There was an interesting article in either Sail or CW on Ellen MacArthur''s sleep schedule for her around alone race. Worth looking into. I believe she also got some sleep time every 2 hrs.
Sleep is an interesting question. Generally, you cannot go for long periods of time without REM sleep. That varies from person to person, but I think an average REM cycle (dream cycle) is 40 min. I believe Ellen had one period per day of 2 hr sleep....but check me on that.
More recently, there was an article in CW on the latest around alone race and one contestants sleep (though I think they simply reported the aggregate). You might be able to get some info off the race web site.
They do make radar''s that have an alarm function that can be set at various distances. Also, a good idea is to select a boat that has nice long, wide cockpit benches that are great to sleep on.
The need for sleep cannot be overstated. Being able to function with a schedule of catnaps on a constant basis is one thing that separates solo sailors from others.
Hope this helps