Who knew that the song Enter Sandman was actually about an interesting biological phenomenon? Turns out many aquatic and terrestrial mammals and birds actually sleep with one eye open! The corresponding hemisphere of the brain maintains wakefulness, while the other sleeps.
For instance, Mallards (pictured above) exhibit unihemispheric sleep as a way to keep an eye out for predators. Some aquatic mammals, such as cetaceans and manatees, keep one half of their brain awake to control surfacing for air while the other half sleeps.
The phenomenon of unihemispheric sleep has called into question the definition of sleep, its function, and whether it is even essential. Cool!
I stumbled upon the rabbit hole of unihemispheric sleep after watching this eerie video of sperm whales sleeping:
It appears that sperm whales undergo complete (bihemispheric?) sleep for 12 minute snaps, sleeping for just about 7% of their day, giving that whale the title of sleeping for the smallest percentage of their day out of any mammal (giraffes come in 2nd place with 8%).
This is why I love biology. Lets assume sleep is a biological necessity. Millions of years of evolution and adaptation has pulled this necessity in as many directions. From sleeping with half your brain at a time, or with the whole brain 7% to 80% of the day to everything in between. Biology is a healthy mix of ubiquitous phenomena and specialized solutions. Sometimes the hardest part is not clicking that one more wikipedia article all day.
Vincent Cannataro, Ph.D. 
