A question about seagulls (probably FAO merkz tbh but feel free to pitch in)

See at night when they are bobbing on a river for a few moments then they as they each reach a certain point downstream they wheel up into the sky, fly further back and float again for a bit?

Why don't they just climb out and sit on the bridge ?! Seems a lot of effort. It didn't look like they were fishing.

a lot of birds can sleep in very short bursts since they need to be aware of predators and although I don’t know specifically if that includes gulls I wouldn’t be surprised 

some birds can do that weird half the brain asleep at a time thing as well