The early bird and the night owl: A look at differences in our sleep behavior

November 21st, 2023

Written by: Jafar Bhatti

We have all heard how important sleep is for our mental and physical well-being. In fact, scientists agree that most adults should receive between seven and nine hours of high-quality sleep per night¹. But what most scientists fail to mention is the particular time of day (or more accurately, time of night) that those hours of quality sleep should fall within. You may know some “night owls” that fall asleep later in the night and wake up later in the morning, or “early birds” that fall asleep earlier at night and wake up earlier in the morning. Formal research on these differences in sleep behavior, which are now referred to as an individual’s chronotype, began in the early 1970s and has since been an active area of research. In this post, I will talk about what defines a chronotype, how we measure chronotypes, and how one’s chronotype may predict their behavioral and health outcomes.

What is a chronotype?

A chronotype refers to an individual’s tendency to sleep at a particular time during a 24-hour period. Although the term chronotype was not used until 1989, the observation of individual differences in sleep behavior has been readily discussed as early at the start of the 20th century. Even at this early point, it was recognized that there are two broad categories of sleepers: morning-type and evening-type. Those who are classified as morning-type (i.e., early birds) tend to sleep later at night and wake earlier in the morning. Alternatively, those who were classified as evening-type (i.e., night owls), tended to sleep later at night and wake later in the morning.

Early research on these two chronotypes (referred to as morningness and eveningness chronotypes) found that while the total amount of sleep received between the two groups did not differ, there were some biological and behavioral differences. Biologically, scientists found that the time at which internal body temperature peaks during the day differs between morning-type and evening-type individuals^2,3. Morning-type individuals show peak internal body temperature significantly earlier in the day than evening-type individuals. There are also differences in when individuals of different chronotypes choose to engage in daily physical activity. As expected, morning-type individuals prefer to engage in physical activity earlier in the day and evening-type individuals prefer to engage in physical activity later in the day⁴. However, in regard to social activity, studies have failed to show any reliable relationship between one’s personality type, such as introversion and extraversion, and chronotype^3,5.

What makes someone a morning person or an evening person? Just like many other aspects of our lives, the factors contributing to chronotype are likely a combination of environmental and genetic factors. In regard to environmental factors, studies have found that the amount of light that individuals view during early and late hours of the day can impact one’s chronotype⁶. This is consistent with the plethora of studies showing that light exposure influences the body’s internal clock that governs waking and sleeping states, called the circadian rhythm. In regard to genetic factors, two recent studies collected data from over 100,000 individuals to identify changes in genes that could explain individual chronotypes. These studies found links between one’s chronotype and changes in 22 genes located near other genes that influence your circadian rhythm. ^7,8

How are chronotypes measured?

Researchers can measure an individual’s chronotype by surveying them using special questionnaires. The most commonly used questionnaire is called the Morning-Eveningness questionnaire (MEQ) and was originally developed in 1976². This questionnaire categorizes individuals into “morning type”, “evening type”, or “intermediate type” by asking participants about their preferences for sleep/wake time as well as how alert they feel throughout the day. Using the MEQ, in addition to other similarly designed questionnaires, researchers can estimate the number people who fall into one category or another. Studies have shown the occurrence of different chronotypes follows what is known as a normal distribution (Figure 1). This suggests that most people sleep around the same time and there are fewer people who sleep much earlier or later than the average person. While this pattern of occurrence is the same across different cultures and societies,⁹ there are differences between younger and older individuals. In general, younger people have sleep behavior more consistent with the eveningness chronotype and older people have sleep behavior more consistent with morningness chronotype¹⁰.

Does chronotype play a role in our health?

Before discussing any of the findings regarding chronotype and health outcomes, it is worth mentioning that these studies are correlational. This means that researchers observe a trend between two variables. In this case, one of the variables is chronotype the other variable is some health outcome. While correlational studies can be useful for making predictions about the world, it is important to keep in mind that these kinds of study do not imply that one variable is causing another.

