Kernix, it's like shooting craps in Vegas. Some people do it, some people don't. Some people win...others lose.
Technically, if you are the only person on board and you are sleeping, you are "failing to keep a proper watch" and that's against both US and international laws regarding vessels and seamanship.
Racers have special needs and take special risks. They also sometimes get away with it, and sometimes don't. A radar set with an "alarm" function will pick up a large target, it may not see a small one at all.
The bottom line is that you do what you are comfortable with, it is your freedom and your responsibility over yourself and your boat.