This is correct. I am a PhD student and I know several undergraduates who get up at 5am (or stay up until 5am) to get started on the AoC problems ASAP.
I'm in central Europe and so puzzles unlock at 6 here. I've often thought that this time is nearly ideal for me. But at the same time, had I lived in the UK, it would have been absolute hell to try and be competitive. So an hour can make a huge difference.
I have had days where I've just caved in to the lack of sleep and decided to sleep in the next day, sliding on leaderboards in the process.
It's very unhealthy.