You know I write a lot about chronotypes—I wrote a whole book about them!
Why is knowing your individual chronotype so important? It’s a roadmap for optimizing nearly every aspect of your daily life. Our daily circadian rhythms regulate sleep, hunger, metabolism, immunity, cognition, desire, creativity, sociability and nearly all of your body’s physiological activity, rising and falling throughout the 24-hour day, each playing their part in the incredibly complex orchestra of your body’s daily functioning.
Understanding the timing of our biological rhythms tells us the optimal time to eat, sleep and exercise, how to organize our daily workflow, when to have sex (for maximum pleasure), when to argue with our partner (and when not to), when to take medicine and supplements, when to brainstorm a new idea and when to pitch that idea in a meeting…when to do just about everything. It’s the difference between working with your body’s biology, and fighting against it—which all too many of us do, when we live our lives out of sync with our circadian rhythms.
Science and medicine haven’t always taken into account the importance of circadian timing—the when of daily life. Far from it. We’re still at the dawn of using our bodies’ daily rhythms to live longer, healthier, happier, more productive and better rested lives.
Our understanding of chronotype is still relatively new, but we’ve learned a tremendous amount in the past 50 years or so. Today I thought we’d take a little step back and look at how chronotype theories have evolved, and why chronotypes had to move beyond morning, evening and in-between types—and beyond sleep itself.
The two types: morning and evening
Scientists began observing evidence of the 24-hour circadian cycle in nature in the 18th century. But it wasn’t until the 1970s that scientists began to put real attention toward studying circadian rhythms in humans, and developed the first tool to measure and assess individual chronotypes.
At first there were two major categories for chronotype: morning and evening. The Morning Evening Questionnaire, developed in the 1970s, gathered information from individuals about sleep and activity to determine whether a person was a “morning” type, an “evening” type, or an “intermediate” type. The MEQ took self-reported information from individuals about their daily sleep-wake habits and analyzed that information with detailed measurements of body temperature changes to determine whether a person had a circadian clock that tilted early or late, or fell somewhere in between. (Remember, body temperature follows a daily rhythm, and evening changes in temperature are a critical part of transitioning to sleep.)
Morning and evening types continue to be used as the two primary chronotypes by many clinicians in assessing patients, and in a lot of the scientific and medical research about how chronotype affects behavior, health, mood, cognition, and sleep.
But many people don’t fit into one of two types. Think about it. If you had to choose, you could probably pick some degree of preference for morning or evening, in terms of your sleep and activity levels. And some people easily and clearly identify as morning people or night people. But a great many of us don’t. Maybe it’s because our preferences don’t fall clearly to either of these extremes. Maybe it’s because we recognize ups and downs of energy and sleepiness, variable moods, and different types of mental focus, throughout the day and night. And it’s also because there are other factors that contribute to identifying with a chronotype, beyond sleep-wake and activity patterns—including personality and behavior.
The three birds: larks, hummingbirds, owls
This general classification system for chronotypes is based on the fundamental distinction between morning-evening chronotypes and also recognizes the great number of people who fall somewhere in between.
Larks are the morning types, people who wake early, keep early bedtimes, and experience their peak productivity and alertness in the morning and early afternoon.
Owls are up and about at night, evening types who wake up later in the morning, go to bed late in the evenings, and hit their natural focus and productivity peak in the evening hours.
Hummingbirds are, well, everybody else. There’s a big middle ground between Larks and Owls, filled with people who don’t fit naturally into either type. As with the MEQ, this three-chronotype categorization theory doesn’t dig deeply into the chronobiological differences for people in this middle-of-the-road category.
As research into chronotype and circadian rhythms progressed in the late 20th century and into the 2000s, we learned a lot more about how individual chronotype influences sleep, but also behavior, personality, and health.
For example, scientific study showed us that morning types (Larks) tend to be more conscientious, agreeable and positive minded, pro-active than other chronotypes. They also tend to be more proactive (willing to take action to achieve advantageous change) and less prone to procrastination.
Research found evening types tended to be more extroverted and more impulsive, and greater seekers of novelty (new and different experiences) than other chronotypes. Being an evening type, studies found, was also linked to greater risk for depression, and more tendency toward risk-taking behaviors, as well as higher consumption of alcohol, nicotine and caffeine.
The four chronotypes: Lions, Bears, Wolves, Dolphins
In my research and my work with patients, I was never really comfortable with the three-category break-out of chronotypes. The MEQ didn’t take into account what we know about sleep drive, and how sleep drive works in tandem with circadian sleep-wake preferences to create our individual sleep profile. Like circadian rhythms for sleep, sleep drive is also genetically determined. Some of us are biologically wired with a low sleep drive, others have a medium sleep drive, and some of us have a high sleep drive.
