Some ducks practice what is called unisphereic sleep.
When they form a line to sleep, the ducks at the end will often have one eye looking to the outside. The half of the brain that controls that eye will stay awake while the other half goes to sleep.
It’s a tactical formation: a line of ducks sleeping, while a duck on the outer end is keeping a lookout for predators while the remaining ducks sleep normally.
It’s remarkable because - ducks aren’t particularly smart creatures - this behavior has been ingrained and then reinforced through reproduction. (Half-Brained Ducks in a Row. Science Magazine. Mackenzie, Dana)
Sleeping with one eye open - is literally a thing - created by evolution.
But I’ll raise the stakes. Here’s something even better.
Some birds, that engage in super-long flights that last for weeks, even months engage in unisphereic sleep.
Scientists strapped a EEG detector to a frigate bird − those birds with strange, inflatable necks.
These birds are capable of flying for weeks on end. The EEG detector monitored their in-flight sleep patterns:
Noting that it would put one side of the brain to sleep as it navigated (just as dolphins do, to go up to breathe while sleeping), avoiding collisions with objects and other birds.
Some birds even have the ability to go completely to sleep, both brains asleep, while still retaining their GPS capabilities. (Evidence that birds sleep in mid-flight.Nature Communications volume 7. Neils Rattenborg.)