experienced at least one lucid dream, with the frequentness of lucid dreams decreasing with age. In a less scientific survey, I was amazed at the number of friends and family members I discovered over the course of writing this article who have lucid dreams.

While it might be possible to broadly estimate how many people have experienced a singular lucid dream, it’s perhaps more difficult to determine how many people are deliberately doing so. Dr. Daniel Erlacher, a sleep researcher and lecturer of sports sciences at the University of Bern who contributed to two of the studies cited above, told me that when we talk about very frequent lucid dreamers—those who have multiple lucid dreams per week—the figure drops to around one percent. These are the people researchers want to get into their labs.

But we needn’t look any further than Facebook pages, blogs, and websites to find a community of passionate lucid dreamers and lucid dreamers in training. Rebecca Turner, now a 30-year-old writer from Kent, England, says she trained herself to lucid dream when she was 14 and has been doing it ever since—up to several nights per week, depending on how much time she devotes to “lucidity practice.” The Facebook page for Turner’s site, World of Lucid Dreaming, has more than 200,000 followers, and on April 12th Turner and some other lucid dreamers are organizing a "Lucid Dreaming Day."

Some people are so good at lucid dreaming, they can indicate to researchers, while they are asleep, that they’ve achieved lucidity. In a method developed in the 1970's by British Psychologist Keith Hearne and in the 1980’s by Stephen LaBerge—a psychophysiologist who did decades of dream research at Stanford and is considered the Godfather of lucid dream research by his fans—sleeping subjects are instructed to sweep their gaze from left to right within their dream twice when they’ve become lucid. Because eye muscles are not paralyzed during REM sleep (unlike the rest of the body’s muscles, which presumably prevents us from physically acting out our dreams, wandering out of the cave and getting eaten by a bear), sweeping your eyes from left to right in dreamland corresponds to doing so in real life.