If you catch yourself nodding off in classes, on the bus, or anywhere else, you can probably think of a reason for your condition. Perhaps you stayed up late studying for an important test, or to watch a movie with your roommate, or spent the night drinking. But do you really know why your body is telling you to sleep? What is the true purpose of that irresistible urge to close your eyes and drift away? Why do we and almost all animals need to sleep?
The truth is, scientists still don't have a clear answer to this question. They know that sleep plays an important role in memory and that cells function better in a rested person than in an exhausted one, but scientists say that so far as they can tell, animals could have evolved mechanisms to accomplish these things in a waking state, so why sleep?
I became interested in this topic after listening to a one of WNYC's Radiolab podcasts, called simply Sleep. The podcast discussed different scientific ideas about why we sleep and what happens when we do and included some really interesting information on research currently being done to try to solve this puzzle. One researcher they spoke to, Stephen Lima, is looking at sleep from an evolutionary standpoint to try to understand why animals evolved the need to sleep. It may seem obvious that animals would evolve to sleep, rest must be good right? Well, not really. Sleep makes an animal incredibly vulnerable, it could be eaten while resting, its offspring could be threatened, anything could happen. There is something about sleep that almost all animals need, and we still don't understand what that is. According to Dr. Lima, "The fact that sleep is so dangerous is the best evidence that it is necessary, because if it weren't necessary, we wouldn't be doing it.".
The evolutionary approach to sleep research is a new one, Dr. Lima states that, "One of the reasons we don't understand sleep is that we haven't taken this evolutionary perspective on it". The idea is that if we can understand why sleep evolved, we can understand why it is necessary and what it really does. He and his team of researchers have been watching animals like iguanas, ducks and, yes, fruit flies sleep. An article published in the New York Times also focuses on Dr. Lima's sleep research, as well as other researchers'. One discovery they have made that is particularly interesting involves the sleeping habits of ducks. When ducks sleep in a row, on a log or wherever, the ducks on the ends of the line will sleep with one eye, the one facing away from the other ducks, open. Every so often they will stand up, turn around and sleep with the other eye open. Dr. Lima discovered that the reason for this is that the ducks on the ends are only allowing half of their brain, the half controlling the closed eye, to sleep at a time. The team is now doing research on iguanas to see if they share this strange behavior. If they do, it will tell scientists that this half-brained sleep probably evolved early in animal evolution, and that early mammals may have been able to do it also, but then lost this ability later in their evolution.
Another model for sleep research is the fruit fly. A study published in the journal Science in 2006 focused on the sleeping habits of Drosophila (click here for abstract). It found that the flies needed more sleep after social interaction and couldn't remember tasks taught to them if they were deprived of sleep for a certain period of time after they learned them.
This article from the Public Library of Science details a study done in 2006 on zebrafish sleep that found "both striking similarities to mammalian sleep and its regulation and intriguing differences.
This new evolutionary approach to understanding human behavior shows that there's more to phylogeny and systematics than endless debate over the seemingly trivial classification of obscure creatures most people have never heard of. Hopefully through more research scientists like Dr. Lima will be able to solve more intriguing mysteries about our past, and help us to understand what has made us what we are.
Sources:
(podcast)
Jones R (2007) Let Sleeping Zebrafish Lie: A New Model for Sleep Studies. PLoS Biol 5(10): e281doi:10.1371/journal.pbio.0050281
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