In my research and my work with patients, I was never really comfortable with the three-category break-out of chronotypes. The Morning-Eveningness Questionaire (MEQ) doesn’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 don’t take into account the personality differences that are 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 are distinct—and who definitely do not fall within any of the two or three established chronotypes.
This group is people with chronic insomnia, who tend to sleep less than 6 hours a night, who have constant trouble falling asleep and/or staying asleep through the night, who tend to be restless and tired during the day, and restless and wired at night. This group of people, who make up a significant share of my patients and about 10 percent of the general population, appear to fall clearly in an altogether different chronotype.
My years-long deep dive into chronobiology research showed that this group is, 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 their brains are biologically hardwired 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 the 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 percent 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 the Bear circadian clock 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 percent 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.
About 15-20 percent 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 that 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 percent of the population are Dolphins.