Chronotype & circadian rhythm

Chronotype and circadian rhythm are closely related, though different. According to the Sleep Foundation, “while circadian rhythm can be ‘trained’ by adhering to a strict schedule, the underlying chronotype exists on a more permanent basis.”

Chronotype is the natural inclination of your body to sleep at a certain time, or what most people understand as being an early bird versus a night owl. In addition to regulating sleep and wake times, chronotype1 has an influence on appetite, exercise, and core body temperature. It is responsible for the fact that you feel more alert at certain periods of the day and sleepier at others.

Chronotype vs. Circadian Rhythm

Sleep chronotype is closely related to circadian rhythm, which controls the day-to-day sleep-wake cycle and releases melatonin in response to environmental cues such as light and temperature. However, while circadian rhythm can be “trained” by adhering to a strict schedule, the underlying chronotype exists on a more permanent basis.

Thus, a natural night owl may be able to wake up at 7 am every day for work, but they may not be productive until later in the day. Conversely, an early bird may wake up bright and chipper for their 7 am shift, but then start to feel sleepy already in the late afternoon.

Chronotype does not influence total sleep time. If most adults need between seven and nine hours of sleep a night, this is usually much easier to accomplish for an early bird than for a night owl, who has trouble falling asleep before 1 am. For this reason, night owls have historically faced more difficulty adapting to typical work schedules.

Scientists consider it very difficult or impossible to purposely change your chronotype, though it may shift throughout the course of your life. When a person’s natural chronotype comes into conflict with the demands of their schedule, this is termed social jetlag2.

People who have a later chronotype may suffer from social jetlag and feel permanently tired if they need to wake up early for work or school. Likewise, those who prefer to go to bed earlier may not do well with social or cultural activities that are programmed later in the evening. For both groups, trying to perform activities that require concentration or creativity may be difficult at non-peak times.

What Determines our Chronotype?

Chronotype can vary from person to person depending on genetics, age, and other factors. Some scientists believe that chronotype may differ according to geographical location as well, due to changes in daylight hours.

As a general rule, most children have an early chronotype. Beginning in adolescence, chronotype is pushed back, leading to the myth that teenagers are lazy because they find it difficult to wake up for school. Chronotype then gradually shifts earlier and earlier starting from the age of 20. The majority of middle-aged American adults do best with a bedtime between 11 pm and 12 am, and a wake-up time between 7 am and 8 am. In older adulthood, our chronotype shifts even earlier.

Females tend to have an earlier chronotype than males, though some studies find that this gap disappears after approximately age 50. It’s possible that the differences between the genders are simply a product of societal factors such as household tasks, career progression, and retirement, which tend to follow different patterns for women and men.

Emerging evidence shows that chronotype likely has a strong genetic component. Among other things, having a longer allele on the PER circadian clock gene has been tied to morningness. Some researchers postulate that the variation in chronotype might have been a survival technique that evolved in hunter-gatherers. The theory is that by taking turns sleeping, there would always be someone awake to keep watch.

While most chronotypes fall within a reasonable range, the total possible range of bedtimes stretches as long as ten hours between extreme morning types and extreme evening types. Individuals whose chronotypes make it difficult to adhere to the demands of a normal schedule may be diagnosed with advanced or delayed sleep-wake phase disorder.

The importance of the Chronotype

Multiple studies have found associations between chronotype and personality, health, and quality of life.

Personality traits associated with morningness include conscientiousness and agreeableness. By contrast, neuroticism and openness to experience are typically related to eveningness. Studies have found conflicting evidence for whether extraversion is more representative of morning or evening types.

Morning people tend to perform better in school, while evening types may have more of an aptitude for creative thinking. It is difficult to say whether these traits are innate or whether they are due to secondary factors, such as the fact that school tends to start early in the day and many creative professions require people to be active in the evening.

