I think that every singlehander has his own way. When I was on crossing alone, I was trying to get short naps of half an hour all the time. Once outside of the shippng lanes , I was getting naps of about an hour. Nevertheless, many times I was waking up because the sound of the water along the hull had changed, or a flapping sail or some new sound unusual for the boat...and sometimes just because I felt there was something dangerous outside (twice a collision course). The fact is that all your senses develop a bigger sensitivity, and you get awake easier. When weather was getting very bad, I was hoving to and going to sleep, as there was nothing better to do. Now that my girlfriend and I sailed together fo six years, we got used to take a 4 hours watch each, except in long offshore crossings where we sleep happily together and wake up occasionally to check that everything is ok (no ships, no nasty clouds and boat on course under windvane). Occasionally you get awaken by a nasty squall and you better handle the reefing in seconds. If sailing along a coast we keep a strict permanent watch.