The function of sleep is one of biology’s 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.
What is sleep?
It’s 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 or torpor, sleeping animals wake up rapidly when stimulated.
The physiological features, timing and duration of sleep vary across the animal kingdom. Some species adopt a stereotypical posture while sleeping – humans lie flat and bats hang upside down – but in other creatures, sleeping postures can be subtle. Octopuses have ‘active sleep’, when their skin shifts in colour and texture, for example, and ‘quiet sleep’, when their eyes and tentacles move more slowly. Mammals and birds have two distinct states, too, called REM and NREM sleep.
What’s the difference between REM and NREM sleep?
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 – hence the name) but slow and synchronised in NREM. Electrical activity during REM resembles what’s seen in animals that are awake, so it’s also called ‘paradoxical sleep’.
Because body and brain temperatures drop during NREM, one explanation for REM’s function is that it’s like the brain is ‘shivering’ 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’t normally act out our dreams.
What are dreams?
. 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.
Do animals dream?
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’re asleep. Meanwhile, the activity in the brains of rats sleeping after navigating a maze suggests they re-run the maze in their sleep.
Why do animals need sleep?
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 – to help with learning and memory without being confused by fresh sensory information.
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 – 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 sleeps just two hours in the wild.
The three theories for sleep’s function aren’t mutually exclusive, and all have weaknesses. Even brainless jellyfish sleep, and you’d expect intelligent elephants – who famously never forget – to sleep longer.
Can we survive without sleep?
Yes, but not indefinitely! Sleep deprivation is ultimately lethal, can cause poor cognition, abnormal development and reduced lifespan.
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 ‘unihemispheric’ 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.
Main image: A sea otter sleeping © Getty Images
