Sleep biology: not a simple lullaby

We all know the comfort feeling of taking a nap, or the relief of a good night of sleep after a long day of work. Mahatma Gandhi said: “Each night, when I go to sleep, I die. And the next morning, when I wake up, I am reborn”. In a less positive point of view, Edgar Allan Poe described sleep as “those little slices of death”, and William Shakespeare, in Macbeth, referred to it as “the death of each day’s life”. But is sleeping really similar to death? Is it a so monotonous and inactive period of each day’s life?

In reality, sleep is a very dynamic physiological state of the brain. When electrical activity in an animal’s brain is measured during sleeping periods, it’s possible to recognize mainly 2 sleep stages: REM sleep and non-REM sleep.

The REM sleep is observed in birds and mammals, and “REM” is the short for “rapid eyes movement”. If you have the chance to observe someone sleeping, you can understand why it’s so-called: this stage is often accompanied by rapid eyes movements beneath eyelids. REM sleep is also referred to as “paradoxical sleep”, because it’s characterized by small and fast waves of brain activity, similar to those of an awake animal (Figure 1). This phase is also accompanied by reduced muscle tone and scientists believe that it is the moment when we dream. Non-REM sleep, on the contrary, is characterized by large, slow waves of electrical activity in the brain. This phase correspond to the majority of sleeping time and, in humans, it’s subdivided in three stages.

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Figure 1. Electroencephalogram (EEG) and Electromyogram (EMG) traces of an awake and asleep mouse. EEG shows brain waves and EMG reflects skeletal muscle tone. Image: Aulsebrook et al, 2016. http://dx.doi.org/10.1016/j.tree.2016.05.004

Have you ever thought about what kinds of animals sleep? Maybe you’ve already noticed your dog or your parrot sleeping some (or many) hours per day. But sleep is not an exclusive behavior of birds and mammals. Research are also being developed on sleep physiology of other vertebrates, like reptiles, and invertebrates, like fruit flies, honey bees and crabs. Invertebrates usually show reduced brain activity during sleep and don’t seem to have REM-sleep.

In humans and in some other mammals, sleep is like a “shutdown” in behavior, since individual lasts many hours immobile, with eyes closed and reduced responsiveness to the environment. In the wild, this can have costs, making the organism vulnerable and unable to perform essential tasks. However, not every species sleep like this. Some birds and cetaceans (like whales and dolphins) can sleep with one side of the brain each time! When birds fly non-stop for days, they literally sleep with only one eye closed, maintaining environmental awareness and aerodynamic control. Surprisingly, a recent research found that great frigate birds (Fregata minor) flying over the ocean for up to 10 days can sleep with either one brain hemisphere at a time or both hemispheres simultaneously!

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Figure 2. How do animals sleep? Images show that species can perform diverse behaviors during non-REM sleep. Image: Aulsebrook et al, 2016. http://dx.doi.org/10.1016/j.tree.2016.05.004

General biology questions concerning sleep are poorly understood. Questions like “why do animals sleep?” or “what are the specific functions of sleep” remain active topics of debate. What we know is that sleep deprivation can lead to numerous health problems in humans, like reduced immune function, memory problems, obesity and diabetes. Also, we don’t know much about sleep physiology or its role on animal’s fitness in the wild, since most of the studies on sleep biology have been performed on laboratory conditions. Moreover, we know that species synchronize sleep with natural light-dark cycles (remember this subject here). Then, could anthropogenic changes in the environment, like artificial light at night, disrupt sleep of numerous species in the wild, leading to implications for adaptive waking behavior, reproductive fitness, and species conservation? Future studies on how animals sleep in diverse, ecologically realistic situations will help to better understand these gaps in knowledge of sleep biology. There are many questions for we to discover on this activity that is far from being similar to death!

Bruna de Oliveira Cassettari

References:

Aulsebrook, A. E. et al. (2016) Sleep Ecophysiology: Integrating Neuroscience and Ecology. Trends in Ecology & Evolution, 31 (8), 590–599. [http://dx.doi.org/10.1016/j.tree.2016.05.004]

Rattenborg, N. C. et al. (2016) Evidence that birds sleep in mid-flight. Nature Communications, 7:12468. [10.1038/ncomms12468]

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