Yes, it's due to the heat (assuming they DO have room in the hive). You can use shims to prop up either the inner cover, or if you have supers on, prop between the top brood box and first super. Even 1 beespace will help create an airflow situation that will help cool the inside of the hive.
I have the exact same situation with one of my hives, the girls are always in by morning, but I've got out at 10:30 - 11pm and looked with a flashlight - they're still hanging out (yes, I'm a little obsessed with my bees).
In my case though, that hive is forced into a very crowded situation due to working Ross Round supers, the other hives have a little 'bearding' in the evening, but nowhere near as bad as the one hive.