New studies reveal that even living beings with no nervous system need to sleep, or at least 'rest'. Other parts of the body are affected, starting with the immune and digestive systems.
Even sponges, in their own small way, rest. This is certainly true of Tethya californiana, as discovered in 2017 by researchers at the University of California, San Diego. The journal Science now takes stock of this and other discoveries pointing in the same direction: that of a radical change in our understanding of sleep.
Until a few years ago, it was thought sleep was something only done by living beings with a nervous system, and that its main and perhaps sole purpose was to consolidate memory and ‘free’ the brain of unnecessary information and stimuli acquired during the day. It was believed it had no effect on the rest of the body. This idea was also reflected in the kind of studies carried out, which were mainly on based recording electrical activity in the brain through polysomnography and did not concern themselves with anything else.
However, in recent years, several discoveries have begun to challenge this view, as it has been shown that very different, but equally primitive, creatures do rest. For example, jellyfish, hydra, placozoans (invertebrates with only two layers of cells), and further up the evolutionary scale, simple worms like C. elegans, midges, birds and fish.
Many animals are able to sleep with one part of their body, while still remaining active and alert: birds do it while flying, fish and marine mammals do it while swimming and cattle do it while standing, as do bats (which sleep an average of 20 hours a day), octopuses, which have two nervous systems and elephants, which get by on just two hours of sleep a day.
The role of light
Studies in recent years have highlighted that primitive creatures with no brain at all slow down their normal activity each day, almost always depending on the amount of light available (which can be likened to the way in which more complex organisms sleep).
This is the case for many marine organisms like deep-sea jellyfish, which are dependent on algae and other photosynthetic organisms for survival. Jellyfish, which have no nervous system, take advantage of darkness, when prey is more difficult to detect, to slow down their activity notably. This has also been seen in the laboratory, by alternating light and dark or by subjecting jellyfish to various types of stimuli that ‘tire them out’, after which they always slow down to rest.
All this has led to the realisation that sleep is not only good for the brain and it is certainly not only the preserve of vertebrates. The whole organism, whatever it is, needs sleep and benefits from it to maintain a good working state. It is especially important for the immune, digestive and musculoskeletal systems, as well as for the metabolism as a whole. But that's not all. There are genes and proteins that have been preserved throughout the phylogenetic tree that are associated with regulating sleep (including the melatonin cycle, which also acts on jellyfish), proving just how important it is.
Sleep disorders are on the rise
Today, sleep researchers no longer have a purely electrophysiological approach that focuses only on the brain. They also analyse other cells, study genes and proteins and try to understand the complex relationships between various parts of the body. The latest issue of Science has a whole collection of articles on the subject, with various studies and reflections and the journal points out that sleep disorders are becoming increasingly common and have serious repercussions on human health ‘as a whole’.
Discoveries in recent years have given us an entirely new vision, which will probably help us identify more effective therapies and treatments