And assessments of chronotype based on sleep-wake activity didn’t take into account the personality differences that were such a clear component of chronotype, as so many studies have shown.
Over the years, I’ve treated thousands of people with a range of sleep problems, and among them are a particular group of people with a set of sleep patterns, activity preferences, personality and behavioral traits that were distinct—and who definitely did not fall within any of the two, or three, established chronotypes.
This group was people with chronic insomnia, who tended to sleep less than 6 hours a night, who had constant trouble falling asleep and/or staying asleep through the night, who tended to be restless and tired during the day, and restless and wired at night. This group of people, who made up a significant share of my patients and about 10% of the general population, appeared to fall clearly in an altogether different chronotype. My years-long deep dive into chronobiology research showed that this group was, in fact, genetically distinct. People with chronic insomnia have a genetically determined low sleep drive, biochemical patterns for hormones and cardiovascular activity that are the inverse of the other chronotypes, and brains biologically hard-wired to be active and aroused at night.
That’s how I came to establish four chronotype categories. I looked to the world of mammals, rather than birds, to name them—we are mammals, after all.
Lions are morning hunters, and people who are Lion chronotype are the early risers of the world. Lions are optimistic, naturally disciplined (including about their sleep routines) practical and goal oriented. Lions are generally good sleepers, with a medium sleep drive. It’s rare to find a Lion who struggles much to stick to a regular, early bedtime. Lions have a natural tendency for routine and moderation in their daily habits, and this shows in their overall health picture: studies show that morning types with early bedtimes have a lower risk for cardiovascular disease, less obesity, and may have lower risks for mental health disorders, including depression, anxiety and others. Lions leap into their days full of energy; the morning and early afternoon are when Lions are at their most productive.
About 15-20% of the general adult population are Lions.
Bears are all-day hunters, and Bear chronotypes are go-with-the-flow types with middle-of-the-road sleep-wake preferences. Of the four chronotypes, Bears adhere most closely to a solar schedule. Bears are most alert and productive during the middle of the day, from late morning through early afternoon.
Bears are easygoing and social, fun-loving team players. They have a high sleep drive and tend to sleep deeply. But many Bears carry a sleep debt—they don’t get enough sleep to meet their needs. Bears are prone to inconsistency in their sleep routines. They often under-sleep during the workweek and sleep extra on the weekend to make up for their insufficient rest. Inconsistent sleep habits can put Bears circadian clocks chronically out of sync. Social jet lag has been linked to higher BMI and greater risk for obesity. Excess weight is a common challenge among Bears, who tend to carry weight particularly around their midsection. Their vulnerability to social jet lag is one highly likely factor.
Bears are the most common chronotype—about 50% of the adult population are Bears. Because it’s the most common chronotype, Bear time has a dominant influence over our broader social time. Six o’clock is the standard dinner hour because that’s when Bears are ready for their evening meal. Remember settling in to watch your favorite TV show at 10 p.m.? That’s when Bears are ready to lay low, but aren’t quite prepared to fall asleep. Amid a majority of Bear chronotypes, modern society has long adopted Bear time as the norm. In large part, all the other chronotypes live on Bear time, when it comes to daily social schedules for school, work, and socializing.
Wolves are nighttime hunters, and the Wolf chronotype has a strong preference for evenings. Wolves are the people who drag themselves out of bed before 9 a.m. and don’t start feeling really tired until midnight or so. Wolves are creative, impulsive, and emotionally intense. They love to seek out new experiences and are natural risk takers. Wolves have a medium sleep drive, with peaks of productivity in the late morning and again in the late evening.
Because of their strong preference for evening hours, Wolves often struggle with living according to the schedule society demands of them. Things like work and school get going too early, and social fun ends too soon. Wolves are highly vulnerable to chronic social jet lag and insufficient sleep—and that can have consequences for their mental and physical health. Research shows evening chronotypes are at greater risk for chronic diseases including cardiovascular illness and diabetes, and for depression.
Wolves typically perform at their best with around seven hours of sleep. Getting that much sleep can be tough for Wolves, because their biological rhythm is so at odds with society’s timetable for daily life. (I’ve written about how some Wolves may have gotten a bit of a break from this struggle during the pandemic.)
About 15-20% of the population are Wolves.
