The single handers claim to sleep 20 minutes at a time, for example those in the mini transat atlantic race. However when you read their accounts, you often hear of them going below for several hours to sleep after an especially tiring day (many sail changes, bad weather, etc). They rely on the radar
with a beep that wakes you up when it sees something passing nearby. That combined with an AIS tranceiver, radar
detector and active radar
reflector (though these only work with the older generation radars
I have heard) seem to cover you pretty well.
However I know that one of these mini 650's had an issue where the sailor accidentally turned down the volume on his radar
and wasn't awakened until it was too late. He lost his rig
in the middle of the atlantic after hitting a tanker. He was lucky he didn't hit it straight on or he would have certainly died.
I often wonder how coastal single handers do it, since it will be rare that you will have nothing on the horizon for 20 minutes at a time, ever...