( I say, Which nation's citizens don't shower regularly? ? )
HHmm ... This sleepwear question does come up quite often and is usually answered by very sloppy people who feel it is quite normal to sleep in the clothes they will be wearing the next day.
Now, this may be permissable for those from some of our younger democracies and newly freed colonies (within the last three hundred years) but for those from a civilised nation (and there is only one of course) this will just not do at all.
At all refugios one is expected to wear a decent pair of flanelette pyjamas at the very least.
Silk can be worn of course, but as tailored pyjamas only, not as skin-fitting long underwear, and only - and I must stress this - only when worn with a Fez, whilst smoking a Turkish cigarette, and holding (languidly) a gin and tonic (in one of the glasses your servant will be carrying for you).
A dressing gown can be considered as optional only by the very avant-garde (how would it be if there were a fire?) so, not knowing who you are, I must tell you that the norm - ask any pilgrim - is a tartan wool dressing gown with gold piping (weighs a mere 2.3765478632 kgs).
This is the absolute minimum that can be worn at night whilst on Camino.
However, to really fit in and to feel quite at home and relaxed in all eventualities allow me to suggest that a pair of plain Morroccan leather slippers would complete the ensemble - but plain slippers mind you, not tooled with any fancy decoration or design - you do not want to appear foolish after all, heaven fordbid.
Take this advice and I assure you that you will have a most memorable pilgrimage (as will the other pilgrims that you meet each evening).
I do hope that this helps.