Understanding How (and Why) We Sleep

Few experiences are as universal as sleep. Few are so misunderstood, either; despite modern advances in medicine, psychology, and basic biology, after millennia of philosophizing and decades of research, we’re still attempting to gain insights into a process that we all undergo every night. Worse, for every legitimate sleep study that comes out, countless news articles and laymen tend to extrapolate “rules” regarding how to enhance our sleep in 21st century society’s  never-ending quest for optimization. 

Rather than conform to this trend of unsolicited health advice, we’d like to break down some of the fundamentals of sleep science that have been gleaned in recent years. Some of these concepts–like sleep cycles and chronotypes–may already be familiar to you from their time in the popular press. Our goal is to flesh out these ideas so that the next time a flashy headline pops up on your timeline (probably something to the effect of “Try this hack to maximize your REM sleep!”) you can decide for yourself whether the article is worth your time. 

How does sleep work?

Psychologically speaking, we could best describe sleep as a state of temporary reduced consciousness. Instead of perceiving the world around us, as we do when we’re awake, we become inactive and unresponsive to external stimuli. There are also clear physiological changes that occur when we sleep: our breathing slows, body temperature drops, and even the way the neurons fire in our brain changes. 

This does not mean we should consider sleep a period of inactivity, however. Particularly in the brain, our body uses sleep as a time of consolidation and reinforcement, ultimately impacting everything from cardiovascular and metabolic health to the longevity of our learning and memories. 

What are sleep cycles?

All of those different functions require different biological modes, or settings, to accomplish. The divisions in sleep scientists have uncovered so far are arbitrary; they’re the product of how we study and measure sleep and thus may not reflect the full underlying reality of the phenomenon we’re trying to understand. That said, this is our current best theory for how we sleep, best matching all the data we’ve accrued so far. 

Overall, sleep occurs in stages. Stage 1 happens as we first exit wakefulness. It occupies the least amount of time and can be thought of as more of a transitional period, during which the body and brain make changes in preparation for the deeper parts of sleep. As such, on a night where we sleep without interruption, we don’t typically return to Stage 1 until we’re on the verge of waking up. 

Stage 2 is where behavior and bodily metrics begin to match up with what we commonly consider sleep. Breathing begins to slow, core temperature starts to drop, and overall awareness of our environment greatly diminishes. The most dramatic changes however are those which occur in the brain: rhythmic synchronization of brain waves begin to appear in this stage and will continue into the next. 

Stage 3 is the deepest part of sleep, where breathing is at its slowest, blood pressure has fallen (sleep scientists describe deep sleep as the greatest blood pressure medicine ever made), and brain waves reach their slowest yet more powerful. Should you wake during this stage, you’ll experience “sleep inertia,” or a sustained grogginess that takes at least half an hour to shake–something we’ve all experienced when rudely pulled back into reality by an alarm. More important, however, are the processes which happen during this deepest part of sleep. The majority of immune system changes, metabolic regulations, and memory consolidations occur at this time, and while no stage of sleep is more important than another, the relevance of Stage 3 to our health cannot be understated. It takes time to reach Stage 3, which is why uninterrupted sleep is so necessary for overall sleep quality. 

Last but not least is the stage everyone knows: REM sleep. This is when dreams typically occur, and for that reason the body is temporarily paralyzed (so you don’t act out your dreams, which strikes us as evolutionarily prudent). Even though your body may be inactive, your brain is not, and during this time further memory and learning consolidation is underway while your breathing becomes fast and irregular. 

It’s important to note that even though we call these combined four stages of sleep the sleep cycle, we do not always progress through the sleep cycle in perfect order. As stated above, we typically only enter Stage 1 when exiting and entering wakefulness; once you’ve exited your first REM stage, you typically return to Stage 2, then reenter Stage 3, and then go back to REM. This will occur several times throughout the night until your body is rested and ready to wake again. Additionally, all stages of sleep lengthen each time you experience them, meaning that your first full sleep cycle in a night will always be shorter than subsequent ones.  

How much variation is there in sleep cycles, and what causes that variation?

