Bestselling author Matthew Walker doesn’t mind if you fall asleep while reading his book. In fact, he encourages it. The UC Berkeley neuroscientist knows that if you sleep after learning something, you’re more likely to remember it. And if you remember what you learn in his book, Why We Sleep — Unlocking the Power of Sleep and Dreams, maybe come nighttime, you’ll turn off Netflix, put away your phone, and go to bed already.

We’d all be better off if we did. For starters, we’d be healthier. The link between sleep and illness is reason alone to hit the sack on time. But another aspect of sleep — noteworthy now that kids are back in school — is its critical connection to learning. Because without adequate sleep, learning and memory are compromised.

But sleep is usually the first thing to go when we get busy. Students, in particular, are challenged with packed schedules, heavy homework loads, early school starts, and brutally early bus pick-ups and “zero-period” classes. It’s a tough schedule, and one that goes against their very biology.

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Starting in puberty, adolescents’ circadian rhythm begins to shift. By high school, it’s running hours later than their parents’. Melatonin peaks later, so teens go to bed later. Come sun up, their bodies are still immersed in vital stages of sleep. It’s a hard-wired biological change, and it’s why the Tahoe Truckee Unified School District recently queried parents to assess whether a later school start might be possible.

“When I filled out the survey,” said Ashley Phillips, local mom of middle- and high-schoolers, “I saw how incredibly hard it would be for most working families to accommodate a later start. I also thought about after-school clubs and sports getting pushed later.”

In the end, the school district decided not to change start times. The issue is complex, and not the focus of this article. Nevertheless, waking early has repercussions for teens, as does sleep loss for everyone, regardless of age.

SLEEP 101 AND LEARNING

Humans have two stages of sleep — REM, or dream sleep, and non-REM, which is deep, dreamless sleep. Non-REM is divided into four stages of increasing depth which, for simplicity, I’ll group into light and deep non-REM.

We cycle through all the stages of sleep multiple times per night, in cycles that last about 90 minutes. But the amount of time we spend in each stage is not equal. Our first couple of cycles are spent mostly in deep sleep, and as we progress toward morning, our cycles shift into more REM, alternating with light sleep.

As a result, most of our deep sleep happens in the first half of the night, and most of our REM and light sleep in the latter half.

This has major implications for learning.

When we memorize facts, such as the state capitals, someone’s name, or where you put your sunglasses, the memory is first captured by a region of the brain called the hippocampus. But the hippocampus has limited storage capacity. So while we sleep, our brain does a data transfer, moving new memories from the temporary storage in the hippocampus to long-term storage in the neocortex.

This transfer happens during both light and deep non-REM, with two distinct outcomes. Deep sleep cements, or consolidates, the new memories, saving them for long-term recall, while light sleep refreshes the hippocampus so it can absorb new information the following day. This refreshing happens via bursts of electrical activity called sleep spindles.

So what happens when you’re short on sleep? If you stay up late, you’ll be losing mostly deep sleep. In experiments, subjects who lost sleep in the first half of the night found their recall plummeted the next day.

But if you lose sleep in the latter half, especially the last two hours of snoozing, you’ll be losing most of your light sleep (and sleep spindles.) The next day, you won’t be as able to absorb new material. Missing those last two hours also means losing most of your REM.

REM — WEIRD BUT ESSENTIAL

When you have a whacky dream about riding a unicorn through Safeway, it’s hard to believe your brain is accomplishing anything. But during REM, your brain takes the vast catalogue of facts that it saved during non-REM sleep and figures out how all the information fits together. It does this by making novel, nonlogical associations between distantly related facts, which allows you to form abstract concepts, hatch creative ideas, and solve problems.

Put another way, non-REM helps you learn information, while REM sleep helps you comprehend it.

PRACTICE PLUS SLEEP MAKES PERFECT

In addition to learning facts, we also need to learn motor skills. When we practice a sport, musical instrument, or the fingering on a new computer keyboard, we learn by chunking the new movements into manageable segments. As we sleep, our brain transfers the new motor memories to brain circuits that function subconsciously. When we wake, our movements have become more fluid, automatic, and precise.

Even in the absence of practice, sleep has been shown to improve speed by 20% and accuracy by 35%.

The sleep stage responsible for improving motor skills? Light non-REM sleep spindles that are rich in the last two hours of sleep … precisely the hours that kids lose when they wake early for sports or zero-period jazz band — which require, ironically, motor skills.

“Maybe our kids are just overscheduled,” said Phillips, “and there aren’t enough hours in the day to fit everything in.”

But would her daughter want to quit jazz band in order to sleep in?

“No,” Phillips said. “It’s too important to her. We just try to get to bed early.”