Evening people tend to have more flexible sleep schedules, be less physically active, and sleep less on weekdays, making up the lost time by sleeping in on the weekend. These unhealthy habits lead to an increased stress response, higher cortisol levels, and a higher resting heart rate, which are risk factors for sleep apnea, obesity, type 2 diabetes, mental disorders, and metabolic syndrome.

Eveningness is also linked to impulsivity, anger, depression, and anxiety, as well as a host of negative habits including risk-taking, skipping breakfast and eating more in the evening, using more electronic media, and use of substances such as tobacco, alcohol, and caffeine.

It’s important to remember that chronotype likely interacts with many other factors to produce these trends. For example, a propensity for substance abuse in evening types may arise as a side-effect of depression and anxiety, which were in turn provoked by sleep deprivation due to social jetlag. Therefore, while some personality traits may depend on genetics, they are more likely the result of irregular sleep schedules caused by forced adaptation to earlier wake times.

Many of these adverse outcomes are also linked specifically to a mismatch between chronotype and work schedule, regardless of whether the person is an early bird or a night owl. This reinforces the idea that the easiest way to improve worker health may be simply to try matching shifts to a person’s chronotype. Unfortunately, this is not always possible, and it may severely limit career choices.

For those who must adhere to a routine that does not match their chronotype, melatonin supplements, light therapy, or careful attention to sleep hygiene habits may help shift circadian rhythm to reduce insomnia and the effects of social jetlag. However, most people find they are unable to permanently change their chronotype.

Types of Chronotypes

Scientists usually describe two chronotypes: eveningness and morningness, otherwise known as night owls and early birds (or “morning larks”).

In truth, chronotypes fall on a spectrum, with most people lie somewhere in between. Researchers refer to these in-between people as intermediate types or “hummingbirds.” Some researchers have now added a fourth category, “bimodal”, to reinforce the fact that some people identify more with morningness in some ways and with eveningness in other aspects.

For research purposes, scientists have developed several questionnaires that categorize subjects by morningness versus eveningness tendencies. Two of the most popular questionnaires are the Morning-Eveningness Questionnaire (MEQ) and the Munich ChronoType Questionnaire (MCTQ). Each of these approaches chronotype from a slightly different angle, with the MCTQ focusing on actual wake and sleep times and the MEQ asking questions that encompass a range of activities such as meal and exercise times.

The variation in test questions is one reason why researchers have found it difficult to make accurate generalizations about the specific traits associated with each chronotype. However, it can still be useful to have an idea of our chronotype, so we can adjust our schedule accordingly.

What Is My Chronotype?

To figure out our chronotype, think about what time we would prefer to wake up on a day that we are completely free to plan, with no work or other requirements.

We already know whether we prefer waking up early or late. If not, many websites offer online quizzes that categorize our chronotype based on questions about our sleeping preferences, energy levels throughout the day, meal timing, and other facets of our circadian rhythm. The Morning Evening Questionnaire (MEQ) and the Munich ChronoType Questionnaire (MCTQ) are both available in an online version.

One of the most popular online quizzes was made by Dr Michael Breus, who describes four kinds of chronotypes, based on sleep-wake patterns seen in animals. Answering his online chronotype quiz will tell whether one is more of a bear, wolf, lion, or dolphin:

• Lion: The lion chronotype stands in for the early bird. These individuals wake up early and are most productive in the morning, but may have more trouble following a social schedule in the evenings.
• Bear: According to Dr Breus, the bear chronotype makes up about 55% of the population. People with this intermediate chronotype tend to follow the sun. They do well with traditional office hours but also have no problem maintaining a social life in the evenings.
• Wolf: The wolf chronotype is equivalent to the classic night owl, and is believed to make up approximately 15% of the population.
• Dolphin: The dolphin chronotype is based on the ability of real dolphins to stay alert even while sleeping. Human “dolphins” are best described as insomniacs.

While these types can give us a general idea of your ideal schedule, there will always be variations from person to person. Whether we identify with an animal chronotype or whether we simply know deep in our hearts that we prefer being awake at night, having a better understanding of how we are wired may help improve our sleep quality and quality of life.