I don't have any experience with crossing seas, but I do have some experience in sailing single-handed, sailing night time and sailing offshore.

In my previous sailing boat the engine was not working. I was frequently stuck throughout the night with no wind. Staying awake throughout the night was just not an option - so I put an alarm to wake me up every twentieth minute, I'd look around to ensure everything was OK, and then sleep again for 20 more minutes.

Today we have radar, that helps a bit; I can put the navigation system to beep if the radar senses something, I can also set it to beep if it's too shallow (not much useful when being on deep water). In addition to a timer, I can sometimes nap a bit in the cockpit while sailing. Of course, there is still the risk of getting entangled with fishing equipment, crashing into floating containers and other objects too small to be seen by the radar. Then again, I probably wouldn't spot those things in the darkness until it was too late even if I kept a watch.