Every American and Canadian is asleep? Lol. It's 7:00am on the east coast and 4:00am out West. Let's reverse it, do you think UK buyers stop trading US stocks at 4:30pm when our market closes?
On the whole it is. You're right in one sense. If a stock is dual listed, like this one and gets good news they both go up. But the old wait till the US opens is a myth. US and Canadians can buy into uk stocks at anytime our markets are open. They don't just Spring into life buying at 2:30pm.