This is the 2816th most frequent French word.
coucher
To put to bed; to lie down; to sleep.
In this sentence, 'coucher' is used reflexively ('se coucher') to mean 'to go to bed', indicating the action of going to rest or sleep.
Je vais me coucher tôt ce soir.
I will go to bed early tonight.
Here, 'coucher' is used to describe the setting of the sun ('se coucher' in this context means 'to set'), connecting it with the natural phenomenon.
Le soleil se couche à l'ouest.
The sun sets in the west.
In this case, 'coucher' is used transitively to refer to the act of sleeping, emphasizing the physical location of their sleep outdoors under the stars.