S'allonger vs. se coucher
While both 'se coucher' and 's'allonger' refer to the act of lying down, they have subtle differences in usage and connotation in French.
S'allonger
'S'allonger' means to lie down or stretch out, often for a short period or without the intention of sleeping.
Je vais m'allonger sur le canapé pour me reposer un peu.
(I'm going to lie down on the couch to rest a bit.)
Il s'est allongé dans l'herbe pour lire son livre.
(He lay down in the grass to read his book.)
Le patient s'est allongé sur la table d'examen.
(The patient lay down on the examination table.)
Se coucher
'Se coucher' typically means to go to bed or lie down with the intention of sleeping for the night.
Je me couche généralement vers 23h.
(I usually go to bed around 11 PM.)
Les enfants se sont couchés tôt ce soir.
(The children went to bed early tonight.)
Il faut se coucher maintenant pour être en forme demain.
(We need to go to bed now to be in good shape tomorrow.)
Summary
In summary, 's'allonger' is used for lying down temporarily or for rest, while 'se coucher' is primarily used for going to bed to sleep. 'S'allonger' is more versatile and can be used in various contexts, whereas 'se coucher' is more specific to bedtime or nighttime sleep.