This is generally true for NQ, ES.
If you trade the metals. Their 8:30 open also provides great opportunity. The overlap into the 9:30 sometimes, that's where the main move is for them.
There are always exception to the rule.
Generally, if you don't get a move in the morning session, the big boys may sometimes run the move over LUNCH. Which means you'll get a wicked move between 11:00 and 12:00.
Doesn't happen all the time, but just general rule to keep an eye out for.