With that said, the studies investigating chronotype and health outcomes tend to find more adverse health outcomes for individuals for the evening chronotype compared to the morning chronotype. One example of this is obesity. Studies have found that individuals with the evening chronotype tend have higher rates of obesity and cardiovascular disease compared to individuals with the morning chronotype^11,12,13. Follow-up studies on this work have hinted at differences in eating patterns and overall nutrition as a possible explanation for this¹⁴. Another outcome related to chronotype that has been studied is mental health. Studies have found that the evening chronotype is associated with a number of mental health disorders including substance use disorder, major depressive disorder, and general anxiety^15,16.

While this body of work does not appear hopeful for those with the eveningness chronotype, another set of studies has a more positive outlook for individuals in this group. One study found that people with the eveningness chronotype have higher cognitive ability^17,18. Furthermore, a different study found that evening-type individuals had enhanced creative thinking compared to morning-type individuals¹⁹.

Readers, especially those who self-identify more with the evening chronotype, should take these studies with a grain of salt. As mentioned earlier, these studies are correlational and do not show that eveningness causes negative health outcomes. Moreover, these studies typically collect data from a small number of individuals which make their results less meaningful to a general population.

Conclusion

To summarize, one’s chronotype is defined by their tendency to sleep at a particular time during a 24-hour window. While two major categories of chronotypes are defined, morning-type and evening-type, most people tend to fall somewhere in between. Research on chronotypes is ongoing with many questions left unanswered. For example, we know that chronotype changes with age, how do these changes take place?  Are these changes due more to our biology as we age or are they caused more by our changing environments? Additionally, more follow-up work is necessary to better understand what underlying factors linking the evening chronotype and adverse health outcomes. With the growing interest in sleep research, I expect that we will learn answers to these questions and many more in the years to come.

