Do birds fly for days while over the ocean? How do they sleep?

There are two methods by which birds fly for a long time without losing their ability to flight. The first method is that birds will fly for long time non-stop and they sleep in bursts of couple of seconds and usually fly when doing so. They do this few times a minute so it will eventually add up.

source https://rb.gy/3jvhkl

Unihemispheric slow-wave sleep (USWS) is a type of sleeping technique some birds and fish possess. By this technique, birds can rest half of their brain while the other half stays alert. While normal organism sleep with both their eyes shut and both brains unconscious

source https://rb.gy/ofpp5v