I am a singehander and break the rules all the time on passage. Section I, Paragraph 5 of the COLREGS:
The rules apply in any visibility (e.g., in sight or in restricted visibility).
Every vessel must at all times maintain a proper look-out by sight (day shape or lights by eyes or visual aids)and hearing (sound signal or Marine VHF radio) as well as by all available means (e.g., Radar, Automatic Radar Plotting Aid (ARPA), Automatic Identification System (AIS), GMDSS...) in order to make a full assessment of the situation and risk of collision
I use my radar and AIS (transponder) but as soon as I get some shut-eye I am breaking rule 5. If anything happens while I am not actively on deck and keeping lookout then I am either partially or completely at fault.
If I am at fault in a collision with a big vessel then I'm also dead. Even if I am the stand-on vessel but not on watch I am both dead and at fault.