KnowHOW team explains: Unihemispherical sleep ? when one side of the brain shows waking activity while the other side is asleep ? is a characteristic of many aquatic mammals as well as several birds and reptiles. The half that stays awake is at a low level of alertness to keep an eye on the predators and to watch for obstacles while the dolphin rises to the surface for air during sleep.

After having rested one half of the brain along with an eye, the dolphin reverses the process, putting to sleep the other half and the eye.

Birds employ this half-brain strategy more effectively while asleep. A recent experiment by a group of researchers at the Indiana State University on a group of mallard duck proved how alert the open eye remains while the birds are asleep.

The experimenters allowed the birds to sleep and then showed a video of an approaching predator to them. The birds were found to detect the danger within a fraction of a second.

Electroencephalograph (EEG) on their brain showed the hemisphere opposite the open eye was alert, while the other hemisphere was getting high-quality sleep.

The question was sent by Samiran Bose from Salt Lake