Most of us start to tire after about half a day without any sleep. Staying awake for five in a row would be extremely difficult and even if you could manage it, you’d be a physical and mental wreck by the end. But not all animals suffer from the same problem. A dolphin can stay awake and alert for at least 5 days straight, chaining together all-nighters without any noticeable health problems or loss of mental agility.

The two halves of a dolphin’s brain can sleep in shifts, “shutting down” one at a time so that the animal is always half-awake. They can truly sleep with one eye open, an essential skill for an animal that has to be constantly