Why are some People “Night-Owls” and others “Early-Birds

Reviewed by Sahil Chopra, MD, and Stacey Gunn, MD.

Research by Savit Malhotra and Theresa Do.

Introduction

For the past few months, we have spent a significant amount of time discussing the different disorders associated with sleep. While we will continue to discuss these disorders in future articles, we want to take a moment to discuss some of the actual biology behind sleep. Although we won’t be discussing this in a series format, all of these independent articles will slowly tie into each other. Previously, we talked a little about circadian rhythms, how they are established, and how they can be influenced. This week, we are going to be talking about what “early-bird” and “night-owl” are, the history behind these two circadian groups, and the stigma surrounding these classifications. 

What are “Early-Birds” and “Night-Owls?”

The phrase “early-bird” is an idiom used to describe a person who goes to bed and wakes up early, typically going to bed around 8 PM or 9 PM and waking up around 5 AM or 6 AM, or even earlier. Contrary to the “early-bird”, the phrase “night-owl” describes a person who goes to bed and wakes up later. Their bedtime is typically after midnight, and they sleep into the morning.^[1] 

At this point, you may be curious what percent of the population falls into each category. Different sources have estimated that anywhere from 10-20% of the population are considered to be extreme “early-birds”/”night-owls.”^[2] A 2024 study conducted on youth (18-24 years of age) in Turkey found that approximately 11.5% were “early-birds,” 22.4% were “night-owls,” and the remaining 66.1% were somewhere in the middle.^[3] A similar study looked at Canadian youth and found that the groupings were approximately 9%, 36%, and 55%, respectively.^[4] As we will discuss later in this article, societal norms might play a significant role in shaping these distributions. 

What Determines these Groupings?

Societal norms aside, there is some biology behind why people have varying bedtimes. The behavioral reflection of external and internal factors on each individual’s circadian rhythm is called their chronotype, or their body’s natural preference for certain sleep times.^[5] Chronotypes are developed largely by genetics, but can also be influenced by other external factors, such as light exposure, work schedules, and physical activity (see figure below for more examples). It is important to remember that chronotypes exist on a continuum rather than in rigid categories like “early birds” or “night owls”. This means that two “early birds” or two “night owls” may not have the exact same sleep times and circadian rhythms. Understanding this continuum may help explain the variations in sleep patterns.

Dr. Matthew Walker, a neuroscientist who specializes in sleep research, gave his theory behind why different chronotypes exist in his book Why We Sleep. In this book, Dr. Walker shared that chronotype determinations might be a result of when humans were still in hunter/gather tribes. In these tribes, there would often be a group of people who went to bed early (say, 9:00 PM) and another group that went to sleep later (around maybe 2:00 AM). The group that went to bed early would often wake up around 5:00 AM and the other group would wake around 10 AM (which closely aligns with our current definition of “night-owls” and “early-birds”). He speculates that this could have been evolutionarily advantageous, as one group would be able to stay awake longer and protect the tribe as a whole and the other tribe would wake up earlier to protect the tribe, resulting in there only being three hours where the group was left unprotected versus eight hours. Dr. Walker says that this longer period of protection could have accounted for a 50% increase in survival (Walker, 2017).

As discussed in previous articles, the body’s 24-hour internal clock, also known as the circadian rhythm, also plays an important role in managing sleepiness and alertness. It is controlled by the suprachiasmatic nucleus (SCN), which has been deemed as “the master circadian pacemaker”. The SCN is located within the brain’s hypothalamus and bases its regulation on light-dark input from the eyes.^[6] The SCN interacts with genes and other physiological systems to regulate when we feel most alert and when we need to begin winding down for sleep.

The SCN runs in the background as the pacemaker for alertness and sleepiness, and can be influenced by external factors (light, exercise, meals, socialization, etc), so that we can adjust to changes in our environment. In 1938, Dr. Nathaniel Kleitman and his graduate student Bruce Richardson from the University of Chicago embarked on a one month research project to see how sleep behaviors may be impacted by the lack of light.^[7] They stayed in a dark, chilly cave in Kentucky named Mammoth Cave, where they allowed their bodies to run on their intrinsic circadian rhythm without cues from light, and at one point attempted to modify their circadian rhythms from the typical 24-hour rhythm to 28-hour rhythm. The researchers recorded their daily activities and physiological reactions during their time in the cave. At the end of their experiment, the researchers had two vital observations. Firstly, their body temperature changes correlated with their cognitive and physical performance throughout their day/night cycle, which laid the groundwork for understanding the role of temperature fluctuations as they relate to circadian rhythms. Secondly, age appeared to be a contributing factor to how well each person adapted to the new internal clock. Richardson, who was 25 years old at the time of the experiment, reported higher adaptability to the 28-hour rhythm opposed to Dr. Kleitman, who was 43 years old, although genetics and other personal differences might have contributed as well. Later research has corroborated the idea that adjusting one’s circadian rhythm gets harder as we age.^[8] The research by Kleitman and Richardson brought attention to the idea that our environment can change our sleep patterns, encouraging further research on other factors impacting the circadian rhythm.

The Surrounding Stigma

At some point in your life, you’ve likely heard the phrase “the early bird catches the worm,” a common cliché used to say that those who wake up early are more likely to succeed at something. Whether it be at work, in school, or at social gatherings, our society has long favored “early-birds” over “night-owls.” Take public high schools, for example. Between 2020 and 2021, the average elementary school had a start time of 8:16 AM while public high schools had an average start time of 8:07 AM.^[9] This is often fine for elementary-aged children who tend to wake up bright and early, but can be particularly problematic for high schoolers, as there is a natural shift in our circadian rhythm to stay up later and sleep later.^[10] In fact, the American Academy of Sleep Medicine (AASM) actively advocates for school start times for middle and high schoolers to be no earlier than 8:30 AM and have either been successful in passing legislation, or have legislation pending, in several states.^[11]

 In 2015, researchers at the Ludwig-Maximilian-University in Germany gave workers in a factory shifts that align more closely to their chronotype.^[12] After implementing the chronotype-adjusted shifts for two shifts, employees reported less disruption with their sleep and higher ratings on self-reported sleep quantity and quality.

Instead of punishing that which is a biological factor, we should instead acknowledge the science behind why humans differ in their chronotypes. Recent evidence and data has begun a movement to push school-start times back further to address this growing issue as well as give people the flexibility to work in accordance with their chronotypes. This is a large topic and one that we will address in a coming article. Unfortunately, these movements are still young and while significant progress has been made, more large scale changes will take time. For the time being, it seems as though those who are “night-owls” are often frowned upon and seen as lazy and unproductive. But this simply is not the case. “Night-owls” are just as productive as “early-birds,” just at different times of the day. Hopefully, the stigma surrounding these groupings fades over the coming years. 

Conclusion

Regardless of if you’re a late sleeper or early riser, the chronotype that you have is something that you are born with.  However it is possible to tap into the things we do have control over (strategic light exposure or avoidance at different times of the day, timing of meals, timing of exercise, timing of social activities), in order to try to realign your circadian rhythm to be closer to where you need to be for school or work. This is not an easy shift, but having proper alignment in your circadian system goes a long way to improving sleep, energy, and overall health, and your sleep specialist can work closely with you to help make an individual plan to optimize your circadian rhythm. 

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