Why do we sleep? What happens when we don't?

Scientists have many theories about why we sleep. New research points to the idea that we sleep in order to clear our neural networks of the unimportant events of the day. To forget, in other words. This makes room for new learning the next day. 

By studying sleep/wake cycles in mice, Dr. Graham Diering and his colleagues could see that synapses — the connections that send signals between neurons — strengthen during waking hours and weaken during sleep. It's a crucial step in forming memories while during sleep. A lack of sleep disrupts the brain's ability to form new memories. 

His paper, published in the journal Science can be found here. 

Bob McDonald interviewed Dr. Robert Stickgold, director of the Center for Sleep and Cognition at the Beth Israel Deaconess Medical Center, about some of the other consequences of sleep deprivation. Part of that interview is transcribed here and has been edited for clarity.  

Bob McDonald: What has your lab found about the lack of sleep and the kinds of memories we retain?

Robert Stickgold: There's a wonderful study that I did with Matt Walker who's now at Berkeley. Matt had subjects come in either after having a good night of sleep or being sleep deprived the night before and showed them a number of stimuli that were positive, negative or neutral pictures or words.

And then they did a test to see how many of those they remembered. They came back a couple of days later, and it turns out that if you're sleep deprived you forget about half of the positive information you originally learned. Half of the neutral information you learned, but you don't forget the negative information and the result is, that after these couple of days, what you remember having happened two days back is only the negative stuff. And not as much of the good stuff.

BM: Why do you think that happens?

RS: Our brain has a piece called the amygdala which is really good at responding strongly to emotional stimuli. And then it has another region in the frontal part of the brain that actually pays attention to whether you should be afraid. So, if you go to see the movie Friday the 13th Part Eight, and you're seeing people chopped up with axes, your amygdala will be screaming "run run." But this frontal piece of your brain, the orbital frontal cortex is telling your amygdala to "chill out,  you're at a movie theater. It's fine."

And that inhibition of the amygdala by the frontal cortex is impaired when we're sleep deprived. And so we don't get to tone down the emotion to a more reasonable level.

BM: Besides the negative thinking that can come from sleep deprivation What are some of the other health consequences of not getting enough sleep?

Sleep isn't just for the brain.- Dr. Robert Stickgold. 

 RS: There are other systems that have, over evolution, become dependent on sleep as well. If you take a group of college students and restrict their sleep to four hours a night for five days, and then give them a glucose tolerance test you discover that they're looking pre-diabetic.

Their regulation of insulin function is dramatically altered and they start to look like they have diabetes. So we might be seeing more Type 2 diabetes in part because people aren't sleeping enough.

Another study shows that if you give people an vaccine for Hepatitis-A and then you sleep deprive half of them for one full night afterwards, and then four weeks later bring them back and look at how much antibody to the Hepatitis-A their bodies are making, the ones who were sleep deprived the night after receiving the vaccine only have half as much antibody in their body as the ones who slept well. So you're going to get sick more likely, you're going to get fat more likely if you haven't slept enough.

If I can paraphrase my mother, if you don't get enough sleep you could end up fat, sick, depressed, and stupid.- Dr. Robert Stickgold

BM: We hear so much about the key to a healthy life is eat right and exercise. How would you rate sleep up there with those other two?

RS: Well the U.S. military now refers to it as a triad, that is sleep, exercise, and nutrition. And what's sad is that nobody likes dieting. Most people don't particularly like exercising, but everybody likes sleeping. It's sort of a shame that it's as hard to get them to do that as it is to get them to do the others.