Yes, it's a learned behaviour Phoebe as it doesn't happen with you. We had exactly the same with GS3. He went down like a lamb and sleeps through at our house, but was just as you describe when at his.
DD and SiL used to joke we must drug him!
He's much better these days for his parents especially when he started school at 4. I'd just let them get on with it aand not worry.
How much do you spend on yourself?
Bought the wrong Hot Cross Buns 😩
Water Pollution -“ A National Disgrace”? A case for renationalisation?