Market order for options filled before market open??

Hi everyone. I’ve traded options for about a year or so at this point. At some point, I set a market order for spy atm calls the night prior, expecting it to fill at market open 9:30 ET, but then when I woke up I found out that it actually filled at 7:something which really confused me because I thought options can only be traded during market hours. Anyone know why this could’ve happened?