Sleep 8.1 Introduction to Circadian Physiology

Name: Sleep 8.1 Introduction to Circadian Physiology
Uploaded: 2025-11-25T08:37:55.048Z
Duration: 9:57
Description: How central & peripheral clocks work together, why zeitgebers (time givers) are essential, and how the suprachiasmatic nucleus (SCN) orchestrates rhythms.

How central & peripheral clocks work together, why zeitgebers (time givers) are essential, and how the suprachiasmatic nucleus (SCN) orchestrates rhythms.

Video Breakdown:

00:00 Introduction to Circadian Physiology

01:04 Objectives of the Talk

02:02 The Importance of Circadian Rhythms

05:23 Technological Advances in Circadian Science

06:49 Timing of Medications and Treatments

08:26 Conclusion and Future Directions

Believe it or not, this is the eighth talk for our wearables and sleep research webinar series. Today we will talk about circadian physiology, central and peripheral clocks, zeitgebers.
And, as many of you know, circadian sciences have seen an explosion in interest in investment in the past five to 10 years. And hopefully from this talk you will get a sense of why that is. Before we proceed with the talk, I just wanted to briefly remind you of the broader webinar series, which spans the full spectrum of sleep science, all from a wearable perspective.
We've talked about actigraphy, PPG, sleep regularity, sleep staging, and today we talk about circadian rhythms with physiology and zeitgebers, which means time givers in German. In the next talk, will be about circadian physiology testing, including cortisol, melatonin, and also some analytical methodologies.
So what is the objectives for today's talk? First is, to answer the questions. What has led to the recent explosion of interest in circadian sciences? How does the suprachiasmatic nucleus (SCN) act as the body's master clock?
And how does the SCN synchronize and coordinate the peripheral clocks? And what are other zeitgebers other than light? And how do they interact with the circadian rhythm? And what are the differences between Process C and Process S and how do they compliment each other? And so I've divided this talk into six categories here:
Introduction, the Central Clock. I would talk about the SCN, peripheral clocks, systemic zeigebers, non-photic zeitgebers, and Process C versus Process S. So let's start off with the introduction and the implications of circadian sciences. So, as you know, circadian rhythms are deeply conserved features across life.
And even simplest organisms like cyanobacteria, have molecular clocks. And these rhythms are seen in fungi, plants, insects in mammals. And interestingly, they find different mechanisms by which their internal circadian rhythms are established. Cyanobacteria, for instance, uses protein phosphorylations, fungi uses these genes, plants, insects, and mammals uses different types of mechanistic genes. So clearly, something about the circadian rhythm is really desirable to have different organisms establish it through different means. But from the scientific research community perspective, clearly, circadian sciences have become more prominent, due to a number of reasons.

And probably one of the big reason was, or at least recognized in 2017, was the awarding of the Nobel Prize of physiology medicine to these three gentlemen, professors Rosbash, Hall, who are Brandeis University in Boston, Dr. Young in Rockefeller University, and they were rewarded for the discoveries of the molecular mechanisms controlling circadian rhythms.
This was a quote by Dr.Stevens at UConn that basically says here, "Until quite recently, circadian biology was considered a quaint little subtopic of biology in general. However, the 2017 Nobel Prize for discovery of the mechanisms controlling circadian rhythmicity highlights the fact that circadian biology is fundamental to all biology in almost all organisms on the planet."
So that seems to suggest the really and strong importance of circadian rhythms. The other reason why circadian scientists have become truly popular is the recognition that modern lifestyle leads to circadian misalignment. You have circadian rhythms, which should ideally be synchronized with our environment, our behaviors, but our modern lifestyles with indoor lightings onto late into the day, our poor sleep patterns, our sedentary behaviors, and then access to food and sometimes not great foods all throughout the course of the day. And also shift work along, along with jet lag, which then subsequently sends messages to our central circadian rhythm, to then lead to reduced amplitude of the circadian rhythm.
This subsequently feeds back and then leads to changes in our behavior, in our interaction with environment as well. And then it sort of feeds it, it intensifies and then leads back again, back to the circadian rhythms, leading to low amplitudes, and poor timing cues. And in fact, I think we find that as people get older, our circadian rhythm amplitudes gets reduced and even the period, and even the phase shifts as well. This leads to a number of chronic diseases that are become a part of modern lifestyle. This includes hormone imbalances, inflammation, altered glucose, lipid metabolism, and the list goes on. So clearly circadian misalignment, secondary to our modern lifestyle has some broad implications for our health.

