We think we know what sleep is. We all need at least a few hours of shuteye to go about our activities the next day.
Though we may try to stay awake through force of will and excessive caffeination, our bodies demand that we shut down for a period before rebooting and getting back to it. Everyone knows the unpleasant effects of sleep deprivation – and the bliss of restful slumber.
Until recently, sleep was believed to be restricted to complex animals that have central nervous systems, like us. Resting was thought to primarily benefit the brain – allowing for consolidation of memories and removal of waste products.
However, studies of extremely primitive animals lacking central nervous systems have reinforced the idea that sleep doesn’t just give the grey matter a break, it likely helps with metabolic and other cellular functions as well.
Take the nematode Caenorhabditis elegans, for example. Growth and other energy-intensive processes appear to increase during rest periods in these tiny worms, which only sleep for perhaps 12 hours across their entire lifespan, in naps that are spaced out across their larval development.
Even the simplest animals, tiny placozoans, enter periods of rest, and they don’t have nerve cells. Sponges, corals and jellyfish also appear to snooze, reducing their activity at night. This may not be sleep in the same way vertebrates experience it, but it suggests that some form of rest has been a constant in animal evolution.
Sleep is thus something of a vague concept, a simplistic characterisation of a phenomenon we do not entirely understand.
Vertebrate sleep takes several strange forms. Dolphins, seals and some birds experience unihemispheric sleep. One side of the brain powers down while the other stays active, allowing the animal to keep moving and remain aware of its surroundings.
- How do dolphins sleep?
- How do birds sleep? Can they sleep when flying? And why don't they fall out of trees when asleep?
These alternating periods of sleep may last for minutes or hours depending on the species – and depending on how sleep is defined, suggest that they are never fully asleep at all.
The animals that require the least amount of sleep as we understand it – dead to the world, off in the land of dreams – are horses, donkeys and elephants.
Horses and donkeys may function with as little as three hours of sleep a day, though five to seven is more common. And African elephants can get by with a mere two hours.
Top image: the nematode Caenorhabditis elegans. Credit: The original uploader was Kbradnam at English Wikipedia.(Original text: Zeynep F. Altun, Editor of www.wormatlas.org), CC BY-SA 2.5 https://creativecommons.org/licenses/by-sa/2.5, via Wikimedia Commons
