Sleep is just as important to birds as it is to us. However, a bird’s sleep is far different from that we experience. One way that it differs is birds can actually sleep with one of their eyes open.
A bird is capable of performing this unbelievable fete because half of its brain is fully asleep while the other half is only partially asleep. In other words, the side of the brain with the open eye is only partially asleep. This enables a sleeping bird to detect the approach of a potential predator.