The term circadian comes from two Latin words, circa, meaning “around” - the alternative translation is “approximately” - and diem, meaning “day”. In its original meaning, the term refers to an internal process that regulates an organism's approximately 24-hour sleep-wake cycles.
Larks and owls exist, even though being on the extreme ends of the circadian continuum is uncommon.
SCN is the master cock, an endogenous timekeeper in our bodies tailored to maintain an approximately 24-hour cycle.
Zeitgebers are external cues, such as light, that entrain human circadian rhythms.
The vast differences in chronotype urge a delicate and individually tailored approach when it comes to school and work schedules.
Maintaining a steady circadian rhythm, i.e., avoiding forced phase shifting is essential for healthy physical and physiological functioning.
The phrase is not explicitly used for humans and can describe the animal, plant, fungi, and even bacterial recurring cyclic processes.
Endogenous and exogenous factors
Several factors govern circadian rhythms, which are in two groups: endogenous and exogenous.
- Endogenous factors. Describe something that comes from within the organism; hence nature is innate. Even though circadian rhythms are, by definition, endogenous, they are affected by external cues, such as exogenous factors.
- Exogenous factors. Literature calls these “Zeitgebers”, the word that comes from the German language. “Zeit” means time, and “geber,” means giver. One of the dominant zeitgebers (time-givers) for humans is light. However, other external cues can influence the endogenous circadian rhythms in humans, such as temperature, activity, and feeding behaviors. Even though the circadian rhythm can be affected by exogenous factors, it is a vital process, which exists naturally regardless of the presence of the zeitgebers.
Several case studies indicate that without exogenous factors, the circadian rhythm maintains an approximately 24-hour cycle. This persistent maintenance of sleep-wake-activity cycles without external cues is called free-running circadian rhythms.
In humans, a single so-called master clock orchestrates the endogenous circadian rhythms. Our “master clock” is a structure in the brain called the suprachiasmatic nucleus (SCN), which is a “bundle” of around 20,000 neurons. This structure receives direct input from our eyes; remember, the main zeitgeber for humans is light. Human eyes perceive the light of the day or darkness of the night and feed signals into SCN, which in turn orchestrates the remaining cells of our body to “act accordingly” in sync with a 24-hour day.
Hormones for day and night
Humans are diurnal, which means they are awake and active during the day, resting throughout the night. This is the opposite of being nocturnal, meaning active during the night and sleeping during the light periods. Every organ in the human body, and nearly every tissue, has a clock, which signals them to turn on and be active or turn off and slow down, preparing for rest. However, all of them are tuned together and controlled by the master clock - the SCN, which projects to the pineal gland.
Two primary hormones impact an individual’s awakeness and sleepiness:
- Melatonin. The pineal gland is responsible for melatonin secretion, a hormone with levels that peak at night. Melatonin is essential for maintaining healthy circadian cycles by preventing the phase shift, increasing the total sleep time, and inhibiting arousal.
- Cortisol. Another hormone, cortisol, secreted in the adrenal gland, produces an opposite effect to melatonin. The SCN governs cortisol through the hypothalamus-pituitary-adrenal axis (HPA). This hormone is at its lowest during the night and starts rising during the morning hours, signaling the body to wake up. The release of cortisol has a negative feedback loop with the melatonin. It inhibits its secretion.
What makes someone a morning person?
Phase shift is a term that refers to individual bedtime and wake-up time. When these times occur earlier, such as earlier to bed and an earlier wake-up, it’s called phase advance, and when these occur later, it's phase delay. The academic literature indicates that maintaining a stable phase shift and total sleep time is essential for healthy psychological and physiological functioning.
The circadian rhythmicity affects not only our physiological processes. Literature indicates that human cognitive functioning varies, depending on this endogenous cyclicity with vast individual differences. Even though the circadian rhythms stick to the approximately 24-hour cycle, the sleep and wake times can vary for everyone. If you ever wondered if there are larks and owls, the answer is yes. The two types of chronotypes include:
- Early chronotypes: They are also referred to as larks. These are the extreme morning types who show a marked preference for waking up at very early hours. Larks tend to have difficulties staying awake after their usual sleep time
- Late chronotypes: Referred to as owls, these are the evening types who prefer to go to bed in the late hours of the night. Owls often have trouble getting up in the morning.
These differences in chronotype show in times of cognitive peak performance. Evidence suggests that individuals will perform better on cognitive tests if they take the test at their optimal time of day. This process is called the synchrony effect and involves the cognitive demand aligning with an individual's cognitive peak based on the time of day. Notably, brain imaging studies investigating the diurnal patterns in cognitive performance offer a more delicate relationship between cognition and circadian rhythms - some but not all tasks vary throughout the day in the same way. Insight-based problem performance is shown to increase at a non-optimal time of day, contrary to the performance of solving analytical problems. Studies have shown the diurnal brain dynamic can affect attention, thus impacting learning, work or school performance, memory, and productivity.
Academic literature urges that psychological and neuropsychological assessments, as well as work and school times, should be scheduled with a consideration of circadian rhythmicity and individual chronotype, rather than based on social and economic factors, given that the former is not as easily adjusted.
Factors shifting the circadian rhythms
Disrupting our circadian rhythms hurts our physical and psychological health and well-being. Factors that can shift the circadian rhythms:
- Using bright lights during the sensitive time of the day, typically 2 hours before bed.
- Consuming food, alcohol, and tobacco.
- Not having a set sleep schedule or bedtime, such as working alternating daytime, night-time shifts.
- Consuming caffeine late at night.
- Performing mentally stimulating activities late in the day, like working out.
- Performing mentally stimulating activities late in the day, such as work and studies.
- Extensive or frequent travel through time zones.
- Frontiers. Living Without Temporal Cues: A Case Study.
- NIH. The biological clock tunes the organs of the body: timing by hormones and the autonomic nervous system.
- Cold Spring Harbor Symposia on Quantitative Biology. Sleep and Circadian Rhythms in Humans.
- NIH. A time to think: circadian rhythms in human cognition.
- NIH. Circadian Rhythms in Attention.