Matt Walker: Essentials: The Science & Practice of Perfecting Your Sleep

This Huberman Lab Essentials episode revisits a conversation with sleep scientist Dr. Matt Walker and starts with the most basic question: what is sleep? One note on the transcript: the introduction says the guest is Dr. Matt Walker, but the speaker labels in the supplied text seem to be swapped. Lines labeled with Walker's name read like the host's questions and sponsor reads, and lines labeled with Huberman's name read like the scientist's answers. To avoid mixing them up, this piece calls the speakers "the host" and "the guest" by their role in the conversation.

The guest calls sleep "probably the single most effective thing you can do to reset your brain and body health," and also "an incredibly complex physiological ballet." He splits it into two main types, non-rapid eye movement (non-REM) sleep and rapid eye movement (REM) sleep. During REM sleep, he says, you are "completely paralyzed."

The brain paralyzes the body so that the mind can dream safely.

His argument for why this matters is evolutionary. Someone who dreams they can fly and acts it out by jumping from an apartment window would have dropped out of the gene pool fast. The paralysis has limits, though. Involuntary muscles keep working, so breathing and heartbeat continue, and the guest describes "bizarre, what we call autonomic storms" during REM. Only two voluntary muscle groups escape the paralysis. One is the extraocular muscles, without which there could be no rapid eye movements. The other, found later, is the inner ear muscle.

The guest attributes one explanation for the spared eye muscles to "some people." The idea is that eyes left still for long periods might develop oxygen problems in the aqueous or vitreous humor. The host adds that the drainage systems of the front of the eye need movement, and notes that people with glaucoma have drainage deficits through the anterior chamber. They present this as a proposed explanation, not a settled finding.

The guest walks through a typical night in the first person. Sleep starts in the light non-REM stages, one and two. After about 20 minutes it moves into the deeper stages three and four. In the lighter stages, heart rate drops a little and brainwaves slow down. Awake, they cycle maybe 20 to 50 times a second. In light non-REM they slow to maybe 15 or 20, and then to about 10 or eight cycles per second.

In stages three and four, heart rate "really does start to drop." Hundreds of thousands of cortical cells fire together and then go silent together. The guest calls this a kind of physiological coordination "that we just don't see during any other brain state." He stays in deep sleep for about another 20 or 30 minutes, putting him 60 or 70 minutes into the first cycle. Then he rises back into stage two, and after about 80 minutes has a short REM period.

The pattern then repeats about every 90 minutes, which he says is "the average for most adults." What changes across the night is the mix. In the first half, most cycles contain a lot of deep non-REM sleep (stages three and four). In the second half, "that seesaw balance changes," and cycles fill up with lighter stage two non-REM sleep and more and more REM sleep.