My own personal routine is to do as Mark suggested and get as many 20-25 minute naps as I can during the day. With practice you'll get good at waking up, looking around for a minute or two and then going right back to sleep. I don't sleep any more than that when I'm within 150-200 miles of California because of the shipping. I have occasionally slept more than that when the fog is so thick that even if I was awake I couldn't avoid a collision with a ship.
That's sort of a fatalistic attitude, I suppose.
I keep up that routine while doing longer passages but when I'm more than 200 miles off the California coast I try to get 90 minutes of sleep between midnight and 3:00 AM. Staying asleep for 90 minutes allows you to get into REM sleep, stay there, and then come out of it again and that makes a big difference in overall alertness. For my body, it's not crucial to get that 90 minutes for a 2-3-4 day trip but on a trip that is longer than that, it's really important.
I have noticed that for me, there's a really big difference between getting an hour of sleep and getting 90-120 minutes.
I know, myself that I can keep this routine of naps and 90 minute sleeps for about 6 days. At that point I have to get about 6 hours of uninterrupted sleep or I lapse into serious stupidity and feel really sick.
1968 Selmer Series 9 B-flat and A clarinets
1962 Buesher "Aristocrat" tenor saxophone
Piper One Design 24, Hull #35; "Alpha"