And the other thing that has developed is the development of new technological and research advances. And I would probably categorize these into two categories. First is genetic and molecular tools, and then wearables and point of care testing. For genetic and molecular tools, probably the biggest
development for circadian scientists were these real-time gene expression reporters, particularly luciferase. And you could assess the changes in gene expressions over time. And that would give us, an idea of the cyclical nature of, the patterns and the cells. And then you also had optogenetics and chemogenetics where you could alter the genes in order to see how changes the genetics can alter the cyclical or circadian nature of an organism.
And then you have multi-omics like transcriptome, proteomics, metabolomics, all the things where you can see how changes in, into these clock genes can lead to systemic changes to the organisms. And then you'll have wearables,with point-of-care testings, including salivary tests such as, melatonin, cortisol, dried blood spots.
Then you have wearable data, which includes actigraphy to enable us to assess sleep patterns as well as activity, and also continuous glucose monitoring. And then, the next big development was the recognition that timing of medications and treatment actually matters a lot. One of the biggest studies that demonstrated this was the Hygia study, published in 2020.
This is a large study conducted in Spain involving over 19,000 hypertensive patients. And they found that patients who took their blood pressure medications at bedtime rather than at waking had significantly better blood pressure control and a dramatically lower risk of major cardiovascular events over median followup of 6.3 years.
And the results, which were published in European Heart Journal in 2019, showed a 45% reduction in the risk of events like heart attacks, stroke and heart failure for bedtime dosing. So you could see this here for the cardiovascular disease outcomes. This is a huge improvement just based on the timing of the medications, and usually to match the blood pressure dipping that happens early in the night.
There's another study, that was also done for cancer and this was a systematic review of 18 randomized control trials, which involved ultimately 2,500 patients and found that 61% of studies reported significant decrease in toxicity with chrono-modulated chemotherapy while maintaining the same level of anti-cancer activity.
And 17% of these studies reported improved efficacy measures such as survival or objective response rates, but no studies actually reported decreased efficacy.
So, I think these reasons I hope you can understand, is the reason why the circadian sciences have really seen an explosion of popularity and interest. And I wanted to show this really interesting video of a plant as it responds to light, just to give you a sense how an organism uses a circadian cycle to go to, sort of day to day.
So, as the sun moves from the right to the left, you can see that the leaves starting to feed the left and then it sort of relaxes at night. And then in anticipation for the sun shining to the right here, all the leaves move to the right.
So there's an anticipatory component of, the circadian cycle and it involves all parts of the plant, as you could see, that all parts of are involved in the circadian management of the plant. The other thing I wanted to talk about later is the importance of the two process of sleep regulation.
There's Process C, which I will largely talk about, which is the circadian component of sleep. But then there's Process S, which is the homeostatic drive that we'll talk a little bit about that later, but I wanted to have that to give you an understanding of the context of Process C compared to Process S.

Course episodes

Sleep 8.1 Introduction to Circadian Physiology

Sleep 8.2 The Suprachiasmatic Nucleus: The Body’s Master Clock

Sleep 8.3 Peripheral Clocks: Synchronizing the Body Beyond the Brain

Sleep 8.4 Systemic Zeitgebers: How the SCN Coordinates the Body

Sleep 8.5 Non-Photic Zeitgebers: Meal Timing, Exercise, and Metabolism

Sleep 8.6 Understanding Process C and S in Sleep Regulation