According to the researchers, the timing of buffalo sleeping

During the day, camera traps documented 70 instances ofyellow-billed oxpeckers riding on buffalo, and just eightinstances of the birds on giraffes

During the day, camera traps documented 70 instances ofyellow-billed oxpeckers riding on buffalo, and just eightinstances of the birds on giraffes. (Snapshot Serengeti and the Serengeti Lion Project)

and waking is likely caused by their migratory habits. Their migratory patterns would push them toward a more northern or southern location in order to avoid harsh weather conditions during winter.