Why are some body functions turned off during sleep but not others?
Elizabeth Lembke, Gerlingen, Germany
Mary Carskadon, director of the Sleep and Chronobiology Research Laboratory at Bradley Hospital in Providence, Rhode Island, answers:
Some of the changes in body functions may help us remain asleep for sustained periods, so that we can recharge. For example, we do not see, we cannot hear very well, and our sense of smell is depressed. As a result, minor environmental changes do not disturb us from our state of slumber. Some physical responses, such as sneezing or coughing, are also lost, perhaps due to our reduced sensitivity to tickles in the nose or throat. At the same time, our senses remain responsive enough so that extreme external stimuli can rouse us to wakefulness.
Illustration by Christoph Niemann
Certain processes that we take for granted when awake may almost vanish during REM sleep, the state in which we dream. One dramatic change is our loss of the ability to regulate body temperature. In REM sleep, the brain no longer initiates the process to lose heat if it is too warm or to produce heat if it is too cold. This shutdown may have originally evolved to conserve energy during sleep. During the REM state, the brain also sends signals to the motor neurons to reduce their excitability, thereby paralyzing many muscles. Such paralysis allows us to experience our dreams, however active they may be, without moving a muscle. If this autoparalysis system fails, a person can act out dreams, sometimes with violent results.