Ever wondered why it’s so hard to wake up on some mornings and easier on others? Or, why you can still fall asleep after napping one day but not another? What the heck is a “coffee nap?”
All of these questions have to do with sleep cycles but more on that in a minute.
A few weeks ago, we talked about sleep here on the blog, specifically, we talked about why sleep is important and six ways to get better sleep starting immediately.
- Establish a routine.
- Make your room cold.
- Eliminate screens and blue light.
- Read fiction.
- Try melatonin.
- Get outside and in the sun.
That’s great, and those tips will help you get better sleep. However, all slumbers are not equal.
Between the time you doze off at night and wake up in the morning, you go through different sleep phases. You probably knew that. What you might not know is that each phase has a specific purpose and certain symptoms. You might also not have known that the duration of each sleep cycle changes with age.
Sleep Cycles – A Brief Overview
Before we dive into the specific phases in detail, let’s take a quick look at what a typical sleep cycle entails.
The 5 Phases of Sleep
- Stage 1 – This is very light sleep where you’re drifting in and out of sleep and can be awakened easily. You might experience sudden muscle contractions.
- Stage 2 – Body temperature drops and heart rate slows. You’re preparing for deep sleep.
- Stage 3 – The first phase of deep sleep. The brain begins to produce slower brain waves (called delta waves). This is also where you might start sleepwalking.
- Stage 4 – The second phase of deep sleep. During this phase, the brain produces those slow (delta) waves exclusively. If you’re awoken from this phase, you’ll feel very groggy.
- REM – Brain activity increases to levels similar to when you’re awake. This is when most dreaming occurs.
Adults can go through a full sleep cycle in as little as 90 minutes. The first sleep cycle of the night is typically the shortest with each subsequent sleep cycle lasting around 120 minutes. If you do the math, that’s somewhere between 3-5 sleep cycles per night.
You’ve probably heard of REM sleep before and how important it is for memory and cognitive function. Each sleep cycle actually plays a critical role though. Let’s go over the three specific phases – light, deep, and REM sleep – in detail.
Stages 1 and 2 – Light Sleep
Have you ever caught a friend dozing off during a movie?
Their eyes are closed. Their breathing slows down. You might even notice their fingers jerking back and forth. They’re clearly sleeping. Yet, when you wake them up, they claim they were never asleep!
This is a definitive feature of stage 1 during light sleep. The body is beginning to transition from being awake to falling asleep. It’s easy to wake someone up during this phase, and as you have probably witnessed, they may actually claim they weren’t asleep at all!
If you let your friend continue to doze off, they’ll quickly transition from stage 1 to stage 2. When spend a comparatively short amount of time in stage 1 sleep throughout the night (about 3% of total sleep time). During stage 2, they’ll still be easy to rouse, but they may dream (although the dreams won’t likely form a full story or narrative).
According to Tuck.com, we spend a good portion of the night in light sleep – half or more in fact. Yet, light sleep feels much less refreshing:
Light sleep is perceived subjectively as less refreshing than deep sleep or REM sleep, and this is part of what drives older people to complain about their sleep quality. (source)
If light sleep isn’t refreshing, why bother with it in the first place?
Even if it doesn’t feel like it, light sleep is essential for bodily repair (from all those tough workouts!). It’s also critical for learning. Tuck.com indicates that the brain transfers memories from short-term memory to long-term memory during stage 2.
Stages 3 and 4 – Deep Sleep
Let’s say you pull an all-nighter like back in college. You’ll probably feel exhausted the next day and daydream about the next time your head can hit the pillow. You’ve accumulated sleep debt meaning your body is missing the 7-8 hours it normally gets per night.
Now, a funny thing happens even if you’re permitted to sleep as long as possible the night after an all-nighter. Math being what it is, you might expect that you would sleep 14 hours or so if you could, recouping the total sleep debt while also getting a full night’s sleep.
That’s not what happens though. You’ll probably sleep 10 hours and wake up feeling refreshed. What gives?
We crave deep sleep more than any other type of sleep. It’s when the vast majority of our body repair and rejuvenation takes place. Hormones like growth hormone are released during this time for exactly this purpose. As the name implies, it’s harder to wake someone up from deep sleep than light sleep. When you’re awoken from deep sleep, you’ll likely feel groggy and have a hard time coming to.
