November 23, 2009 by Shawn Smith
Q: Can you explain why, when I have no external forces determining my schedule, I tend to gravitate naturally toward a nocturnal state? -Marty
Humans are unique in the amount of variability we display in our sleep schedules. Several of my immediate family members, for example, prefer to rise at an obscenely early hour, while I prefer to rise late and work late. If it weren’t for the time constraints imposed by civilized society I might never see them.
It seems that most people express a preference toward one schedule or the other. But biologically speaking, most people don’t fall into either camp. Only 30 to 40 percent of us have a strong enough preference to be classified by psychologists as either “evening types” or “morning types.” The majority of us are adaptable to a variety of sleep schedules.
Which means you, Marty, are a bit of an oddball. I’m sure you have noticed that being a night-owl involves sacrifices, not the least of which is the McDonald’s breakfast menu. But don’t despair – there are advantages associated with your sleeping preference.
Before we get to that, let’s talk about what might be causing your biological sleep preference in a little section I like to call…
What Might Be Causing Your Biological Sleep Preference
Sleep and wakefulness are so complex that it’s a wonder they function at all (see, for example, Rosenwasser, 2009). Out of this complex system, there are a couple of important biological markers that can be used to measure sleep preference.
The first is melatonin release. This hormone is associated with sleep onset and makes its appearance after the onset of darkness. The second is body temperature, which decreases with sleep and reaches a low point during the second half of the sleep cycle. Research on “morningness” and “eveningness” tends to rely on those markers, along with a commonly accepted sleep preference questionnaire. That is not a terribly interesting point, but I wanted to assure you that there is standard methodology behind this research.
According to a reasonably thorough study, roughly 50% of sleep-preference variability is accounted for by genetics (Markku et al. 2007). Other factors include:
Gender: Men tend toward evening preferences more than women, and women tend toward morning preference more than men. Adan & Natale (2002) and others have suggested that men and women possess slightly different circadian systems (the parts of the brain that maintain sleep rhythms). Women tend to be more sensitive to environmental cues concerning day and night, and men’s sleep systems seem more adaptable to changes in sleep schedules.
Age: Aging is associated with a shift toward morningness. Actually, that’s not quite accurate. Aging is associated with a decreasing interval between the minimal sleep temperature and habitual wake time, but let’s not get too technical. Suffice to say that, with age, we sleep less and wake up earlier. If you and I live long enough, Marty, we might become morning people. Children also tend toward morningness, and adolescence is characterized by a shift toward eveningness.
Geographic latitude: If you want to be a morning person, you should plan ahead and be born in a location where the winter days are long. Latitude affects both light and temperature, which in turn affect sleep patterns. Ciarleglio et al. (2008) noted genetic sleep phase differences between populations in different latitudes. Northern latitudes, for example, have been associated with delayed sleep phase syndrome (which is not to say that latitude causes it). This syndrome is a condition in which a person prefers to go to sleep quite late – sometimes as late as 4:00 AM – and awaken mid-day, after a normal but delayed sleep period. That is not a problem by itself, but it can become one in a world that keeps banker’s hours.
Season of birth: The time of year in which you were born can have a small but statistically significant effect on sleep phase preference. Those born in summer months show a slight tendency toward evening preference.
Social habits and culture: As a night owl, I’m sure you have noticed the pressure (or the desire) to conform to a more normative sleep schedule. Forcing yourself into a schedule that doesn’t jibe with your natural preference can be difficult at first, but humans are nothing if not adaptable.
So, Marty, your natural nocturnal preference could be a result of any of those factors. Or all of them. Or none of them. I hope that clears things up. My advice: don’t fight too hard against Mother Nature.
Of course, sleep preference research doesn’t end there. As always, psychologists want to find out what it all means.
What does your sleep preference say about you?
As with any trait that distinguishes one person from another, psychologists have tried to identify personality traits that correspond with sleep preference. Here are a few of the findings from the literature:
Conscientiousness, neuroticism, and extraversion: Tonetti et al. (2009) reported that morning types score higher on measures of conscientiousness than evening types, while evening types score higher on neuroticism scales. Some studies have found a positive correlation between eveningness and extraversion, though a few studies have contradicted that finding.
Intelligence: Kanazawa & Perina (2009) have suggested that “more intelligent individuals are more likely to be nocturnal than less intelligent individuals.” They base their theory on the evolutionary principle that adopting “evolutionarily novel values and preferences” is a characteristic of more intelligent people. For example, prior to major technological advances, it made sense to move about during the day when a person could be most effective. The authors contend that unique behaviors, like running around in the dark when everyone else had the good sense to be asleep, required a higher level of cognitive complexity. Bringing the question to the present day, the authors were able to identify correlations between intelligence and waking time, with higher intelligence associated with slightly later waking time.
A few miscellaneous findings: In a literature review on the topic of sleep preference and personality, Cavallera and Giudici (2008) found, among other things:
- Evening types may be less dependable than morning types.
- Morning types perform better on intelligence tests in the morning, while evening types do better in the afternoon – except on spatial tests, in which case the results may be reversed.
- Evening types display better immediate recall and commit fewer recognition errors on intelligence tests.
- Evening types display more flexible sleep habits than morning types.
- Evening types are less emotionally stable than morning types.
- Evening types tend toward depression, addiction, and eating disorders.
There were a several other gems from a fairly broad body of research, with some findings being more dependable than others. It’s an interesting research overview, if you are looking for some bedtime reading.
Take this personality research with a grain of salt. Sleep habits don’t define a person. If these findings suit you, then play to your strengths. If not, forget about them. As for me, the sun is rising which means it’s time for bed.
Adan, A. & Natale, V. (2002). Gender differences in morningness-eveningness preference. Chronobiology International, 19(4), 709-720.
Cavallera, G.M. & Giudici, S. (2008). Morningness and eveningness personality: A survey in literature from 1995 up till 2006. Personality and Individual Differences, 44, 3–21.
Ciarleglio, C.M., Ryckman, K.K., Servic, S.V., et al (2008). Genetic differences in human circadian clock genese among worldwide populations. Journal of Biological Rhythms, 23(4), 330-340.
Kanazawa, S. & Perina, K. (2009). Why night owls are more intelligent. Personality and Individual Differences, 47, 685-690
Korczak, A.L, Martynhak, B.J., Pedrazzoli, M., et al. (2008). Influence of chronotype and social zeitgebers on sleep/wake patterns. Brazilian Journal of Medical and Biological Research, 41(10), 914-919.
Markku, K., Hublin, C., Markku P., et al. (2007). Heritability of diurnal type: a nationwide study of 8753 adult twin pairs. Journal of Sleep Research, 16, 151-162.
Rosenwasser, A.M. (2009). Functional neuroanatomy of sleep and circadian rhythms. Brain Research Reviews, 61, 281-306.
Tonetti, L., Fabbri, M., and Natale, V. (2009). Relationship between circadian typology and Big Five personality domains. Chronobiology International, 26(2), 337-347.
Mongrain, V., Lavoie, S., Selmaoui, B., et al. (2004). Phase relationships between sleep-wake cycle and underlying circadian rhythms in morningness-eveningness. Journal of Biological Rhythms, 19(3), 248-257.
Natale, V., Adan, A., & Fabbri, M. . Season of birth, gender, and social-cultural effects on sleep timing preferences in humans. Sleep, 32(3), 423-426.