Sleep Links Memories, Drives Immunity, Hikes Height—and More

As we spend one-third of our lives asleep, it better be for a good reason, actor and science host Alan Alda said at the World Science Festival (WSF) in New York City last week.

It is. According to sleep specialists talking on Alda’s WSF panel “What is Sleep?,” the last decade of research has revealed that the sleeping brain links current and past memories, re-rehearses and finesses activities tried during the day, and even secretes chemicals that make teens taller—among other things.

“What functions does sleep serve?” asked panelist and Harvard University Medical School neuropsychiatrist Robert Stickgold, Ph.D. “In the last 10 to 15 years we’ve become much clearer about what those are. Everybody has a sense that sleep helps us recover from events of the day, but that turns out to be a small part of what sleep is doing.”

Read More: Dreams and REM Sleep May Be About Memory Consolidation

Height

Perhaps the most dramatic function of sleep involves height, Stickgold said. In the young, “what makes you grow taller is growth hormone, secreted in your brain when you are in deep slow-wave sleep. Eighty to 90 percent of all the growth hormone you release, is released during this sleep stage. So if you are not getting deep enough sleep, you might not end up being as tall as you might otherwise.”

Immune function

Sleep is also involved in immune function and disease protection. “If we sleep-deprive you after you get a flu vaccine, you only get half the protection from that vaccine that you would have if you had had a good night’s sleep.”

Instant 20 percent memory boost

Stickgold’s life’s work has focused on the role of sleep in memory. “You probably all have this awareness that, if you are really tired, it is hard to learn,” he said. But most don’t realize that “the night after you learn is at least as important as the night before. All different types of memory we’ve studied display this effect.”

Stickgold had Alda type the same five letters over and over. “Many of you may have had the experience of trying to learn a certain passage in a violin or piano piece, and you give up due to frustration,” he said. “You say, `I will never learn that.’ Then the next morning you wake up, and the first time you try it, you get it perfect. That works because you got better at it while you were sleeping.”

When asking a “huge number of subjects” to type five numbers over and over, as Alda was doing, the pattern is always the same. Over 12 minutes of practice, people get 14 sequences right in first 30 seconds. By the end of 12 minutes, they get about 22 sequences right. “Everybody follows this same course,” Stickgold said. At the end, it plateaus. Sending subjects off to dinner and back changes nothing: they will still get 22. “The good news is they don’t forget it; the bad news is, they don’t get better.”

But that is during the day. Sending subjects home to sleep results in a 20 percent improvement in the typing sequence the next morning “for no reason other than having slept on it,” Stickgold said. “Sleep improves memories. It is true that practice makes perfect, but we have to change that now. It is practice with sleep.”

Stickgold also showed a clip of his young son, Adam, playing with a video maze. He gave him 10 minutes of “moseying around trying to get to the exit door.” When Adam was later asked to retrace his steps without a nap, it took him a minute longer to make his way out of the maze than it did right after he learned it. “But if he has had a nap, he will be one minute faster,” Stickgold said. Post-sleep, Adam’s brain “has created a better image of how that maze is laid out. We think now that, after you sleep, your brain is producing a better image of how the world works, and of how you can use what you learned that day to help you do better tomorrow.”

The bottom line, Stickgold said: Sleep makes people alert. It makes the brain “work better”—especially in the morning—and “it helps us use today what we learned yesterday.” It helps strengthen new memories, integrate them with old memories, extract patterns, and learn new things. Also, he said: “It feels great.”

REM sleep rehearsal

Massachusetts Institute of Technology neuroscientist Matthew Wilson, who studies sleep and memory, noted that he was simply studying memory—sans sleep—initially. He was in the business of inserting hair-thin wires into rat’s brains, next to memory and place neurons in the hippocampus of rats, and recording the different patterns of the neurons’ electrical activity as the rats physically engaged in different tasks. The sound was similar to the crackle and pop of Rice Krispies, with different—predictable—patterns emerging.

One day, Wilson let a rat sleep while he was at the computer, going over the data. He realized he had left the rat’s system on when he heard something unexpected. “I suddenly hear activity that sounds like the animal is up and running around,” Wilson said. “I hear place cells firing in a rhythm associated with running and I am a little bit concerned, I am thinking the rat has jumped off the platform and is taking off across the room. I look and see he is sound asleep. He is asleep but his mind is active. He is in REM sleep.” The rat was re-running—rehearsing—in his mind the courses he had run that day.

Wilson was hooked. He got involved in sleep research and discovered, with others, that rats possess all the sleep stages of humans. “In all these stages we see memory patterns reactivated. In fact, not only do we see memory patterns in the hippocampus reactivated, when we look at various parts of the brain, perception areas—involved in hearing and seeing—are active, too. So when the hippocampus is activating memory patterns of being in a particular location, the visual part of the brain is activating the images, seeing what they are remembering.” This, he said, is what occurs when we engage in the activity we call dreaming.

But sleep and dreaming are not just about memory consolidation, he said. Importantly, “sleep is unique in that it seeks out connections between different memories. Sleep gives us the opportunity to explore memory space. We learn by exploring the `memory palace,’ as people refer to it. When we wake up, we can actually know more, do more, than we could before we went to sleep. We are gaining something, not just strengthening something we already had.”

Teens and sleep

The last speaker, Brown University psychiatrist Mary Carskadon, Ph.D., noted that sleep patterns also differ according to age. While children need more sleep than adults, and teens need more sleep than children, new studies have established that teens’ circadian rhythms are also different. They skew later.

“During puberty things change, the timing of the internal clock is reset,” she said. It is as if teens “have east coast brains but are living on west coast time.”

This explains “slack jawed droolers,” or teens sleeping at their desks, she said. Some researchers are trying to get high school starting times pushed back, to fit teens’ unique circadian rhythms.

Sleep as homework

An award was given to chemist Brandon Aldinger for most clearly explaining sleep research in his writing for children. His writing reflected highlights from both teen and memory sleep work.

“The next time your mom or dad yells, `Wake up, it’s time to go to school,’ you can explain to them that you are still studying from yesterday,” read Aldinger’s award winning essay.