It almost sounds like a luxury that we humans spend one-third of our life sleeping. Sleep allows our body to repair and restore itself. Chronic deprivation of sleep inevitably results in death. For other members of the animal kingdom, however, sleep comes at a high price. As they doze off, they lose control of their muscles and become less responsive to their surroundings, where predators may be watching. Therefore, a variety of shuteye strategies have been developed by different animals, to keep them safe while taking a rest.
Giraffes, because of their height and size, cannot hide from their grassland predators’ eyes. To keep alert at all times to potential attacks, giraffes take short naps of a few minutes, adding up to around 40 minutes of sleep in a day—the shortest of all large mammals. Giraffes mostly sleep standing up and seldom lying down.
Frigatebirds are large, ferocious seabirds that often mix in bird flocks to attack and rob smaller seabirds for food. The “pirate bird” can fly incessantly—and fast too—over the sea for a dozen or more weeks. Scientists have studied the electroencephalography of frigatebirds, and found a sudden decrease in their brainwaves for a few seconds during their long flights—the first official recording of a bird sleeping in flight. The “mini-sleep” is a rather extreme way of taking a rest!
Even more extreme may be the case of American bullfrogs (Rana catesbeiana), which never exhibit sleeping behaviour according to scientific studies. The bullfrog is exceptional as, apart from entering a hibernation stage in winter, it shows similar physiological responses to external stimuli whether it is active or otherwise. This means that the bullfrog is on alert at all times—although some scientists postulated that the species undergoes Non-Rapid-Eye-Movement sleep that allows it to keep the sensory perception during shallow sleep. Before the theory is confirmed, the American bullfrog is still the representative of “sleepless” animals.
Humans carry out spontaneous breathing during sleep, while ocean mammals such as whales and dolphins cannot. They must undergo conscious breathing or they will suffocate and die. A special sleeping mechanism has evolved in whales and dolphins—unihemispheric slow-wave sleep. This is to ensure that half of the brain is working to maintain consciousness while the other half is taking a break. What an amazing expertise that “caffeine-addicted” people would love to acquire to fight off doziness!