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. At night, cortisol is the main stress hormone, and as you can see, its concentration in the body is what this axis is supposed to represent. Think of it as the concentration of these hormones. I'm sorry for using a graph that doesn't have labeled axes. 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, they peak early in the day, and then steadily drop over the course of the day and into the night, where in the middle of sleeping they start to increase again in preparation for the next morning. Now, cortisol is the main stress hormone, and it would make sense that you would want high levels of stress or alertness early in the day when you're getting up, when you have to look for food. Of course, we just go to the fridge these days, but we used to have to actually struggle to get our food. Anyway, 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. Melatonin levels drop precipitously right as you are about to wake up, in part so that you don't feel super groggy in the morning. Though 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, and it's just related to sleepiness and sleep. Now, levels of melatonin stay low throughout the day. You don't want to be sleeping during the day, but as nighttime sets in, as you should be getting to bed, as an organism that's not living in the age of electricity and works based off a day-night cycle, those levels shoot up to make you sleepy. So you go to bed, 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 in the morning, and at the same time, your cortisol is popping up in order to make you nice and alert when you wake up.

Now, these are 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 do what's called torpor, or experience what's called torpor, 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 sleep for a really long time and wake up when it's spring again, so that they can kinda wait out the winter when there's not a lot of food and conserve energy. Well, that's not actually a sleep; it's hibernation. It's not a long nap. It's an actual state of depressed metabolic activity. And it's something that's specific to endotherms. We need lots of energy daily in order to sustain ourselves. And when energy in the environment, you know, food is really scarce like in the winter, this is a nice way to, you know, get it so that we can live until the spring, and then wake up, and start to eat again.

Now, organisms can't just let their metabolic, their metabolic, and physiological processes run wild. They have to be very tightly controlled and maintained. Regulation of physiological processes is super important in order to stay alive, because things are changing around us, things are 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 is called homeostasis. And a great example of why this is so important is, for example, enzymes, right? Those proteins that basically do everything in our cells, they make the magic of life happen in a large part, function best in very specific physiological conditions. 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 are other reasons why we need to maintain homeostasis.

Now, there are two strategies that animals will take when they are trying to maintain their internal environments. And those two strategies are conforming, like being a conformer, and regulating, like being a regulator. Conformers don't actively regulate what's going on. Instead, they'll kind of conform to their environmental conditions. 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, 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'll have fluctuations in environmental temperature, and conformers will just kinda go along with that. They'll let their body temperature fluctuate more with the environment, whereas regulators will, you know, if it gets colder, for example, burn more energy to maintain that desired body temperature. So, you know, they're kind of fighting against what's happening in the environment instead of being like Zen with it.

Now homeostatic systems are often conceived of as having 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 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 the same idea really. These are just generic terms. So, you know, don't worry about necessarily memorizing these names. Just kind of understand the ideas. That's what's really important. So a homeostatic system will be based off a set point. This is kind of like the temperature that you set your thermostat to. In your home, if you have a thermostat and 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 to get to. Right? It might go a little above sometimes, and then compensate, and go a little below, and back and forth. It's the ideal point in the system. Now, a sensor is going to detect stimuli related to the property of the homeostatic system. So, for example, if we're talking about temperature, there will be 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. They're looking for something, and they detect a property that's related to that. So it's not always 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 wiring system in the thermostat that goes, uh-oh, I'm detecting that it's 2 degrees colder than the optimal temperature, and so here's what I need to do.

Lastly, you have the effector, which is the thing that actually generates a response to restore the homeostatic system to ideal conditions. And if I jump out of the way, as you can see, we have a nice example of body temperature.RELATED TABLEBehind me. You know body temperature, usually you 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 regulatory center in the brain for temperature that's going to decide what to do based on the information coming in from these sensors. And the response, let's say it's getting a little too hot. Well, you're gonna want to sweat. So it's going to stimulate those sweat glands throughout the bodies to secrete sweat, which will evaporate and cool you down. So that's just a nice generic example of a homeostatic 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.