They probably slept in yesterday and went to bed later. Your body is still on a different schedule even after clocks have changed. And today is probably their first day to wake up on time and to an alarm clock.
Because if you are used to going to bed at 9 pm, and that is the time you started to feel tired, you wouldnt actually feel it till 10 pm. And when you are used to waking up at 6 am, well what used to be 6 am is now 7 am, so you would have to wake up at what feels like 5 am, though it is only 6 am. I know its confusing.