Understanding the Sleep Cycle

Before the 20th century, the common belief among scientists and doctors was that our brain basically shut down during sleep and the only parts that remained active were those essential for keeping us alive. However, with the invention of electroencephalography (EEG) and the ability to measure brain waves, that belief changed.

We now know that there are two major types of sleep: 1) Non-REM (quiet sleep) and 2) REM (dreaming sleep). These two cycles alternate with each other over the course of the night.

Non-REM (Quiet) Sleep

Non-REM sleep consists of four stages (or three, depending on who you ask):

• Stage 1: We’re technically not asleep during Stage 1, but we’re well on our way. We spend about 5 minutes in this phase, though it can last longer. Brain activity begins to slow down; body temperature starts to drop; muscles relax; eyes move slowly from side-to-side. During Stage 1 sleep we lose awareness of our surroundings but we’re still easily jarred to wakefulness. If you’ve ever woken up from a sleep but claimed you were just “resting your eyes,” you were likely roused during Stage 1.
• Stage 2: This is the first stage of honest-to-goodness sleep. The first time it occurs during the night, it lasts 10-25 minutes before you go to the next stage. During Stage 2 sleep, your eyes are usually still, and breathing and heart rate are slower than when you’re awake. Brain activity is irregular; the EEG will pick up intermediate-sized brain waves intermingled with bursts of fast activity.
• Stages 3 and 4: Stages 3 and 4 are typically smooshed together by researchers and are called “deep sleep” or “slow wave sleep.” Deep sleep lasts anywhere from 20 to 40 minutes, but declines with age. In this stage, brain activity primarily consists of large, slow waves. Breathing slows and blood pressure drops 20 to 30 percent below awake levels. You’re much less responsive to external stimuli during deep sleep and thus harder to rouse. It’s in this stage that our bodies also repair and renew themselves. At the beginning of deep sleep our pituitary gland releases a pulse of human growth hormone to help with tissue repair and growth. Levels of substances that activate our immune system, like interleukin, increase in our blood.

REM (Dreaming) Sleep

The 4 stages of sleep alternate with REM (Rapid Eye Movement) sleep, which is basically when your mind has a party. Here’s what happens to our body and mind during REM sleep:

• Brain releases a small amount of a substance called DMT which has intense psychedelic properties
• Brain activity skyrockets because we’re dreaming
• Muscles not needed for breathing or eye movement are paralyzed; we can’t move during REM sleep
• Eyes move back and forth rapidly (hence the name)
• Blood pressure increases
• Heart and breathing levels increase to daytime levels
• Testosterone levels increase
• Erections regularly occur in men
• Despite being paralyzed, our sympathetic nervous system is twice as active as when we’re awake

We typically experience three to five REM sessions a night. The first episode lasts just a few minutes, but REM sleep gets progressively longer as the night goes on. The final REM session can last up to an hour.

Sleep Architecture



A normal sleeper moves between these two sleep patterns in a fairly predictable manner over the course of the night. If you were to get 8 hours of uninterrupted sleep, you’d go through four to five alternating non-REM and REM sessions. Above is a chart that shows a typical sleep cycle during 8 hours of sleep. Notice how REM and Stage 2 begin to alternate as the night progresses.

What Controls Our Sleep

Circadian Clock

Inside your brain there’s an internal clock that governs your sleep and wake cycle. It’s called the circadian clock, and it’s located in the suprachiasmatic nucleus (SCN), a cluster of cells that are part of the hypothalamus. The cycling of the clock (called the circadian rhythm) in humans follows a 24-hour cycle that responds to light and dark in the environment. The circadian rhythm for sleep and wakefulness in humans dips and rises at different times of the day. The desire to sleep is generally stronger between midnight and dawn, as well as in mid-afternoon. The exact cycle will change from person to person.

