We say that a healthy circadian rhythm is the foundation for good sleep. Exactly how our sleep is impacted by the circadian rhythm?
The Sleep-Wake Cycle
Most people are familiar with circadian rhythm primarily as it relates to the sleep-wake cycle. This cycle dictates when we feel alert and awake versus when we feel sleepy and in need of rest.
On a typical day, our alertness level rises in the morning, dips in the early afternoon (often leading to a post-lunch slump), and peaks again in the early evening before gradually declining toward bedtime.
Level of wakefulness throughout the day
How Does Circadian Rhythm Regulate Sleep?
Sleep scientists believe that our sleep-wake cycle is regulated by two main forces, referred to as two processes: the homeostatic sleep drive, and the circadian rhythms. The interplay between these systems ensures we get adequate rest and maintain alertness during waking hours.
Homeostatic sleep drive, or sleep pressure, can be understood as the hunger for sleep. Just like our hunger for food increases the longer we go without eating, our need for sleep builds up higher the longer we stay awake. This drive is regulated by adenosine, a chemical that accumulates in the brain during wakefulness. Adenosine level increases as we stay awake, making us feel progressively sleepier the longer we are awake.
When we finally go to sleep, adenosine levels decrease, reducing the pressure to sleep and allowing us to wake up feeling refreshed. This homeostatic process ensures that we get the amount of sleep necessary to recover from wakefulness and maintain optimal functioning. In essence, the homeostatic sleep drive operates as a balance system, increasing the desire to sleep in response to prolonged wakefulness and reducing it once sufficient sleep is obtained.
However, even after sleep pressure builds up from being awake all night, we don't immediately feel tired; instead, we might experience a second wind of alertness before eventually crashing. The second wind of alertness is the effect of the circadian rhythm. The circadian rhythm is in charge of making us feel sleepy and awake at certain consistent times each day, even if we haven't been awake long enough to build a strong homeostatic sleep drive or have been awake for too long.
These two processes work together to regulate our sleep-wake cycle. The combined effect of the homeostatic sleep drive and circadian signaling ultimately determines our overall level of alertness throughout the day. We all have experienced a natural dip in alertness in the early afternoon. Many may have felt the surge of energy in the late afternoon or early evening. These ebbs and flows of energy level are the result of the net effect of the two processes. When these two processes are in sync, we have long, restorative sleep overnight and wake up feeling refreshed.
Homeostatic sleep drive and circadian signaling together regulate sleep and wale cycle
However, disruptions to either process can lead to sleep problems. For example, staying awake late at night despite fighting against a strong homeostatic sleep drive not only leads to sleep deprivation and the accumulation of sleep debt but also introduces unwanted light exposure that disrupts the circadian rhythm. Similarly, circadian rhythm disruptions, such as those caused by shift work, travel across time zones, or irregular sleep schedules, can lead to poor sleep quality and difficulty falling asleep or waking up at desired times.
Where Is the Control Center Located?
Now we know that circadian rhythm is the invisible force behind all oscillating, 24-hour behavioral and physiological processes. What exactly drives the circadian rhythm? Scientists believe that a master clock, usually referred to as the circadian clock, is the driver behind all these 24-hour rhythmic phenomena.
Scientists believe that the circadian master clock resides in the suprachiasmatic nucleus (SCN), which is a peanut-size structure located in the hypothalamus of the brain. The SCN consists of about 20,000 neurons and is situated just above the optic chiasm, where it receives direct input from the eyes. SCN responds promptly to changes in light, making it the primary regulator of circadian rhythms. Circadian signals from the SCN are transmitted to an area in the brain called the pineal gland. The pineal gland is a tiny endocrine gland in the brain that produces several hormones, one of which is melatonin, a hormone that induces sleep.
While the SCN is where the master clock is located, other tissues and organs have their own clocks, referred to as the peripheral clocks. Like the commander giving orders to the subordinates, the SCN synchronizes these peripheral clocks, ensuring that various bodily functions, such as hormone release, metabolism, immune functions, and sleep-wake cycles, are well-coordinated.
Impact of Light and Darkness
Because SCN responds fast and strongly to light stimulation, light and darkness are the most powerful cues that help synchronize our internal circadian rhythms with the external environment. Similar to light and darkness, environmental cues that can influence circadian rhythm are called the zeitgebers, which literally means time giver or time cue. Light and darkness are the strongest zeitgebers.
In the absence of light, the SCN triggers the release of melatonin from the pineal gland, signaling to the body that it is time to prepare for sleep. This process helps sleep initiation and ensures that we get restful, restorative sleep. When there is light exposure, the light activates the photosensitive receptors in the eyes (including the ipRGCs, which stands for intrinsically photosensitive retinal ganglion cells). The photosensitive receptors send the signals to the SCN that it is daytime; the latter then sends signals to the pineal gland to suppress the production of melatonin.
Before the invention of electric lighting, humans relied solely on natural light from the sun and fire. People woke up with the sunrise and went to bed shortly after sunset. The natural progression from daylight to twilight helped maintain a strong, regular circadian rhythm. In the late 19th century, the invention of electric lighting revolutionized human activity, extending the day beyond natural limits. While this brought significant benefits in terms of productivity and lifestyle flexibility, it also introduced new challenges for circadian health. Artificial lighting enabled people to stay awake and active well into the night, disrupting the natural alignment between the circadian rhythm and the external environment. This disruption of alignment is referred to as circadian rhythm misalignment.
Two Key Hormones: Melatonin and Cortisol
The final piece of the puzzle is the hormones. We hear a lot about melatonin and cortisol. What exactly are they, and what are their roles?
As we mentioned before, melatonin is a hormone produced by the pineal gland in response to darkness. Its level rises in the evening (melatonin onset), peaks during the night, and decreases in the morning (melatonin offset). The release of melatonin signals that it is time to sleep. Because of this, melatonin has often been referred to as a “sleep hormone.” While melatonin isn’t essential for sleeping, you sleep better when you have the highest levels of melatonin in your body.
Cortisol, on the other hand, is highest in the morning upon waking and gradually decreases throughout the day, reaching its lowest levels at night. It helps regulate metabolism, immune response, and stress. While melatonin promotes sleep, cortisol promotes wakefulness and energy. Disruption in this balance can lead to sleep disorders, fatigue, and stress-related health issues.
Because these two hormones play key roles in the circadian cycle, taking melatonin supplements is a popular way to try to fix sleep problems related to circadian rhythms. This can be helpful for those dealing with issues like jet lag, shift work, or delayed sleep phase disorder. However, melatonin supplements don't always work for everyone. We will discuss the details and reasons why in another blog post.
