When all of a sudden you realize
something terrible has happened to you, it feels like being
slapped in the face with a wet fish. In instances where you gain this
insight right after waking up, you have obviously 'slept with a wet fish' only to realize what a fool you've been after you wake up.
When I woke up and realized those conmen had
tricked me out of a lot of
dough last night, I really knew I had slept with a wet fish
all night long!