Understand and Use Dreams to Learn and Forget

The episode opens with the host introducing himself as Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine. He sets out the agenda: dreaming, learning during dreaming, and unlearning during dreaming, with particular attention to the unlearning of troubling emotional events.

His interest goes back to childhood. A friend brought over a mask, bought through a magazine ad, with a small red light in the corner that was supposed to trigger lucid dreaming. Lucid dreaming is defined as dreaming during sleep while being aware that one is dreaming, and in some cases being able to direct the dream, for example choosing to fly, turn into an animal, or move to another place within the dream. He wore the mask awake while watching the flashing light, then wore it to sleep. He reports seeing the red light while asleep, "presumably through my eyelids," while admitting he cannot be sure his eyes were closed. Recognizing the vivid experience as a dream, he says he could then direct some of its events. This is a personal anecdote, not a controlled test of the device.

He states that lucid dreaming occurs in about 20% of people. A small share of those people lucid dream almost every night, and many of them report that their sleep is less restorative as a result. He also notes how attached people feel to their dreams, and how strong the urge can be to tell someone about an intense dream. He says he does not really know what drives this behavior, offering only that sharing may help people process the dream and make sense of it.

On historical attempts to interpret dreams, he mentions Sigmund Freud's symbolic representations and says a lot of that "has been kind of debunked." He still sees some interest in dream symbols, but says the episode will not focus on Freudian theory. His chosen route is to start from what is known concretely about the physiology of sleep.

The physiological account starts at sleep onset. Autonomic centers in the brain contain neurons that close the eyelids as we get sleepy. Sleep is then generally broken into a series of 90-minute ultradian cycles, regardless of how long a person sleeps.

Early cycles are made up mostly of shallow sleep (stage one, stage two) and slow-wave sleep, with relatively little rapid-eye-movement (REM) sleep. With each successive cycle, REM takes up a larger share of the 90 minutes and slow-wave sleep a smaller one. He says this holds whether or not sleep is interrupted, for instance by waking to use the restroom. The practical point is that the more total sleep a person gets, the more REM sleep they get.

The central claim of the episode follows: REM and non-REM sleep play distinctly different roles in learning and unlearning, and they handle different types of information. He links this to motor-skill learning, to unlearning traumatic events, and to processing both emotionally challenging and emotionally pleasing events. He also previews an argument that daytime activities can be used to get more non-REM or more REM sleep depending on a person's emotional and physical needs, along with later topics: lucid dreaming, and how drug-induced hallucinations resemble dream states in surprising ways while differing in important ones.

He flags a simplification. Slow-wave sleep and non-REM sleep "aren't exactly the same thing," but for clarity he will treat them as one category ("I am lumping right now") while acknowledging the distinction. Readers should keep this simplification in mind for everything said about slow-wave sleep later on.

Slow-wave sleep is described as a state in which the brain stays metabolically active but shows large sweeping waves of activity across much of the brain. He points to evidence of such waves across association cortex and large parts of the brain stem, naming a pathway from brain stem to thalamus to cortex (transcribed as the "ponds geniculate occipital pathway"). He adds that more of this pathway's activity occurs in REM sleep.

To explain why the stages differ, he turns to neuromodulators: acetylcholine, norepinephrine, serotonin and dopamine. These chemicals act relatively slowly and bias some brain circuits toward activity and others away from it. His analogy is a music playlist that pushes toward a genre, such as classical versus third-wave punk or hip hop, mellow versus aggressive, music with lyrics versus without.

His summary of each neuromodulator's role in waking states: - Acetylcholine amplifies circuits for focus and attention. - Norepinephrine amplifies circuits for alertness and the desire to move. - Serotonin amplifies circuits in brain and body associated with bliss and the desire to remain still. - Dopamine amplifies circuits for pursuing goals, pleasure and reward.

In slow-wave sleep, he says, there is essentially no acetylcholine. Its production from two major sites, a brain stem nucleus (named in the transcript as the parabigeminal nucleus) and the forebrain's nucleus basalis, drops almost to zero. Since acetylcholine supports focus, he describes slow-wave sleep as broad sweeping activity with "a kind of distortion of space and time so that we're not really focusing on any one thing."

His account of norepinephrine in this stage is qualified. He first calls it one of the molecules "very active at that time," then says "there's not a ton of norepinephrine around in slow-wave sleep. But it is around," and takes that as a sign that something in the movement circuitry is active. Serotonin, by contrast, is abundant. His summary: no acetylcholine, very little but some norepinephrine, and a lot of serotonin, all in the early part of the night when slow-wave sleep dominates.

People are not paralyzed in slow-wave sleep. They can roll over, and sleepwalking typically happens during this stage. The evidence he cites for its function comes from deprivation studies, which he calls "kind of sadistic." In these studies, people are selectively deprived of slow-wave sleep either by being woken as soon as electrode recordings show they have entered it or by chemically shifting their sleep away from it. He says these studies show that motor learning generally takes place in slow-wave sleep.