JV Chamary explores the mysteries of sleep, from why we need it to whether animals dream or not… <\/p>
The function of sleep is one of biology\u2019s biggest mysteries. Being unconscious leaves an animal vulnerable to predators and leads to missed opportunities for activities such as foraging or mating. And yet all animals with a nervous system seem to engage in some form of sleep, meaning the benefits clearly outweigh the costs.<\/p>\n <\/p><\/div> <\/section> It\u2019s a behavioural state in which an animal is less alert to its environment. During sleep, a stimulus (such as a loud noise) that would prompt a rapid reaction while awake, must be more intense to provoke a response. Unlike other resting states, such as hibernation<\/a> <\/strong>or torpor<\/a><\/strong>, sleeping animals wake up rapidly when stimulated.<\/p>\n The physiological features, timing and duration of sleep vary across the animal kingdom. Some species adopt a stereotypical posture while sleeping \u2013 humans lie flat and bats hang upside down \u2013 but in other creatures, sleeping postures can be subtle. Octopuses have \u2018active sleep\u2019, when their skin shifts in colour and texture, for example, and \u2018quiet sleep\u2019, when their eyes and tentacles move more slowly. Mammals and birds have two distinct states, too, called REM and NREM sleep.<\/p>\n Rapid-eye movement (REM) and non-REM (NREM) define sleep based on brainwaves, or oscillations across the central nervous system, captured in electroencephalogram (EEG) readings. The waves are quick and chaotic during REM (along with eye twitches \u2013 hence the name) but slow and synchronised in NREM. Electrical activity during REM resembles what\u2019s seen in animals that are awake, so it\u2019s also called \u2018paradoxical sleep\u2019.<\/p>\n Because body and brain temperatures drop during NREM, one explanation for REM\u2019s function is that it\u2019s like the brain is \u2018shivering\u2019 to keep warm. Indeed, large mammals, whose mass can better retain heat, spend less time in REM than small ones. Sleep also paralyses most muscles so we don\u2019t normally act out our dreams.<\/p>\n . Dreaming occurs during REM sleep and is captured by EEGs. A dream is a series of images, thoughts and emotions that the brain turns into a story.<\/p>\n Yes there is evidence animals dream. We may never know whether other animals experience dreams as stories, but they do include past events. In zebra finches, brain activity during the day matches readings at night, meaning they replay and learn songs while they\u2019re asleep. Meanwhile, the activity in the brains of rats sleeping after navigating a maze suggests they re-run the maze in their sleep.<\/p>\n Ah, this is the big mystery! There are three general theories, two involving the brain. The first theory is that shutdown enables restoration and detoxification, producing molecules and removing waste products. The second is that sleep improves cognitive performance by providing a period in which the brain can reinforce or prune synaptic connections among its neurones \u2013 to help with learning and memory without being confused by fresh sensory information.<\/p>\n The third theory, not involving the brain, is that sleep saves energy. Animals must balance a need to find food with spending energy to forage. This is shaped by their ecological niche \u2013 small mammals may have only a short window to hunt insects, whereas large herbivores gain few calories from vegetation. That may explain why the little brown bat snoozes for 20 hours a day, while an African bush elephant<\/a><\/strong> sleeps just two hours in the wild.<\/p>\n The three theories for sleep\u2019s function aren\u2019t mutually exclusive, and all have weaknesses. Even brainless jellyfish<\/a><\/strong> sleep, and you\u2019d expect intelligent elephants \u2013 who famously never forget \u2013 to sleep longer.<\/p>\n Yes, but not indefinitely! Sleep deprivation is ultimately lethal, can cause poor cognition, abnormal development and reduced lifespan.<\/span><\/p>\n But though sleep typically involves physical inactivity, certain species have evolved a way to stay safe, by shutting off one of their two hemispheres. Such \u2018unihemispheric\u2019 or half-brain sleep is found in whales, dolphins, fur seals and great frigatebirds, among others, which all keep one eye open to watch for predators and potential dangers while swimming or flying.<\/span><\/p>\n\n
What is sleep?<\/b><\/span><\/h2>\n
What\u2019s the difference between REM and NREM sleep?<\/b><\/span><\/h3>\n
What are dream?<\/b><\/span><\/h2>\n
Do animals dream?<\/b><\/span><\/h2>\n
Why do animals sleep?<\/b><\/span><\/h2>\n
Can we survive without sleep?<\/b><\/span><\/h2>\n