joanna is more alert in the morning and her brother david is more alert in the afternoon.

the difference stems frim a difference in the siblings what?

School children are more alert and have quicker reaction times in the afternoon, according to a study whose findings appear to fly in the face of current thinking. Many primary schools design timetables around the assumption that children are more focused and open to learning in the morning and therefore tend to teach maths and literacy lessons before lunch.

It could be from age difference or the fact that she's a female and he's male.