April 10, 2019
What’s Up! It’s Wednesday April 10th!
How Sleep Works
By James Clear
The Sleep-Wake Cycle
The quality of your sleep is determined by a process called the sleep-wake cycle.
There are two important parts of the sleep-wake cycle:
Slow wave sleep (also known as deep sleep)
REM sleep (REM stands for Rapid Eye Movement)
During slow wave sleep the body relaxes, breathing becomes more regular, blood pressure falls, and the brain becomes less responsive to external stimuli, which makes it more difficult to wake up. This phase is critical for renewal and repair of the body. During slow wave sleep, the pituitary gland releases growth hormone, which stimulates tissue growth and muscle repair. Researchers also believe that the body's immune system is repaired during this stage. Slow wave sleep is particularly critical if you're an athlete. You'll often hear about professional athletes like Roger Federer or LeBron James sleeping 11 or 12 hours per night.
As one example of the impact of sleep on physical performance, consider a study researchers conducted on the Stanford basketball players. During this study, the players slept for at least ten hours per night (compared to their typical eight hours). During five weeks of extended sleep, the researchers measured the basketball players accuracy and speed compared to their previous levels. Free throw shooting percentage increased by 9 percent. Three-point shooting percentage increased by 9.2 percent. And the players were 0.6 seconds faster when sprinting 80 meters. If you place heavy physical demands on your body, slow-wave sleep is what helps you recover.
REM sleep is to the mind what slow wave sleep is to the body. The brain is relatively quiet during most sleep phases, but during REM your brain comes to life. REM sleep is when your brain dreams and re-organizes information. During this phase, your brain clears out irrelevant information, boosts your memory by connecting the experiences of the last 24 hours to your previous experiences, and facilitates learning and neural growth. Your body temperature rises, your blood pressure increases, and your heart rate speeds up. Despite all of this activity, your body hardly moves. Typically, the REM phase occurs in short bursts about 3 to 5 times per night.
Without the slow wave sleep and REM sleep phases, the body literally starts to die. If you starve yourself of sleep, you can't recover physically, your immune system weakens, and your brain becomes foggy. Or, as the researchers put it, sleep-deprived individuals experience increased risk of viral infections, weight gain, diabetes, high blood pressure, heart disease, mental illness, and mortality.
To summarize: slow wave sleep helps you recover physically while REM sleep helps you recover mentally. The amount of time you spend in these phases tends to decrease with age, which means the quality of your sleep and your body's ability to recover also decrease with age.
Age-Related Sleep Changes
According to Harvard Medical School researchers, “As people age, it takes longer to fall asleep, a phenomenon called increased sleep latency. And sleep efficiency – the percentage of time spent asleep while in bed – decreases as well.”
Based on my calculations of the above data, the average 80-year-old gets a whopping 62 percent less slow wave sleep than the average 20-year-old (20 percent of the average sleep cycle versus 7.5 percent). There are many factors that impact the aging of body tissues and cells, but it stands to reason that if your body gets less slow-wave sleep to restore itself each night, then the aging process will accelerate as a result.
In other words, it seems reasonable to say that getting good sleep is one of your best defenses against aging quickly.
The Circadian Rhythm
What is your sleep-wake cycle dictated by?
Answer: the circadian rhythm. The circadian rhythm is a biological cycle of different processes that happen over a time span of about 24 hours.
Here are some key points in the typical 24-hour cycle:
6 A.M. Cortisol levels increase to wake your brain and body
7 A.M. Melatonin production stops
9 A.M. Sex hormone production peaks
10 A.M. Mental alertness levels peak
2:30 P.M. Best motor coordination
3:30 P.M. Fastest reaction time
5 P.M. Greatest cardiovascular efficiency and muscle strength
7 P.M. Highest blood pressure and body temperature
9 P.M. Melatonin production begins to prepare the body for sleep
10 P.M. Bowel movements suppressed as the body quiets down
2 A.M. Deepest sleep
4 A.M. Lowest body temperature
Obviously, these times are not exact and merely display the general pattern of the circadian rhythm. The exact times of your circadian rhythm will vary based on daylight, your habits, and other factors we will discuss later in this guide.
The circadian rhythm is impacted by three main factors: light, time, and melatonin.
Light. Light is probably the most significant pace setter of the circadian rhythm. Staring into a bright light for 30 minutes or so can often reset your circadian rhythm regardless of what time of day it is. More commonly, the rising of the sun and light striking your eyes triggers the transition to a new cycle.
Time. The time of day, your daily schedule, and the order in which you perform tasks can all impact your sleep-wake cycle.
Melatonin. This is the hormone that causes drowsiness and controls body temperature. Melatonin is produced in a predictable daily rhythm, increasing after dark and decreasing before dawn. Researchers believe that the melatonin production cycle helps keep the sleep-wake cycle on track.
The 2-Process Model of Sleep Regulation
In 1982, Dr. Alexander Borbely published an article in the journal Human Neurobiology describing something he called the 2-process model of sleep regulation. This conceptual framework for sleep describes two processes that occur simultaneously to regulate sleep and wake states.
Process 1 is sleep pressure. Basically, sleep pressure mounts from the moment you wake up, to the time when you go to sleep. While you're sleeping, pressure decreases. If you get a full night of sleep, you start the next day with low sleep pressure.
Process 2 is wake drive, which counteracts sleep pressure and is controlled by a 24-hour rhythm that repeats in a wave pattern.
It's important to understand this process because it helps to reveal an important point about sleep in our modern world that I learned from sleep scientist Dan Pardi :
For millions of years, humans and our ancestors have evolved to sleep at night (when it is dark) and wake during the day (when it is light). However, in the modern world, we work inside all day, often in areas that are darker than the outside world. And then, at night, we look at bright screens and televisions. Low light during the day, more light at night: It's the opposite of naturally occurring cycles and it seems quite likely that it could mess up your wake rhythm and circadian rhythm.
The result of this shift? Drowsiness and impaired function throughout the day. We'll talk more in just a minute about how to sleep better, including actionable steps you can take to anchor your rhythm, but it pretty much comes down to this: Use common-sense light habits. Get outdoor light exposure during the day, and turn down the lights and turn off your screens after dark.
When Should I Go to Sleep?
If you're getting the recommended 8 hours of sleep, does it matter when you get it?
“The time of night when you sleep makes a significant difference in terms of the structure and quality of your sleep,” said Dr. Matt Walker, head of the Sleep and Neuroimaging Lab at the University of California, Berkeley.
The ratio of REM to non-REM sleep changes through the night, with non-REM sleep dominating cycles earlier in the night and REM sleep kicking in closer to sunrise, Walker said. That means a late night could result in insufficient amounts of deep, non-REM sleep. As we discussed earlier, it's crucially important to get healthy amounts of both REM and non-REM sleep.
So how early do you need to be to bed to get enough of each type of sleep? Walker says there's a window of several hours, about 8 p.m. to midnight.
The best time for you, though, will vary.
Till Roenneberg, a professor of chronobiology at Ludwig-Maximilian University in Munich who studies the biological roots of sleep, says each person has a unique internal timing profile called a sleep chronotype that determines where on the scale from “early bird” to “night owl” we fall. Your chronotype is largely genetic.
When choosing your bedtime, try not to fight your physiology. The best bedtime will differ a little bit for everyone, but it's crucial that you pay close attention to your internal clock and what your body is telling you. As long as you're getting the recommended 8 hours of sleep, just focus on finding the time that works best for you.