Try taking a photo during the day like this, you would have no hope because they would fly away.
SO WHY IS IT THEY DON'T MOVE MUCH AT NIGHT?
As like most insects, flies shut down as the temperature drops. When it cools down they become inactive and hybernate.
They choose somewhere safe to hide or lodge for the night. If it's cold during the day they will continue to hibernate until it warms up to start eating again.
what about winter?
Some flies die because they cant handle the cold change and others bunker down.
In winter some flies shut down and hide until spring hits and the weather warms up. Then the fiestas start, food glorious food.
The more they eat, the more eggs they will lay.
Except what we consider as food, and they think is food, are genrally two different things.