Back to our math problem surrounding sleep debt mentioned above. When you’re recovering from your all-nighter, your body will preferentially recover all of the deep sleep you lost.
During your night of slumber bliss where you’re knocked out for 10 hours, your body will automatically transition your sleep cycles to get more deep sleep and less light and REM stages. While you might only sleep 10 hours instead of the expected 14, you’re regaining all of the deep sleep you missed! This further illustrates just how important deep sleep is to our bodies and to recovery.
Even if you know very little about sleep cycles in general, you probably have heard of REM sleep. It stands for rapid eye movement, and it accounts for roughly 20% of adult sleep time.
REM sleep occurs at the end of a sleep cycle so a normal cycle would go like this:
Stage 1 – Stage 2 – Stage 3 – Stage 4 – Stage 3 – Stage 2 – Stage 1 – REM
As the name implies, during REM sleep, your eyes may jerk back and forth. Compared to other sleep stages, your breathing will also be more rapid. Certain parts of the brain (visual, motor, emotional, parts of memory) speed up activity, similar to waking periods. Other parts of the brain like the ones involved in rational thought actually decrease in activity. This all means that you’re more likely to remember dreams from REM sleep, but those dreams are also more likely to be a bit weird and nonsensical.
REM sleep cycles tend to be shorter at first and then get longer throughout the night. Your first REM cycle might only be 10 minutes (remember, the body preferentially gets deep sleep), but later cycles can last up to an hour. This means if you’re only sleeping 4-5 hours a night, you’re cutting your total amount of REM sleep short.
There are many theories as to why REM sleep is beneficial. A main hypothesis right now is that REM sleep is critical for helping us regulate our emotions. Time.com summarizes some recent research and thoughts from Matthew Walker, author of Why We Sleep:
Several studies in recent years have suggested that REM sleep can affect how accurately people can read emotions and process external stimuli…“I think of dreaming as overnight therapy,” Walker says. “It provides a nocturnal soothing balm that takes the short edges off of our emotional experiences so we feel better the next day.”
For example, Walker’s research has indicated that when people get enough REM sleep, they’re better at judging the emotion from facial expressions and controlling their own emotions (i.e. not acting compulsively) throughout the day.
What to Do Next?
Why does any of this even matter? How is it going to help you with your goal of losing weight, building strength, or improving your 5k time?
By now, we hope the answer is clear. Sleep is absolutely critical to your performance! Each sleep cycle plays an important role in your recovery and how you function the next day. For example, we learned about how the body will sacrifice other phases of sleep to get you more deep sleep since it’s so essential.
It’s fun to know a bit more about the various stages of sleep and why they’re important, but if we had to pull out one overarching takeaway, it would be this:
Set aside 7-8 hours of continuous sleep per night.
This will ensure you get the right amounts of deep sleep and REM sleep necessary.
Random Facts About Sleep
While reading all about the various sleep stages, I came across a few facts that I thought were interesting:
- As you age, you tend to spend more time in light sleep compared to deep sleep. According to Tuck.com, REM sleep will also decrease 10 minutes per night for every decade of life.
- Our body has a natural drive to get more deep sleep, and there’s a quota of sorts we try to get on a daily basis. If you take a short nap during the day (that doesn’t get you into deep sleep), you’ll still be able to sleep easily at night. If you take a longer nap that gets you some deep sleep, you’ll have a harder time falling asleep because you’ve fulfilled some of your deep sleep quota.
- Infants spend almost 50% of their sleep time in REM sleep.
- Polyphasic sleep is an approach to “sleep hacking” wherein you try to get many, shorter bursts of sleep in a 24-hour period instead of 8 straight hours per night. So, someone may get 4-5 segments of 1-1.5 hours a piece throughout the day with the goal of being more productive.
- Between 2-4pm, your body temperature naturally drops as part of your circadian rhythm. This signals your body to produce melatonin, a hormone responsible for sleep. So, you naturally feel tired!
- Naps are super beneficial and can make you more productive. Keep them to 20 minutes in the afternoon to maximize the effect and minimize the chance of grogginess upon waking. The “coffee nap” is also a thing! Drink a cup of coffee then immediately nap for 15 minutes so the caffeine kicks in immediately upon waking.
Many thanks to Tuck.com for being an amazing resource for this post. They have a ton of research-backed articles on sleep that I would encourage anyone interested to check out.