Like people everywhere who use the sun as their clock/calendar, Native Peoples started their day just before the sun was up and finished the day as the sun went down.  They were adept at striking a camp and loading the horses for movement in near darkness. When moving to a new encampment site, they were well on their way before the heat of the day became burdensome.  You can see the horse herd moving some distance away from the main group of travelers since the dust could be thick and choking.