The Circadian Rhythm and the Zeitgebers: Understanding Your Inner Clock

Understanding the Circadian Rhythm

Sleep and wakefulness are regulated by two categories of processes (processes S and C): a homeostatic process (S = sleep pressure), dependent on the duration of prior sleep and wakefulness, and a cyclic process (C = circadian rhythm).

Circadian rhythms are a type of biological clocks (natural timing devices of organisms) that tie physiological processes to the day-night cycle. The term 'circadian' comes from Latin 'circa diem', meaning, 'around a day'. According to the 1999 study by Czeisler et al. at Harvard, the precise length of the human circadian rhythm in healthy adults is 24 hours and 11 minutes ± 16 minutes. This internal timekeeper of ours ensures that vital bodily functions are optimally synchronized with the external environment, allowing us to adapt and thrive in a rhythmic world.

The most well-known circadian rhythm is the sleep-wake cycle, which regulates our alertness and energy levels, causing the [human] body to naturally fall asleep at night and awake during the day. However, this system is not the only one that has a circadian rhythm. Besides sleep, almost everything in our bodies has a rhythm, including

• eating habits, digestion, and energy expenditure, which in turn influence metabolism and weight;
• blood pressure and body temperature;
• cognitive performance and mood;
• hormonal regulation and fertility;
• the immune system (through cyclical recruitment of immune cells);
• DNA repair and cancer prevention.

Of course, circadian rhythms are not exclusive to humans. Chronobiological ('chrono-' = 'time') cycles, especially those that primarily respond to light and dark, are found in most living things, such as non-human animals, plants, fungi, and even fruit flies and bacteria. For example, certain flowers open and close at a certain time, and nocturnal animals are kept safe from daytime predators by sleep exactly due to their circadian rhythms.

How does that work?

Enter the Timekeepers

Since each individual has their own inherent circadian rhythm that deviates from the 24-hour day-night cycle of the Earth, we need external cues to synchronize it to the environment. These signals that adjust our circadian rhythm are called zeitgebers (German for literally time givers, but apparently anglophones like to adopt words from other languages simply like that).

The strongest zeitgeber for the human body is light. Your retina contains special non-visual light-sensitive cells that report straight to the master clock of your body, the suprachiasmatic nucleus (SCN). This master clock analyzes the information on light exposure, aligning your internal rhythms with the day-night cycle.

During the day, when you're exposed to light, this tiny cluster of 20 000 neurons (nerve cells) keeps you alert and active. As the evening arrives, though, dimming the lights, the SCN commands your pineal gland to secrete a neurohormone called melatonin, which relaxes your body and makes you sleepy. For this reason, it is highly advisable to go outside and receive sunlight in the first 2 hours after waking, or, ideally, in the first 30 minutes, and to watch the sunset in the evening, dimming artificial lights as your bedtime approaches. Since the light-sensitive cells are especially sensitive to blue light, light from your devices at night can confuse your biological clock, making falling asleep more difficult, so if you tend to have any sleep problems, avoid using your smartphone before you go to bed.

Image source: https://openbooks.lib.msu.edu/app/uploads/sites/87/2023/01/Pineal-gland-and-melatonin-1024x440.png

Unfortunately, nowadays people spend too much time inside (yep, I'm guilty of that, too), which results in exposure to low levels of light in the daytime and high light levels in the evening/night, obviously, the exact opposite of what is natural and beneficial for the human body. Besides, some people are blind, or they live in the polar zones with their year-long day-night cycle, which frequently results in circadian disorders. We're lucky then that, though light is the major zeitgeber, other factors play a role in biological timekeeping as well.

The main zeitgeber in the absence of light is nutrition. You see, as it turns out, your circadian system actually has a quirk which can potentially result in a mismatch between your central and peripheral clocks: your SCN doesn't directly control your peripheral clocks, nor do those directly influence the SCN. This is where nutrition comes in. Though meal times don't directly entrain the SCN, they act as a strong cue for peripheral clocks, and, for this reason, it does well to consciously synchronize nutrition with your circadian rhythm.

To keep your metabolism (and overall health) in order, it is advisable to restrict food intake to the light phase and refrain from snacks between meals. What is more, the same meal eaten at various times of the day can have different effects on the body. More specifically, evidence seems to confirm the validity of the advice of the Jewish philosopher and doctor Maimonides: "Eat like a king in the morning, a prince at noon, and a peasant at dinner.” Or, as I have heard people say here, eat your breakfast, share your lunch with a friend, and gift your dinner to an enemy. A meal schedule that is heavier towards the early part of your waketime appears to correlate with a decreased risk of obesity and metabolic diseases such as diabetes.

Another such factor, though weaker and in some ways dependent on meal timings, is physical activity. Regular exercise during specific phases helps maintain circadian rhythms. Studies with both humans and animals demonstrate that intense physical activity during the day and the early evening (before melatonin onset) causes a forward shift in the inner clock, triggering the subject to go to sleep and rise earlier. On the contrary, exercise at night can cause phase delays. One way this works may be by increasing your core body temperature (CBT). A higher body temperature (of course, as long as it's normal) is associated with greater alertness. As the time for sleep approaches, though, your body temperature should drop. Thus, exercise in the morning helps you wake up, while if you do it in the evening, your CBT rises, forming an obstacle to generating and sustaining quality sleep. Therefore, it's best to exercise either early in the morning or during the notorious afternoon crash, which occurs when your CBT dips a little.

