Hit the hay/sack American informal verb phrase
The assumption is that the fact that mattresses used to consist of cloth sacks stuffed with hay comes from the fact that they hit the hay and hit the sack. Some historians go so far as to suggest that the practice of fluffing up the hay inside the mattress before lying down to sleep comes from hitting the hay and hitting the sack.
Go to bed
You are too tired, I think you should hit the hay.
Today I'm too busy, so I should hit the sack to provide energy.
Sleep continually for a long period of time
The first idiomatic expression, a variant from around 1940, dates from the early 1900s.
To be very wet
Because of forgetting bringing an umbrella, I look like a drowned rat when it rains.