"I would always ask her "is this winter or summer time?" just to show how this does not really make sense."

If the rest of your schedule keeps the same numeric time, then this does make sense. If your schedule allows for 9 hours of sleep before waking up for school, then you're still getting 9 hours. If you changed the numeric value, then it would either 8 or 10 hours since the school start time retains its numeric value.

"I believe most of these studies are performed in the USA."

The study being discussed is from Singapore.