this is a huge topic. I sailed solo along the east coat of Australia
and knocked up many overnighters while port hopping. I too do 20 minutes asleep and check topsides put a quick entry in the log accuratly indicating the time then set the alarm and climb back into the cot.
I try to get a couple of hours real sleep during daylight an hour after sunrise then late afternoon again. Then I continue thru to 11ish pm awake before starting the 20 minutes off and 5 minutes up routine. I use a dreadful alarm that vibrates as well on my mobile phone
and an oven
timer as a back up.
I have woken a few times to the sight of ships a few miles away and once onlly on a collision
course at night but not what you would call that close (about three miles but he made no effort to avoid me) I dropped a few points off the wind
and passed comfortably behind him.
The biggest fright though came at 3pm on a beautiful day when preparing the evening meal in the galley
. A 40-50k ton ship that I was aware of and was comfortable with, way off collision
course and about 4 miles away. He changed course without my noticing until the fog
horn (5 blasts) went off at a distance of half a mile. This startled me and got me into action very quickly. He was in Morton Bay with a limiting channel available to him (I had lots of bottom space and could go where ever I wanted) but I did not expect a sudden change of direction.
The issue here is that there are safer times to get a cat nap but really another body to help with watches is the real answer.
Until now I didn't have a radar (it's now on a spare bunk awaiting installation) and my wife suffered very badly from sea sickness
so she drove to the destination
and I'd sail solo. We have found a solution to the sea sickness
which gets me a companion but for those rare (I hope) times when I am solo the target alarm on the radar will help.