What is the origin of idiom- take a chill pill?

The idiom 'take a chill pill' is often used to mean 'relax' or 'calm down', however, the origin is not linked to actual medicinal pills. Instead, it is thought that the phrase originated from the 1960s counterculture, where it was a colourful, slang way to tell someone to stay relaxed and not get too stressed or worked up.