The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
19. (Should it be said that the soul does) not (follow the rays) by night; (we reply) not so, because the connexion (of veins and rays) exists as long as the body; and (scripture) also declares this.
It might perhaps be said that the veins and rays are connected during the day, so that the soul of a person who dies during the day may follow those rays; but not the soul of one who dies by night when the connexion of the veins and rays is broken.--But this is a mistaken assumption, because the connexion of rays and veins lasts as long as the body exists. This scripture also declares, 'They (the rays) stretch out from yonder sun and slip into these veins; they stretch from these veins and slip into yonder sun' (Kh. Up. VIII, 6, 2). We moreover observe that the rays of the sun continue to exist in the nights of the summer season; for we feel their warmth and other effects. During the nights of the other seasons they are difficult to perceive because then few only continue to exist; just as during the cloudy days of the cold season.--This the following scriptural passage also shows,' Day he makes in the night.'--If, moreover, he who dies at night mounted upwards without following the rays, the following of the rays would be generally meaningless. For the text gives no special direction to the effect that he who dies by day mounts upwards by means of the rays, while he who dies by night mounts without them.--Should, on the other hand, even he who knows be prevented from mounting upwards, by the mere mischance of dying by night, knowledge would in that case produce its fruit eventually only, and the consequence would be that--as the time of death is not
fixed--nobody would apply himself to knowledge.--If, again, a man dying at night should wait for the dawn (to mount upwards), it might happen that, owing to the action of the funeral fire, &c., his body would, at the time of daybreak, not be capable of entering into connexion with the rays. Scripture moreover expressly says that he does not wait, 'As quickly as he sends off the mind he goes to the sun' (Kh. Up. VIII, 6, 5).--For all these reasons the soul follows the rays by night as well as by day.