‘Hit the hay’ is a US American colloquialism meaning ‘to retire for the evening’ or ‘to go to bed’. Like many expressions that enter into common use from colloquial language, the exact origin of the phrase is virtually impossible to pin down. However, we can pinpoint the approximate time by checking the first occurrences in print. Based on the proliferation of use at the turn of the 20th century, and the multiple references from sports, it’s probably safe to say that the origin of the phrase is likely to have arisen sometime in the late 19th century. This would also place it within the first rural-urban migration in the US (1870 – 1920) and would therefore also make sense in relation to another commonly held assumption that hitting the ‘hay’ is a reference to the plumping of straw mattresses.

Source: Dreams