If you ever see three ducks sleeping on a log, watch them for a bit. The duck in the middle will comfortably sleep, pretty much staying in one position. The two ducks on the outside will sleep with the eye closest to the middle closed and the eye on the outside open then, after a little while, the ducks on the outside will stand up and turn 180 degrees, close the eye that was open and open the eye that was closed. Only half of the duck’s brain is sleeping at one time. Other birds, reptiles, insects, sea mammals… do this as well, (it’s called logging in dolphins because they look like logs floating in the water). The animals sleep half a brain at a time (uni-hemispheric sleep) so the other half can watch for enemies (Dolphins also need to make sure they continue breathing since they’re conscious breathers, they have to come to the surface to breath). Uni-hemispheric sleeping raises a couple of questions.

1) How do you get to be the duck in the middle? Seems like it’s a lot more restful.

2) How do ducks, dolphins and other uni-hemispheric sleepers solidify long term memories whose neuronal maps span both hemispheres?

The current working assumptions, as I understand them, is that sleep is universal (every animal from insect to mammal sleeps or does something that looks like sleep) and sleep seems to serve multiple purposes; cleansing of neurotransmitters to allow more learning the next day, REM sleep allowing the brain to consolidate what is learned during the day and mechanisms during REM and Non-REM sleep for forgetting the non-useful data. If something is learned it will be represented by a “neural map”, effectively a grouping of neurons that may span different areas of the brain, including both hemispheres. During sleep this grouping (since it has been deemed valuable) will be strengthened. For example, if a rat is run through a maze during the day and is rewarded with food, then, during sleep, it will dream it’s running through the maze in order to consolidate its learning on how to get the reward. What confuses me, is how is this learning effected by uni-hemispheric sleeping? In other words, if a given neuronal grouping crosses hemispheres is learning consolidation improved, lessened or is there no difference?

This post was inspired by a podcast by one of my favorite radio shows Radiolab. Another of my favorite shows, This American Life did a less scientific, though quite entertaining look at sleep.