The compound adjective 'well-rested' is a transparent modern English formation. Its first element, 'well', is an ancient Germanic adverb of manner with cognates across Germanic languages, ultimately from a Proto-Indo-European root meaning "to wish, will." The second element, 'rested', derives from the verb 'rest', which entered English from Old English and is related to a Proto-Germanic word for "rest" or "bed." The compound logically combines these elements to describe a state of being that has benefited from good or sufficient rest, directly linking the quality of the rest ('well') to the resulting physical condition ('rested').

Structure: well (in a good manner) + rested (having rested) - well: From Old English wel, meaning "in a good manner, satisfactorily." It functions as an adverb modifying the adjective 'rested'. - rested: From the verb 'rest', originating from Old English ræstan (to rest, be at rest), with the suffix '-ed' forming a participial adjective meaning "having been rested."