Giulio Tononi is a psychiatrist and neuroscientist who focuses on sleep and consciousness research at the University of Wisconsin. He has measured brain waves in sleeping fruit flies, identified genes that are active in humans during sleep, and demonstrated that sleep enhances learning and memory. He believes sleep allows us to recalibrate our brains after each day’s experience.
What’s so fascinating about sleep?
T: I want to understand the neural basis of consciousness, which is my first love, and sleep is the most obvious and dramatic modification of consciousness. I also want to know why sleep is a universal phenomenon. Every creature, with a few very strange exceptions, sleeps. That means sleep fulfills a very basic function, yet it is one of the few areas in biology that is still an unsolved mystery. If you wake somebody up from deep, stage-four sleep, they will usually have nothing to report about the experience. And yet the brain is not any less active than in waking. Where does consciousness disappear to during sleep? And what creates consciousness? You can remove someone’s cerebellum—an ancient part of the brain that has about 50 billion neurons—and they may have motor problems and find it hard to walk around, but they will still be fundamentally the same person. However, if you lose your cerebral cortex, which has about 30 billion neurons, you don’t exist. You have no consciousness. These are very basic mysteries that we need to understand, and sleep is a good way to study them.
Your research shows that even tiny fruit flies sleep. What does that mean for humans?
T: If we can demonstrate that fruit flies sleep, that’s very strong evidence that sleep is fundamental. And we can use the powerful genetic tools already in place to study fruit fly genes because so many fruit fly genes are conserved in humans and because fruit flies reproduce so rapidly. We already know a lot about the genes that control circadian rhythms in fruit flies. Since sleep is first and foremost a behavior, the first thing we did was prove that fruit flies have long periods when they’re quiet and immobile and have a decreased arousal threshold—two hallmarks of sleep. It turns out they rest about 10 hours a night, and when you try to rouse them with air puffs, light, vibrations, or heat, it’s more difficult. That looks very much like sleep. Fruit flies respond to caffeine by staying awake longer and to antihistamines by going to sleep earlier. And if you insert electrodes into the middle of the fly brain, you discover that the electrical activity in the brain changes when flies sleep.
You can actually put electrodes into the brains of fruit flies?
T: It’s very difficult, but you can insert electrodes into a part of the central brain called the mushroom body, and you find EEG-like activity, which changes a bit when the fly goes from waking to sleeping. Another interesting discovery is that young flies that are just a few days old sleep a lot more than older flies. Older flies have more fragmented sleep. All these behaviors are just like human sleep. We even did gene-expression studies in flies showing that genes in their brains change their level of activity in waking and in sleep. Another key similarity between flies and humans is what is called sleep pressure. The longer you stay awake, the longer and more intensely you will sleep. Flies do that too. Now we’re looking at mutant flies that sleep only two to three hours a night. If we can study the biochemical pathways their brains are using and find out how these flies manage with so little sleep, we might eventually find a way to make human sleep more efficient and restorative.