Well, since we've touched on temperature, it is common knowledge that it tends to be higher during the day than at night, that's why it can serve as an excellent indicator of time. For this reason, instead of keeping a steady temperature in your room, cool it down at night, lowering the heat if possible and opening the window (don't freeze, though, of course, you still need to feel comfortable). As the temperature of your environment drops, so does that of your body, preparing you for sleep. Alternatively (or, ideally, in a complementary fashion), you could do almost the opposite - take a short warm shower or bath in the evening, as the heat would force your body to counteract, cooling itself down. By the same mechanism, splashing your face with cold water or taking a cold shower in the morning can help you wake up faster by rising your core body temperature. 

What else do we do during the day? Oh, right. Socialize. Extraverted or not (*wink*), you are most likely surrounded by other people daily at home, school, university, workplace, store, church, or whatever place you happen to attend or spend your days in. Even during the COVID-19 lockdown, you probably held somewhat regular Zoom calls with your colleagues. Consequently, social interactions contribute to adjusting your inner rhythms. Thus, such major life events as divorce, job loss... or a lockdown can not only create the obvious effect (emotional distress) but also disrupt your circadian clock. That's why it is crucial, especially in difficult times, to meet with your loved ones and have stable social interactions, ideally at roughly the same hours every day (of course, no need to get too rigid with this, though).

Besides these five major zeitgebers, there are, of course, many other factors that influence your circadian rhythm, for example, noise, napping, medication timing, and intake of stimulators, such as caffeine, or sleep supplements, as well as your age and physical and emotional health. Generally, it is good for your health to live a lifestyle that is as natural as possible, meaning,

• during the day: expose yourself to natural light, work and interact with people, adhere to a (somewhat) stable meal schedule, and be physically active;
• during the night: sleep in a dark, cool, comfortable place, falling asleep and waking up at a consistent, regular time.

Ok, now that you've gotten a gist of the various factors that regulate your circadian rhythm, let's move on to investigating to a deeper level what exactly happens to your body throughout the day-night cycle.

Phases of the Circadian Cycle

Morning

• Cortisol Release: Upon waking, the levels of a hormone called cortisol start to rise, increasing by 50% in the first 30 minutes after waking. Although cortisol is often referred to as " the stress hormone", it also plays a crucial role in promoting alertness and helping you wake up.
• CBT Rise: As you wake up, your core body temperature (CBT) gradually increases, aiding in alertness and cognitive function.
• Release of Various Neurotransmitters: Have you noticed that you feel happier when the sun is shining? Well, as it turns out, it's due to the consequent increase in serotonin levels. Serotonin, commonly known as the happiness chemical, not only makes you willing to get up in the morning but will also be necessary for the formation of melatonin in the evening. All kinds of other neurotransmitters besides serotonin are released after awaking, too, such as, for example, norepinephrine (aka noradrenaline), a mobilizing hormone that heightens blood pressure, and histamine, a neurotransmitter associated with activity of the immune and nervous systems.

The Circadian Rhythm of Cortisol Levels. Image source: https://www.zrtlab.com/landing-pages/diurnal-cortisol-curves/

Noon

• Peak Alertness: The mid-morning to early afternoon (roughly 10:00 to 14:00 for people who sleep from 23:00 to 07:00) is typically when people experience their highest levels of alertness, focus, and cognitive performance. One of the reasons behind this may be that the peak of noradrenaline occurs at noon. Thus, morning to noon is the best time for doing productive work.
• Digestion: In response to previous eating patterns, the circadian clock controls various factors associated with feeding, such as the levels of ghrelin, the hormone that makes you feel hungry, and digestive enzymes, as well as nutrient absorption and gut wall permeability. Most people are accustomed to eating lunch, usually their biggest meal, in the middle of the day, so this is when their digestive system is active the most.

Afternoon Slump

• CBT Dip: Body temperature slightly decreases in the afternoon, often stimulating a tiny melatonin release and contributing to feelings of drowsiness.
• Decrease in Cognitive Function: Many people experience a natural drop in alertness and energy levels about two hours after lunch. Even in cases where a person hasn't eaten lunch and has no idea about what time it is, they might still experience this afternoon crash in the early to mid-afternoon (frequently between 14:00 and 16:00).

Late Afternoon to Evening

• Cognitive Recovery: After a time that is for many a bit of a stagnating slump of sleepiness, your mind starts to work better again, improving problem-solving, decision-making, attention, and task execution abilities in the afternoon and early evening hours (approximately 16:00 to 22:00 h). Thus, this is the second best time for work.
• CBT Peak: After the slight dip, your core body temperature increases again, reaching its maximum in the late afternoon, at about 18:00.