References

1. Watson NF, Badr MS, Belenky G, Bliwise DL, Buxton OM, Buysse D, Dinges DF, Gangwisch J, Grandner MA, Kushida C, Malhotra RK, Martin JL, Patel SR, Quan SF, Tasali E. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015 Jun 1;38(6):843-4. doi: 10.5665/sleep.4716. PMID: 26039963; PMCID: PMC4434546.
2. Ho0072ne JA, Ostberg O. A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms. Int J Chronobiol. 1976;4(2):97-110. PMID: 1027738.
3. Horne JA, Ostberg O. Individual differences in human circadian rhythms. Biol Psychol. 1977 Sep;5(3):179-90. doi: 10.1016/0301-0511(77)90001-1. PMID: 922076.
4. Vitale JA, Roveda E, Montaruli A, Galasso L, Weydahl A, Caumo A, Carandente F. Chronotype influences activity circadian rhythm and sleep: differences in sleep quality between weekdays and weekend. Chronobiol Int. 2015 Apr;32(3):405-15. doi: 10.3109/07420528.2014.986273. Epub 2014 Dec 3. PMID: 25469597.
5. Kerkhof GA. Inter-individual differences in the human circadian system: a review. Biol Psychol. 1985 Mar;20(2):83-112. doi: 10.1016/0301-0511(85)90019-5. PMID: 3888298.
6. Beute F, Aries MB. The importance of residential dusk and dawn light exposure for sleep quality, health, and well-being. Sleep Med Rev. 2023 Oct 14;72:101865. doi: 10.1016/j.smrv.2023.101865. Epub ahead of print. PMID: 37864914.
7. Jones SE, Tyrrell J, Wood AR, Beaumont RN, Ruth KS, Tuke MA, Yaghootkar H, Hu Y, Teder-Laving M, Hayward C, Roenneberg T, Wilson JF, Del Greco F, Hicks AA, Shin C, Yun CH, Lee SK, Metspalu A, Byrne EM, Gehrman PR, Tiemeier H, Allebrandt KV, Freathy RM, Murray A, Hinds DA, Frayling TM, Weedon MN. Genome-Wide Association Analyses in 128,266 Individuals Identifies New Morningness and Sleep Duration Loci. PLoS Genet. 2016 Aug 5;12(8):e1006125. doi: 10.1371/journal.pgen.1006125. PMID: 27494321; PMCID: PMC4975467.
8. Hu Y, Shmygelska A, Tran D, Eriksson N, Tung JY, Hinds DA. GWAS of 89,283 individuals identifies genetic variants associated with self-reporting of being a morning person. Nat Commun. 2016 Feb 2;7:10448. doi: 10.1038/ncomms10448. PMID: 26835600; PMCID: PMC4740817.
9. Chauhan S, Norbury R, Faßbender KC, Ettinger U, Kumari V. Beyond sleep: A multidimensional model of chronotype. Neurosci Biobehav Rev. 2023 May;148:105114. doi: 10.1016/j.neubiorev.2023.105114. Epub 2023 Mar 1. PMID: 36868368.
10. Duffy JF, Czeisler CA. Age-related change in the relationship between circadian period, circadian phase, and diurnal preference in humans. Neurosci Lett. 2002 Feb 1;318(3):117-20. doi: 10.1016/s0304-3940(01)02427-2. PMID: 11803113.
11. Ekiz Erim S, Sert H. The relationship between chronotype and obesity: A systematic review. Chronobiol Int. 2023 Apr;40(4):529-541. doi: 10.1080/07420528.2023.2180385. Epub 2023 Feb 20. PMID: 36803075.
12. Makarem N, Paul J, Giardina EV, Liao M, Aggarwal B. Evening chronotype is associated with poor cardiovascular health and adverse health behaviors in a diverse population of women. Chronobiol Int. 2020 May;37(5):673-685. doi: 10.1080/07420528.2020.1732403. Epub 2020 Mar 4. PMID: 32126839; PMCID: PMC7423711.
13. van der Merwe C, Münch M, Kruger R. Chronotype Differences in Body Composition, Dietary Intake and Eating Behavior Outcomes: A Scoping Systematic Review. Adv Nutr. 2022 Dec 22;13(6):2357-2405. doi: 10.1093/advances/nmac093. PMID: 36041181; PMCID: PMC9776742.
14. Phoi YY, Rogers M, Bonham MP, Dorrian J, Coates AM. A scoping review of chronotype and temporal patterns of eating of adults: tools used, findings, and future directions. Nutr Res Rev. 2022 Jun;35(1):112-135. doi: 10.1017/S0954422421000123. Epub 2021 May 14. Erratum in: Nutr Res Rev. 2023 Jun;36(1):176. PMID: 33988113.
15. Taylor BJ, Hasler BP. Chronotype and Mental Health: Recent Advances. Curr Psychiatry Rep. 2018 Jul 23;20(8):59. doi: 10.1007/s11920-018-0925-8. PMID: 30039327.
16. Zou H, Zhou H, Yan R, Yao Z, Lu Q. Chronotype, circadian rhythm, and psychiatric disorders: Recent evidence and potential mechanisms. Front Neurosci. 2022 Aug 10;16:811771. doi: 10.3389/fnins.2022.811771. PMID: 36033630; PMCID: PMC9399511.
17. Franzis Preckel, Anastasiya A. Lipnevich, Sandra Schneider, Richard D. Roberts,  Chronotype, cognitive abilities, and academic achievement: A meta-analytic investigation, Learning and Individual Differences, Volume 21, Issue 5, 2011.
18. Kyle SD, Sexton CE, Feige B, Luik AI, Lane J, Saxena R, Anderson SG, Bechtold DA, Dixon W, Little MA, Ray D, Riemann D, Espie CA, Rutter MK, Spiegelhalder K. Sleep and cognitive performance: cross-sectional associations in the UK Biobank. Sleep Med. 2017 Oct;38:85-91. doi: 10.1016/j.sleep.2017.07.001. Epub 2017 Jul 14. PMID: 29031762; PMCID: PMC5930168.
19. M. Giampietro, G.M. Cavallera, Morning and evening types and creative thinking, Personality and Individual Differences, Volume 42, Issue 3, 2007.

Cover Photo by Claudio Scout (left) and Jill Wellington (right) from pixabay.com

Figure 1 made in BioRender by Jafar Bhatti