Sleep might seem pretty simple, but scientists are still scratching their heads over questions surrounding this slumbering state. It's unclear exactly why organisms need to sleep, let alone whether every last species actually settles down for siestas.
Sleep itself isn't all that straightforward, so let's start by describing what's typically defined as sleep. During times of sleep, there's usually a reduction in physical activity and a decreased response to outside stimuli. Sleeping creatures often assume a customary posture -- whether that's lying down for people, hanging upside down for bats, or standing up as is sometimes the case for horses, giraffes and elephants. Sleep is also easily reversible, meaning it's a relatively simple matter to wake up, especially when compared to other states along the continuum of reduced consciousness, such as hibernation or a coma.
It seems most species do sleep, although differences exist between the sleeping patterns of different types of animals. Giraffes require very little sleep; they enjoy only about 30 minutes a day of deep sleep split into several separate sessions. Conversely, brown bats average close to 20 hours a day. Most mammal species need significantly more sleep when they're young, although baby orcas and baby bottlenose dolphins appear to not sleep at all during the first few months of life.
There are also examples of animals that can disrupt their normal sleeping patterns for certain special events, such as migratory birds which can survive with significantly less sleep during migratory seasons without building up any sleep debt. Some have been shown to take extremely brief power naps of just a few seconds, sometimes using unihemispheric sleep to remain semialert to their surroundings. During unihemispheric sleep, which is also practiced by some marine mammals like whales and dolphins, half the brain powers down into various sleepy-time modes, while the other half remains ready for action.
When it comes to invertebrates and other simpler forms of life, they're rarely studied with the electroencephalograms that look for telltale EEG patterns, or the brain's electrical activity, so the definition of sleep tends to focus more on behavioral cues such as physical inactivity, typical postures, regular cycles and a return to activity when roused. Fish and amphibians are among the creature types for whom it's still a little unclear whether actual sleep occurs, or if they're simply exhibiting signs that suggest a resting state. Insects, on the other hand, do set aside time to slumber.
For more articles on alluring topics like lucid dreaming and the evolution of life, visit the links on the next page.
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More Great Links
- "Asian Elephants Frequently Asked Questions." Smithsonian National Zoological Park. (3/3/2010) http://nationalzoo.si.edu/Animals/AsianElephants/elephantfaq.cfm
- "Baby dolphins never sleep." Reuters. June 30, 2005. (3/3/2010) http://www.abc.net.au/science/news/enviro/EnviroRepublish_1403878.htm
- Bäcker, Alex. "Evolution of Sleep." California Institute of Technology. (3/3/2010) http://www.its.caltech.edu/~alex/Evolution%20of%20Sleep%20Lecture.htm
- Biello, David. "Fruit Flies Reveal Sleep Secrets." Scientific American. Sept. 21, 2006. (3/3/2010) http://www.scientificamerican.com/article.cfm?id=fruit-flies-reveal-sleep
- Choi, Charles. "New Theory Questions Why We Sleep." Live Science. Aug. 25, 2009. (3/3/2010) http://www.scientificamerican.com/article.cfm?id=what-do-we-know-about-the-1997-09-15
- "DNA of sleep?" Science Daily. June 24, 2009. (3/3/2010) http://www.sciencedaily.com/releases/2009/06/090624093647.htm
- "Dolphins Keep an Eye Out While Sleeping." Live Science. May 1, 2009. (3/3/2010) http://www.livescience.com/animals/090501-dolphins-vigilance.html
- "Ever wondered why horses sleep standing up?" Washington Post. July 14, 2008. (3/3/2010) http://www.washingtonpost.com/wp-dyn/content/article/2008/07/13/AR2008071301635.html
- Graham, Sarah. "Songbirds Can 'Stash' Sleep during Migrations." Scientific America. July 13, 2004. (3/3/2010) http://www.scientificamerican.com/article.cfm?id=songbirds-can-stash-sleep
- Mahan, Rachel. "Bird Naps: How Migratory Flyers Catch Up on Sleep." Scientific American. April 2009. (3/3/2010) http://www.scientificamerican.com/article.cfm?id=napping-is-for-the-birds
- Marshall, Jessica. "To Sleep, Perchance to Dream." Smithsonian National Zoological Park. December 2007. (3/3/2010) http://nationalzoo.si.edu/Publications/ZooGoer/2007/6/To_Sleep_Perchance_to_Dream.cfm
- Mascetti, Gian et al. " Monocular-unihemispheric sleep and visual discrimination learning in the domestic chick." Department of General Psychology, University of Padova. 2007. (3/3/2010) http://cat.inist.fr/?aModele=afficheN&cpsidt=18409138
- Meadows, Robin. "A Neck Up on the Competition." Smithsonian National Zoological Park. August 1996. (3/3/2010) http://nationalzoo.si.edu/Publications/ZooGoer/1996/4/neckuponcompetition.cfm
- "The Characteristics of Sleep." Division of Sleep Medicine at Harvard Medical School. Dec. 18, 2007. (3/3/2010) http://healthysleep.med.harvard.edu/healthy/science/what/characteristics
- Tobler, Irene. "What do we know about the evolution of sleep--when it arose and why? Bacteria surely don't sleep, do they?" Scientific American. Sept. 15, 1997. (3/3/2010)http://www.scientificamerican.com/article.cfm?id=what-do-we-know-about-the-1997-09-15
- Van Oostendorp, Kyle. "Experimental analysis of sleep behavior in rana sphenocephala; Southern Leopard Frog." Washington College. Oct. 20, 2009. http://dspace.nitle.org/handle/10090/10732
- Viegas, Jennifer. "Top 20 Animal Sleepers and Why We Sleep." Discovery News. Aug. 21, 2009. (3/3/2010) http://news.discovery.com/animals/top-20-animal-sleepers-and-why-we-sleep.html
- White, Margaret. "The Evolution of Sleep." California State University Fullerton. (3/3/2010) http://psych.fullerton.edu/mwhite/473pdf/473%20Evolution%20of%20Sleep.pdf
- "Why do we Sleep? Scientists are Still Trying to Find Out." NatGeo News Watch. Aug. 26, 2009. (3/3/2010) http://blogs.nationalgeographic.com/blogs/news/chiefeditor/2009/08/why-we-sleep-is-a-mystery.html