I haven't been out in the early morning so maybe I am just missing the part where they open or something.
I think you guessed it. I've noticed that pumpkins/squash bloom very early -- as soon as it's light and bees start to fly and melons tend to bloom later in the morning.
Another thing... I tend to get outside when it's light -- sometimes before sunrise, and I used to see the squash blossoms not opening yet all the time during the summer. We're talking 5:30-6:30am. Lately, the sun doesn't rise until almost 7am (6:45am this morning) so I don't get outside until 6:30-7:30 or so at earliest and the blossoms are already open. Sometimes, if I don't get out until 8-9am and the squash blossoms are sometimes already closing.