Zeitgebers (External Cues for Sleep)

While our circadian rhythm is internal and will cycle through sleepiness and wakefulness on a pretty regular basis, external cues called zeitgebers (German for “time giver” or “synchronizer”) also play a role in governing our naturally occurring rhythms. If you change or disrupt certain zeitgebers, you can cause shifts in your circadian rhythm, making it difficult to get to sleep when you’re ready to hit the hay. Below are a few of the most influential external cues that affect our sleepiness/wakefulness cycle:

• Light. Light is the most influential zeitgeber. Our retinas actually have a direct connection to the SCN. When we’re exposed to light, our brain produces more hormones, like serotonin, that make us feel awake; when it starts to get dark, our brain starts ramping up the sleepy hormone melatonin. The creation of artificial lights in the late 19th and early 20th centuries have been blamed for messing up natural sleep cycles. While exposure to any kind of light can affect circadian rhythms, research has found that exposure to light with blue wavelengths is the most disruptive to melatonin production. Unfortunately, most of us put electronic devices right in front of our faces every night that primarily emit this blue light. If you’ve been having trouble falling asleep, it may be because you’re surfing Reddit on your laptop and looking through Instagram photos on your smartphone late at night. Never fear. There are some things you can do that will allow you to use your electronic devices without their disrupting your vital sleep. We’ll talk about those next week.
• Exercise. Strenuous activity like exercise can be a cue to delay sleep and stay awake. One study found that nightly exercise was an effective way to delay the onset of melatonin secretion at night, making it more difficult to get to sleep. This can of course be a benefit if you actually don’t want to sleep, which is why a late bout of exercise is helpful when you’re aiming to pull an all-nighter.
• Negative social interactions. Negative social interactions can contribute to interrupting your internal circadian clock. Disruptive social events like break-ups or the death of a loved one have a tendency to cause shifts in sleep patterns, causing you to toss and turn at night.

One takeaway from this information is that because our sleep cycle is evolved to sync up with something as reliable as the rising and the setting of the sun, varying your daily and nightly routine as little as possible plays an important role in improving the quality of your sleep. I know, easier said than done, but we’ll provide some tips next week that can help.

How Much Sleep Do I REALLY Need?

Every person is different and will require different amounts of sleep to run on all eight cylinders (see, I didn’t say four cylinders). The need for sleep generally follows a bell curve: a few people can get by perfectly fine on as little as 4 to 6 hours of sleep (some folks actually have a genetic variation that gives them more intense sleep sessions, which allows them to get by on less); the vast majority of people need 7 to 8 hours; and a few need as much as 9 to 10 hours of sleep.

You’ve likely read articles highlighting super successful individuals who supposedly get by on just 4 hours of sleep a day. While it’s possible that they’re one of those fortunate mutants who don’t need much sleep, it’s likely that they’re actually sneaking in a nap or two during the day, and thus getting more sleep than they let on. (Thomas Edison was like this; he liked to boast that he only got 3-4 hours of sleep every night…while neglecting to mention that he took one or two 3-hour naps every day.) Either that, or they’re a walking zombie, who could be performing way better in their day-to-day life than they are.

As you get older, you need less sleep. Generally, total sleep decreases from about 7.5 hours at age 20 to 7 hours at age 40 to about 6 hours at age 60. Not only do we sleep less, our sleep architecture changes as well. Stage 1 and 2 sleep increases while Stage 3 and 4 decreases. You’ll also find yourself waking up more at night. Why is this? Well, melatonin production decreases as we age and the body’s overnight dip in temperature, which helps with staying asleep, becomes less pronounced as well.

The best way to calculate how much sleep you need is to simply listen to your body. If you find it easy to get out of bed, aren’t sleepy during the daytime, don’t have any problems concentrating, are generally in a good mood, and you feel fully rested after a night of sleep, well, then you found your optimal amount of sleep. Next week we’ll provide a neat tip on how to discover the right amount of sleep for you as well as your optimal sleep/wake schedule.

The Horrible, No Good, Very Bad Consequences of Sleep Deprivation

A whole host of things go wrong when you don’t get the amount of sleep your mind and body needs. Sleep deprivation comes in two types: complete and partial.

Complete Sleep Deprivation

Complete sleep deprivation happens when you go a night without sleeping at all. Complete sleep deprivation can cause immediate decreases in concentration and hurts long-term and working memory (which means that all-night cramming session for your history test might actually make you more forgetful). You’ll also experience increases in cortisol, which makes you feel stressed and lowers testosterone levels.

