circadian rhythms are those daily cycles that result in regular physiological and metabolic fluctuations. One of the most famous and highly studied is the fluctuation between cortisol and melatonin over the course of the day and the night. Cortisol is the main stress hormone, and as you can see, it's concentration in the body. That's what this axis is supposed to be, you know? Think of it as like concentration of thes hormones. You know, I'm sorry for using a graph that doesn't have labeled access. I know that's a real no no, but it's a pretty picture. So the point is that cortisol levels shoot up right? As you're about to wake up and they peak, uh, you know, early in the day and then steadily drop over the course of the day and into the night where sort of in the middle of sleeping, they start to increase again in preparation for the next morning. Now, court is all, as I said, is the main stress hormone, and it would make sense that you would want high levels of stress or alertness. You could think of it as early in the day when you're getting up when you have Thio, you know, look for food. Blah, blah, blah. You know, of course, we just go to the fridge these days. But, you know, we used tohave toe actually struggle to get our food. Anyhow, melatonin is almost like a counter to that in a really interesting way. See, melatonin promotes sleep and sleepiness. And as you can see, while cortisol is shooting up in the morning right here melatonin is actually chilling out, right? Melatonin levels drop precipitously, right? As you are about to wake up. And that's, you know, in part so that you don't feel super groggy in the morning. So don't be, You know, don't be thinking that melatonin is the main thing that makes you groggy in the morning. There's actually a lot of other stuff going on there. Uh, it's it's just related to sleepiness and sleep. Now it levels of melatonin stay low throughout the day, right? You don't wanna be sleeping during the day, But as night time sets in, as you should be getting to bed right as a, you know, an organism that's not living in the age of electricity and works based off a night day night cycle those levels shoot up to make you sleepy, right? So you go to bed, you have a nice good night's sleep, and right when you're about to wake up, all that melatonin dries up so that you're not all sleepy and morning and, you know at the same time your cortisol is popping up in order to make you nice and alert when you wake up. Now these air just daily cycles. But organisms can show interesting fluctuations in their metabolism and physiology over longer terms. You've probably heard of hibernation, but a lot of animals also, uh, do what's called tor poor or experienced what's called tor poor, I should say, which is a short term state of decreased physiological activity and metabolic rate. It's not as long as hibernation, but you can think of it working to the same effect, essentially hibernation. Of course, I'm sure you're familiar with animals fattening up before winter, where they'll go to sleep for a really long time and wake up when it's spring again, You know, so that they can kind of wait out the winter when there's not a lot of food and conserve energy. Well, that is not actually a sleep. You know its hibernation. It's it's not a long nap. It's an actual state of depressed metabolic activity. And it's something that's specific to end of therms. Right? We need lots of energy on on the daily in order to sustain ourselves. And when. Energy in the environment food is really scarce like in the winter. This is a nice way Thio get us so that we can live until the spring and then wake up and start to eat again. Wake up right now it z you know hibernation is going to be again like tor poor but on a longer term basis. Now organisms can't just let their metabolic, their metabolic physiologic processes run wild. They have to be very tightly controlled and maintained. Regulation of physiological processes is super important in order. Thio stay alive because you know things are changing around us. Things were changing within us, and our bodies need to be able to cope and to maintain ideal conditions for our survival. So this regulation of physiological properties eyes called homo Stasis and a great example of why this is so important is, for example, enzymes, right, Those proteins that basically do everything in ourselves that they make the magic of life happen. In a large part, they function best in very specific physiological conditions. Um, and in fact, proteins are very sensitive to temperature changes and changes in pH. And if you can't maintain these specific conditions for enzymes, they can actually cease functioning, which could obviously be very dangerous and potentially lead to death. That's, of course, just one example. There's other reasons why we need to maintain homeostasis now. There's kind of like two strategies that animals will take when they are trying to maintain, um, you know, their internal environments basically. And those two strategies are confirmation, like being a conform ER and conformers don't actively regulate what's going on. Instead, they'll kind of conform to their environmental conditions. They it's more like making do with what's around them instead of trying to fight against it. Like regulators, which actively control their internal environment, Uh, regardless of what fluctuations are occurring in the external environment. So one way you could think of this is in terms of body temperature. You know you'll have fluctuations in environmental temperature, and conformers will just kind of go along with that. They'll let their body temperature fluctuate mawr with the environment. Whereas regulators will, uh, you know, if it gets colder, for example, burn more energy to maintain that desired body temperature. So they're kind of fighting against what's happening in the environment instead of being like Zen with it. Now, homie, a static systems, uh, are often conceived of as having, um, certain properties. And we're gonna talk about those properties in a very generic way right now. And depending on who your professor is or you know what book you look at, they might use different terms here. So if you see different terms, come up in your course or something, don't worry. It's It's the same idea, really thes air, just generic terms. So, you know, don't worry about necessarily memorizing these names just kind of understand the ideas. That's that's what's really important. So a homo static system will be based off of a set point. This is kind of like the temperature that you set your thermostat to write in your home. If you have a thermostat, you say, like Okay, I want my house to be 70 degrees now. Obviously your house isn't going to be exactly 70 degrees all the time. That's the set point that your heating system is trying. Thio Get to write. It might go a little above sometimes and then compensate and go a little below and Mac forth back forth. It's It's the ideal point in the system. Now a sensor is going to detect stimuli related Thio the property of the home yo static system s O, for example, if we're talking about temperature, there will be, uh, sensors that will pick up on body temperature cues. Now, it's not always as direct as that. For example, the sensors in your brain that look at oxygen concentrations in your blood actually detect pH right there looking for something, and they detect a property that's related to that. So it's it's not always, you know, so directly connected. But the point is, they're looking at some particular property and using some sort of stimulus to keep track of that property. Now the integrator is going to evaluate the sensory information that comes in and determine the appropriate response. This is going to be like the, you know, little, uh, wiring system in the thermostat that goes, Oh, I'm detecting that it's two degrees colder than the optimal temperature. And so here's what I need to dio now. Lastly, you have the effect, er, which is the thing that actually generates a response to restore the Homo static system to ideal conditions. And if I jump out of the way here, as you can see, we have a nice example of body temperature behind me. You know, body temperature usually want to keep it around 37 degrees Celsius. You have cells in your skin and your brain that can detect temperature, and you have a A. You know, you could think of it a za ah regulatory center in the brain for temperature, and that's going to decide what to do based on the information coming in from these sensors and the response, Let's say that it z getting a little too hot. Body temperature exceeds 37 degrees, so it's a little too hot. Well, you're gonna want to sweat, right? So it's going to stimulate those sweat glands throughout the body to secrete sweat, which will evaporate and cool you down. So that's just a nice generic example of a homo static system, and we will be looking into some more specific examples as we examine different physiological systems in the animal body. With that, let's turn the page.