I single hand sail nearly every trip, mostly in Chesapeake Bay. However, I have spent an enormous amount of time offshore in powerboats, large and small, distances to several hundred miles on a single trip, and spent four years in the U.S. Navy. So, I guess if you add up all my sea time, which is more than 55 years, a dozen pond crossings on war ships, I guess I have a little bit of experience.
However, I base my statement on sleep deprivation upon several first hand experiences.
When I was a young man, 22 years old, I worked in the medical field as the chief cardio-pulmonary technician for two major teaching hospitals in Baltimore. I, like many younger men, figured I was invincible, didn't need to sleep, and could live on a couple hours sleep a night. At the end of 15 years I was down to 140 pounds, looked like a zombie, and realized that I was leading a pretty unhealthy lifestyle. One night, about 3 a.m., I was in the operating room, running the heart-lung machine, which bypasses the patient's heart and lungs so surgery can be performed on the non-beating heart. This was my third case that day, I had been awake 22 hours and only had two hours sleep prior to this stint. I thought I was doing just fine, the patient was fairly stable, and all the sudden I felt a sharp pain in my forehead. I had fallen asleep while adding blood to the oxygenator, fell forward, and my head slammed into the stainless fitting that held the pump housing in place. I had no idea I was falling asleep, and fortunately, the patient was not endangered during the few minutes I was in la-la land. My head was bandaged by one of the thoracic surgery residents, and when the case ended, the same resident stitched up my head just above my left eye. I still have the scar from 50 years ago.
One of the places in the University of Maryland Hospital I worked was the R. Adams Cowley Shock Trauma Center. You would be amazed at the number of automobile accident victims we saw nearly every hour, many of which were the result of individuals that fell asleep behind the wheel. A significant number of these folks freely admitted they lived on just a few hours sleep a night, and that they were confident they were fully functional and alert, despite their lack of sleep. In reality, they were not capable of operating a motor vehicle for the 30 minutes it normally took them to commute to work.
Bottom line - while people sincerely believe they are fully functional, both mentally and physically, while catnapping, and that they are not placing themselves or others in any danger because they are in the vast expanses of the ocean, in reality this is never the case. If the boat is moving and the captain is asleep at the helm, then Houson, there is a problem.
Now, I anticipate that those that routinely do take those catnaps will argue that THEY never have a problem. If they feel that confident, that's just fine. As Clint Eastwood once said "Opinions vary."