(On the flip side, skipping a night’s sleep can create feelings of euphoria, and boost your brain’s motivation and desire for reward. This can lead to poor decision-making and risky behavior, but I’ve also found that an occasional all-nighter is exactly what I need to crack the code on a tough post I’m working on. The key here is occasional; remember that a low dose of short-term stress can actually be good for you, while chronic stress is nothing but bad news.)

You likely won’t experience very many periods of complete sleep deprivation in your life (unless you’re one of those all-nighter-loving architects). Our bodies and brains just can’t take it. After a few days of continual non-sleep, your body will simply force it upon you. In fact, you’d die of sleep deprivation before you’d die of starvation.

Partial Sleep Deprivation

Partial sleep deprivation happens when you get some sleep but not 100% of what you need — basically a bad night’s sleep. When you experience one night of partial sleep deprivation, you can function at normal levels the next day, but you don’t feel great. Problems with partial sleep deprivation start showing up after two or three nights of less-than-ideal sleep. You’ll become irritable and you’ll notice a drop in your concentration and work performance; in fact, after only two weeks of getting 6 hours of sleep a night, your reaction time will be similar to someone with a blood alcohol level of .1%, which is considered legally drunk. If you go several weeks, months, or years (we’re talking insomniacs here) with partial sleep deprivation, well, you’re going to be in a world of hurt. Here’s a breakdown of some of the things that will happen when you’re partially sleep deprived for an extended period of time:

• Impaired nerve cell generation
• Increased risk for depression
• Decrease in pre-frontal cortex activity, which makes us more impulsive and unable to control our emotions
• Increased emotional and mental stress
• Reduced grey matter in brain
• Increased blood pressure and chances of heart disease
• Increased chance for obesity and diabetes
• Reduced testosterone levels
• Increased mortality rate

So yeah, if you adopt the maxim that you’ll sleep when you’re dead, you may get there faster that you’d anticipated. Unfortunately, people have this idea that they can just adapt to less sleep; that all those bad things won’t happen because their body will simply get used to being short-shrifted on slumber. But the research suggests that’s simply not true. Whenever you don’t get the sleep your body needs, you slowly begin to accumulate “sleep debt” (the amount of sleep your body’s in the hole for). The more sleep debt you have, the more your cognition and health decreases. To reverse the problems that come with sleep deprivation you have to repay the debt that has accumulated. If your sleep debt has been building for days, weeks, or months, paying it down is possible by consistently increasing your sleep time.

If you’ve had a string of shortened sleep sessions over a week, several days of getting the sleep you need, plus an additional hour or so per night, should get you square with your body’s sleep debt.

If you’ve accumulated hundreds or thousands of hours of sleep debt due to months of bad sleep, you’ll need several weeks of getting the sleep you need plus an hour or more with each session to get your debt paid down.

We’re able to repay our sleep debt, in most cases, because our deprived bodies sleep in a more efficient manner when we start getting the sleep we need by skipping through Stages 1 and 2 faster and getting to the more restorative Stages 3 and 4.

If you’ve been carrying your sleep debt for years, researchers aren’t sure restoring it is even possible; it may cause permanent damage to your physical and mental faculties.

The Benefits of Sleep

So we know that not getting enough sleep is bad for us, but what are the benefits of getting a good night’s sleep? There are a myriad:

• Strengthens immunity
• Increases muscle mass
• Decreases body fat
• Increases testosterone
• Lessens crankiness
• Cleans out your brain of toxins
• Decreases inflammation
• Spurs creativity
• Improves athletic performance
• Increases focus and attention
• Decreases stress
• Improves memory

Everybody’s searching for that magic pill or obscure hack that will improve their life on every level. Meanwhile, the secret to being stronger, happier, and smarter has been staring them in the face the whole time. They just can’t see it because there’s a computer or tablet blocking their view. So if you’re reading this late at night, and you’re tempted to check out just a “few more things” before your turn in, shut the lid of your laptop, and get to bed, my friend.