Sleep in Animals

Not only can ducks sleep with one eye shut, the same happens with their brains. Much of what a bird sees in one eye goes to the other side of its brain, which allows it to sleep with the side of the brain connected to the closed eye, while the other side of the brain stays awake.

Dolphins and some other whales have developed this "half-asleep" technique to even more sophisticated degree, so that they can continue swimming, surfacing and breathing. After an hour or so like this the roles reverse, the sleeping side waking up and the other side going to sleep. Like ducks, dolphins sleep with one eye open.

Migrating birds and albatrosses at sea fly for days at a time; it's not yet known whether they sleep on the wing or simply deprive themselves of sleep.

The record for the longest sleeper goes to the South American two-toed sloth, which spends around 20 hours a day asleep, up in the highest branches of the rainforest. Antelopes are fitful sleepers, sleeping in herds, with only those at the centre of the herd able to sleep safely. Being on the perimeter means either no sleep or becoming someone's dinner.

There are no permanent non-sleepers in the animal kingdom, which points towards sleep having essential life-preserving purpose.

I do however wish I had the ability to sleep with one eye open - could be very useful!