Nighttime

• Dim Light Melatonin Onset (DLMO): As the sun sets and natural light decreases, your pineal gland starts releasing the previously discussed sleep-promoting hormone melatonin, signaling your body that it's time to wind down and prepare for sleep.
• CBT Drop: Having reached its peak at about 18:00, your body temperature begins to drop, promoting sleep and reaching its lowest point (nadir), as the end of your sleep phase approaches.
• Decrease in Attention: As you get sleepy, your task execution abilities decrease again, reaching their lowest level at dawn and early in the morning. This correlates with the drop in your norepinephrine and cortisol levels.
• Sleep: Well, duh.
  Your sleep consists of about 70 to 120-minute cycles starting with non-REM sleep (commonly known as deep sleep) and ending with REM sleep (REM stands for rapid eye movement sleep, often referred to as dream sleep). The beginning of the night is heavier in non-REM sleep, but with each cycle, the proportion between these two kinds of sleep shifts towards REM, providing you with the free virtual reality experience known as dreams. Each of these stages of sleep has its own purpose: while non-REM sleep is crucial for physical recovery and memory consolidation, REM sleep forms creative connections between data and restores your emotional health.

Early morning

• Cortisol Release: Before you wake up, your body's cortisol levels rise again, preparing you to get up.
• CBT Rise: After having hit the nadir at about 04:00 depending on your sleep schedule, your body temperature increases again, setting you up for the morning.
• Decrease in Melatonin: Once you reach the peak of melatonin somewhere between 02:00 and 04:00, the levels of this hormone gradually lower, reducing your sleepiness levels.

And repeat.

Variations in Individual Circadian Rhythms

Obviously, nobody has the same circadian rhythms as another person: each of us has an innate preference for a certain length and timing of sleep and going through our daily cycles. The natural inclination of your body to sleep at a particular type is called your chronotype. You may have heard of the morning bird or morning lark, which is a chronotype characterized by rising and going to sleep early, and the night owl, but the truth is that most people fall somewhere in between and are quite flexible and able to shift their chronotypes to adapt to their life situation. An important factor that plays into this is age: teens over the world experience a backward shift in their circadian rhythm, prompting them to fall asleep and wake up one or two hours later than their parents. Meanwhile, children and young people are more on the early bird side of the spectrum. In most cases, even though you do have a natural inclination that should ideally be honored, your habits are what ultimately shape your life. The variety and flexibility in individual circadian rhythms provide both humans and humanity as a whole with a greater ability to adapt and thrive in diverse environments.

When the Clock is Broken

When properly functional, the circadian rhythm synchronizes the body's functions and sustains consistent and healthy sleep-wake cycles. However, certain factors such as heredity or harmful lifestyle can disrupt the circadian rhythm, which can in turn lead not only to sleep disorders (e.g., insomnia or simply lower-quality sleep) but also to chronic health conditions, for example, obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.

Here are some examples of circadian sleep-wake disorders:

• Jet lag disorder: When you travel to a time zone very different from your own, chances are, some time would pass before you could adjust to the new day-night cycle. In the meantime, you might suffer from low-quality sleep and experience fatigue.
• Shift work disorder: Work is a major factor that influences your circadian rhythms. If you happen to work at night and sleep during the day, your sleep schedule contradicts the natural day-night cycle, confusing your circadian rhythms and causing desynchronization between the many biological clocks of your body.
• Advanced sleep phase disorder: This is like the extreme, radical version of the morning lark chronotype. People with this rare disruption get tired early in the evening and wake up very early in the morning, and they are unable to shift their rhythms backwards.
• Delayed sleep-wake phase syndrome: This disorder is at the other end of the chronotype spectrum, resembling an extreme night owl. A person with delayed sleep-wake phase syndrome stays up too late at night and sleeps in late in the morning.
• Non-24-hour sleep-wake disorder: People with this health condition find their sleep-wake rhythms drifting backwards and backwards, going to bed slightly later each day. More than half of blind people are reported to have N24SWD due to their inability to perceive cues from the main zeitgeber, light. In some rare cases, people with normal vision also get this disorder, though, if they live like vam- er, don't receive enough light stimulation.
• Irregular sleep-wake rhythm disorder: People with this rare disruption have very inconsistent or next to non-existent sleep-wake rhythms, for example, instead of normal ~8-hour night sleep, they may have multiple naps throughout the day-night cycle.

Concluding Thoughts

The circadian rhythm is truly a biological marvel, orchestrating a whole symphony of bodily processes to keep you in synchrony with the world around you. To make the most of it, embrace morning light, time your meals, exercise during the day, align your work times with your peaks of cognitive performance, chat with your loved ones, and, what is probably most relevant to this blog, sleep consistently. For those of you who are more go-with-the-flow people (like me), this is not some dystopian-style, externally imposed, robotic punctuality. This is your rhythm. By adjusting your habits and lifestyle and reconciling the life around you with your internal rhythms (or the other way around if your current sleep-wake cycle doesn't serve you), you can enjoy a stable cycle of restorative nights and days full of energy and live a life of improved wellbeing, productivity, and overall vitality.

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