The awake half reminds the whale to breathe when it reaches the surface, as it continues to swim, generally side by side with other pod members.
Humpback whale whose most obvious feature is the large hump on back! So how can they sleep and not drown? This resting state is more like a very light nap and the whale can still swim at the same time.
On average, larger whale species can hold their breathe for 80-90 minutes, giving them plenty of time to sleep before needing to surface for air.