Unihemispheric sleep has got to be one of the coolest adaptations around. Animals like dolphins have evolved to breathe air but live under water. This makes sleep a risky endeavor, unless you develop a mechanism that allows you to always be a little bit awake. By keeping one hemisphere of their brain awake at all times, they are able to swim to the water’s surface and get fresh air into their system while also maintaining their beauty sleep.
Read more about how some animals adapted to unihemispheric sleep (and why humans don’t) in my article published on That’s Life [Science].