The dolphins of the mammalian world are uni-hemispheric sleepers. That means they sleep with one-half of their brain at a time, with the other half awake and active. That’s a pitch-perfect analogy for this fourth chronotype of restless, light sleepers. As I’ve said, Dolphins are “wired and tired” types—chronically tired during the day and wired with restless, nervous energy at night. Dolphins are light and restless sleepers with a low sleep drive, who tend to wake frequently during the night. Their minds are active in the evening, with often racing thoughts, and they feel physically keyed up.
There are biological reasons for Dolphins’ nighttime restlessness and agitation. It turns out, Dolphins have a circadian biology that is turned upside down. In contrast to other chronotypes, Dolphins’ brain activity increases at night, in areas of the brain that promote alertness. And also unlike other chronotypes, Dolphins’ blood pressure and cortisol levels rise in the evening, which leaves them in a state of physiological arousal at bedtime. Come morning, when other chronotypes are experiencing elevations to blood pressure and cortisol that are fueling their morning alertness, Dolphins’ levels are plummeting.
Personality-wise, Dolphins are highly intelligent, cautious, detail oriented (perfectionism is a common Dolphin trait), and often anxious. As I’ve said, about 10% of the population are Dolphins.
Do you know your chronotype yet? Take this short quiz: www.chronoquiz.com.
Even though chronotypes are genetically determined, they do change over our lifetimes. As young children, we start life as Lions, before becoming Wolves as teens. In young adulthood, usually around mid-twenties, we settle into one of the four chronotypes for a large chunk of adulthood. As we age, chronotypes sometimes change; more older adults become early-rising Lions, or restless-sleeping Dolphins.
And not every person fits comfortably within one of the four chronotypes. Some people have a hybrid chronotype. Here’s how to know if you’re a hybrid chronotype. And here’s where you can read about to handle the sleep and productivity challenges facing each chronotype during the coronavirus.
Are there six chronotypes?
Chronobiology is a hot topic in science—with good reason. Gaining a stronger, more nuanced understanding of how circadian timing works and varies among individuals has enormous implications for sleep and performance, for mental and physical health and the treatment of disease, for extending longevity.
Recently, scientists in Russia have suggested there are six different chronotypes. They came to their new chronotype model through a series of studies that assessed sleep-wake routines—including the timing of sleep as well as sleep duration and the ability to wake up and fall asleep when needed— and patterns of alertness and energy throughout the day.
Two of the chronotypes in this model are the long-identified early and late types:
Morning types, who experience high alertness and energy early in the day, with a gradual decline in alertness throughout the afternoon, and low alertness in the evening. The research found 13% of people were morning types.
Evening types, who have low alertness early, followed by a gradual rise in alertness and energy throughout the afternoon, and high alertness at night. 24% of people were evening types.
The four additional chronotypes reflect variable alertness and energy throughout the daytime, in contrast to the linear decline in alertness (morning types) and linear increase in alertness (evening types) of the other two types.
Daytime sleepy types start the day with high alertness, experience a dip in alertness and energy during the middle of the day, and rebound in the evening to medium alertness. 18% of people were daytime sleepy types, in the study.
Daytime active types experience low alertness and energy in the morning, a peak of high alertness in the middle of the day, and medium alertness and energy in the evening. 15% of people were found to be daytime active types.
Moderately active types experience low energy and alertness throughout all phases of the day, morning, afternoon, and evening. The researchers found that 15% of people were moderately active types.
Highly active types have high energy and alertness all day long, from morning to evening. In the study, 9% of people were highly active types.
There’s a lot that’s interesting in this research and I look forward to seeing more investigation from these scientists. A few thoughts. The fact that the population of the study group was so broadly spread across six chronotypes further supports the idea that most people don’t fit a classic morning or evening chronotype, that there are a range of chronotypes, with distinctly different sleep and waking activity patterns and preferences–and distinct needs for schedules and routines that align with those preferences, in order to live healthier, happier, more productive lives, and to sleep better.
And personality matters a great deal to the process of accurately identifying chronotype. I’ve seen it in research, and in the thousands of patients that I’ve treated over the years. Our chronotype identity is complex and multi-factorial, and goes beyond sleep patterns, sleep drive, and daily rhythms of alertness. How we think, act, and feel, how our moods express themselves (and how we respond to our emotions and emotional challenges), how we function in one-to-one relationships, in groups and teams, and in society more broadly—all these facets of personality (and more) are in part expressions of chronotype, and it’s critical we take personality into account.
We’re going to keep learning more about chronotype and how it affects our biology, our sleep and health, our relationships, and our performance—and we’ll keep talking about it ALL here.
Michael J. Breus, PhD, DABSM
The Sleep Doctor™