The main area where sleep misinformation tends to spread centers around individual variations in sleep cycles and needs. Many sources try to apply a one-size-fits-all approach to sleep science: studies uncovering the existence of an average length of 90 minutes for sleep cycles morph into headlines reading “Scientists Uncover that Humans Sleep in 90 Minute Cycles”, obscuring differences that exist between individuals. One person may have a cycle that lasts an hour; another’s may go on for two. 

The same variation applies to the quantity of sleep required, where some people function just fine on 6 or 7 hours while others need 9 or 10 to feel their best. Either way, a person’s genetics determines their individual needs. As long as you’re falling within established parameters (and you feel reasonably healthy), you don’t have to conform to some magic number to guarantee your continued wellbeing. 

What constitutes good sleep? 

Most of us assess how well we slept by focusing on how many hours of sleep we got and, more subjectively, how we actually feel when we wake up. Even within the field of somnology itself, it was only within the last few years that a shift in emphasis has occurred from overall quantity to sleep to include other metrics: quality of sleep, regularity, and timing. 

Sleep quality is the most subjective of these. Scientists have tried to circumvent this by devising more quantifiable aspects to study, landing on parameters such as continuity (how many times, if any, did you wake?) and efficiency (of all the time you spent in bed, how much of it was actually spent sleeping versus being awake?), in addition to measuring the brain’s electrical activity throughout the night. By combining all of these, it has given somnologists a better understanding of how to determine sleep quality and its effect on our health. 

Regularity of sleep is something any of us with a set routine can easily understand; deviating from the set times you like to sleep and wake can have a profound impact on your sense of wellbeing, even for temporary deviations. In general, humans are creatures of habit, and we sleep best when we can reliably stick to a sleep schedule.

Of course, the trick is picking a sleep schedule which actually suits our needs, and some of these are beyond our control. The idea of chronotypes has gained prevalence in pop culture lately, but there is truth behind the personality quizzes and morning news buzz: people have different internal clocks which are the product of genetics, environment, and culture, and there is only so much we can do to change our own clock. No one is truly nocturnal–that would defy human biology–but in general, we can confidently say that all human beings fall into one of five chronotypes:

• Extreme morning type: comfortable going to sleep by 8PM and waking up around 4AM

• Moderate morning type: comfortable going to sleep by 9PM and waking up around 5AM

• Neutral: comfortable going to sleep by 11PM and waking up around 7-8AM

• Moderate evening type: comfortable going to sleep by 12-1AM and waking up around 9-10AM

• Extreme evening type: comfortable going to sleep by 2-3AM and waking up around 10-11AM

As you can tell, the times given above are structured around the assumption of an 8 hour night which for most people is neither necessary nor realistic, but these ranges give you a good idea of the large variation in sleep needs that could exist between an extreme morning type that prefers 7 hours of sleep and an extreme evening type that needs 10 hours of sleep to feel their best.  

Why does all of this matter?

Everyone can speak to how awful it can feel to undersleep, but there are graver consequences to lack of sleep than just feeling tired. As we’ve improved our ability to measure sleep, we’ve also improved our ability to measure the bodily effects of a lack thereof, and the data only serves to reinforce how vital quality and regularity of sleep is to our health, even more than quantity. Meta-analyses have strongly correlated these metrics with decreases in mortality from heart disease, cancer, and other illness, with an effect twice as strong as the decrease achieved by quantity of sleep. These strong correlations also suggest the inverse is true: having irregular, low quality sleep presumably plays a role in increasing our susceptibility to things like heart disease and cancer, not to mention the effects of a weakened immune system and compromised stress tolerance. Understanding how we sleep and what sleep variables we should truly care about could perhaps be the best health insurance you never have to pay for.

On a societal note, the validated existence of chronotypes should cause us to stop and wonder if our culture’s continued preference of “morning people” is doing lasting damage to the many among us who sleep better on a later time frame. As we continue to experiment with work from home and flexible work weeks, perhaps we should innovate in other areas, such as schooling, rather than hold enter subgroups of our population to rigid schedules which actively harm their health.