Deliberately losing consciousness for prolonged periods of time is not the best survival strategy, yet many animals do just this, spending hours on end asleep. Despite the prevalence of sleep, scientists are still unsure why this seemingly counterproductive behaviour evolved. Sleep has a number of functions, including strengthening or weakening synapses (the connections between neurons), clearing away waste produced by neurons, and strengthening memories. However, with so many functions, it is unclear what the original reason was that drove the evolution of sleep.
Hope for a resolution of this conundrum comes from recent research led by Caltech’s Paul Sternberg, which indicates that even jellyfish, one of the most ancient animal groups, sleep.
Sleep is characterised by three behavioural characteristics: a period of decreased activity, reduced responsiveness to stimuli, and regulation to ensure the animal gets enough sleep. Researchers were able to demonstrate that the jellyfish Cassiopea satisfies all three criteria for sleep.
Firstly, Cassiopea show a quiescent state, decreasing their activity during the night. Importantly, to distinguish sleep from other sleep-like phenomena such as paralysis and coma, quiescence must be rapidly reversible. Indeed, addition of food rapidly woke the sleeping jellyfish.
Secondly, Cassiopeia is less responsive when asleep. To show this, researchers made use of an unusual feature of this jellyfish – they are upside down. Being upside-down, when they swim, they move towards the sea floor, where they rest their tentacles pointing upwards. When raised above a surface they will swim downwards until they adopt this position. Researchers showed that when jellyfish were lifted from the bottom of their tank when asleep they were slower to begin moving back to the bottom than when awake.
Thirdly, Cassiopea regulates how much sleep it gets. Much like if you go out late you might feel tired the next day, Cassiopea also becomes more sleepy if it misses out on sleep the previous night. Keeping the jellyfish awake for six or twelve hours by squirting them with water caused them to be less active the following day. The same treatment applied during the day had no effect on the jellyfish, indicating that they were suffering from sleep deprivation rather than physical fatigue.
These findings are important because jellyfish possess a very simple nervous system. While many animals have a centralised nervous system (where neurons are concentrated together into an area that forms the brain and nerve cord), jellyfish have no brain, instead having a diffuse network of nerve cells throughout the body called a nerve net. The fact that sleep exists in an animal with such a simple nervous system suggests that sleep evolved before the evolution of a centralised nervous system. This suggests that the original role of sleep is nothing to do with the brain, but is instead a more fundamental requirement of a nervous system.
One possible reason is that sleep is required to clear away waste products produced by neurons during the day. A study in mouse brains show that during sleep, the space between brain cells may increase, allowing toxins to be washed away by a flow of cerebrospinal fluid (a clear fluid found in the brain and spinal cord). However, it is unclear whether, without a brain, the same thing might occur in a much simpler nervous system.
Another theory is that sleep may provide a time for damaging molecules produced by metabolism called free radicals, to be removed. In other words, sleep would act as an antioxidant. At the end of the day, we still do not know why sleep first evolved, but with the discovery of sleep in such a simple nervous system as the jellyfish’s, we may be one step closer.