to lie down: to put one's body flat, whether to rest, to sleep, to have sex, or in death
One simple action sits under every use of this word: a body going from upright to flat. From that single picture the Bible draws a whole range of situations. A person lies down to rest on the ground or a bed, lies down for the night and sleeps, lies down with another person for sex, and lies down for the last time in death.
The shades are not separate words but the same act read by its setting. Lying down at evening shades into sleep. Lying down beside someone becomes the ordinary plain way the Bible speaks of sexual relations, often without naming the act outright. And the long sleep of death is pictured as lying down too, so kings "lie down with their fathers" when they die.
Because the core stays constant, the right English word depends entirely on context, and a reader has to follow the situation rather than fix on one translation.