Media reports quoted Aargau cantonal police as saying the accident happened on the central Bahnhofstrasse, close to the Manor department store, in the early hours of the morning.
The street lighting was dimmed because of the Christmas lights, the 20 Minuten website reported.
“Lighting on Bahnhofstrasse has been reduced owing to Christmas illuminations and visibility was poor,” a police spokesman told radio SRF.
The woman van driver reported hearing a bump.
When she went to investigate she found the man lying on the street.
Police said it appeared the dark-clothed man had been on the ground when he was hit.
Investigators are appealing for witnesses.
Alcohol tests were being carried out on both the driver and the victim.