How Your Body Knows What Time It Is - Circadian Rhythm

[00:00:00]If you were blindfolded and then stuck in  a dark windowless room for weeks on end, you would still have a pretty decent idea  as to what time of day it is. And that is because of your circadian rhythm,  which is your literal biological clock. Circadian actually translates  to about a day. So this isn't perfect.

[00:00:22]I'm not sitting here saying like you're  gonna be like, oh, it's, you know, 03:43PM.

[00:00:26]Like, you're not gonna have that level  of precision, but you're going to have a pretty decent idea because of some very specific  things happening in your body. In today's video, we're gonna be using this brand new  physiology study unit from Kenhub to figure this out. This is a premium feature that  I'm gonna show you, but there is plenty that you can get for free the moment you sign  up. If we scroll down and look at this, you're gonna see that we have these  related articles here of on, like, the suprachiasmatic nucleus, the pineal gland,  all of our images. There's so many articles.

[00:01:01]All of that is gonna be 100% free, but this is a  premium feature. And what this is showing us is basically an in-depth lesson all about, let's  get up here, sleep and wakefulness. So while, yes, the videos and the quizzes are  premium, there's still so much value.

[00:01:17]So go ahead and check the link in the description  below, and we're actually going to link a separate but equally awesome study unit on reflex arcs  so you can actually see what they are like.

[00:01:27]But while you're there, you can also just check  out everything else that Kenhub has to offer.

[00:01:31]Now what we're gonna do is basically just look at  this whole process for the circadian rhythm. And the first thing we're going to be taking a look at  is these two drives. It's called the homeostatic sleep drive and then the circadian wake drive.  So the blue line is going to be your sleep drive. And, basically, what we're seeing here  is you can see it says day, night, day, night.

[00:01:56]Right? And you can see that we have  times here. This is gonna be very, very helpful. And then you can also see there's gonna be two thresholds. We have a sleep  threshold and then a waking threshold.

[00:02:05]So what's happening over the course of  a day is your homeostatic sleep drive is increasing. This is quite literally your  body compelling you to sleep. There's an urge to sleep that is building. Now this is  directly linked to adenosine. So adenosine is basically what is left over as your  body uses ATP or adenosine triphosphate.

[00:02:28]So as your body's naturally using the body's  energy currency, adenosine is gonna start to build up in the neurons of the brain,  and that is going to pretty much just start to make you feel tired is more or  less what's gonna happen. Interestingly, caffeine will block those receptors,  meaning the adenosine can't bind to it, but it blocks the receptors. It actually doesn't  activate them. So caffeine doesn't really give you energy as much as it just doesn't  let your body realize it's tired. And, that caffeine's gonna go away and the adenosine  crashes and, well, that's all she wrote.

[00:03:00]But what's happening here is this homeostatic  sleep drive is increasing, and, eventually, you're gonna get to this point where the adenosine  buildup is just massive. And there's other factors at play here, and you're gonna cross what's called  the sleep threshold. And this is usually around, like, whatever you typically whenever you  typically go to bed. So maybe it's if you're like me and you're just a verifiable old man, it's  around 9PM. But if it maybe for you, it's 11PM.

[00:03:25]Maybe if your circadian rhythm has adjusted  to other hours, like shift work can be chaotic on this. Jet lag, we're gonna see why  jet lag can mess with this like crazy, but some people can push this out quite  significantly, and and they can actually, basically push it like, 3AM, so on and so forth.  But the point is here, adenosine builds up, and then you cross this sleep threshold. But at  the same time, we also have to understand you have this circadian wake drive. So the circadian  wake drive is a combination of several things.

[00:03:54]So this is where it seems like cortisol is  gonna be at play here, your body temperature, right, ATP consumption. So, basically,  what's gonna happen here is it's gonna increase. And so you can see,  like, in the middle of the day, your circadian wake drive is just rocking.  You are good. You're not feeling tired.

[00:04:11]Yes. Your wake your your sleep drive is kinda, like, growing. Right? That homeostatic  sleep drive. But the the circadian wake drive is still doing pretty good,  but then it's gonna start to go down.

[00:04:22]This is when you're starting to sleepy.  And then as soon as that sun sets, this is when it's really gonna start  taking off. And this is where you are then crossing the waking threshold.  So the sleep threshold and the waking threshold are basically just where these  two things are basically meeting. Right?

[00:04:38]Where it's like your body temperature is gonna  start to decrease. Your cortisol level is gonna start to go down, which we're gonna see, and it  happens at about the same time. And this is where you sleep. And so as you drop into sleep, what's  gonna happen is your adenosine is gonna start to be, like, take you know, cleaned up. We'll  just put it that way inside of the neurons.

[00:04:57]And that is gonna be very important because  there's other things happening with sleep too, by the way. It's not just adenosine  cleanup. There's a whole bunch of, like, flushing that's happening inside of your  brain, and it's just kind of progressing throughout the night. And at the same time,  you can see that around this is about, like, two, one, two, 3AM or so on a typical night.  You can see you're just, you're bottomed out.

[00:05:19]Right? Your circadian wake drive is all the way  down, but you're cleaning up. And then eventually, cortisol is gonna start to rise as we're  gonna see. Your body temperature is gonna start to rise, and sure enough, you wake,  and then you repeat the same thing the next day. So this is one essential  component of our circadian rhythm.

[00:05:38]But the the question you might be wondering is,  well, how does your body know that it's actually bright or night, right, day or night or anything  like that? So if we go to this next image, there's a lot to look at here, but I just  want to just take our time with this so we fully understand exactly what's happening here  because we have this sagittal view of the brain.

[00:05:59]So you can see the folded part is going to be the  cerebrum. Then this right here is the brain stem, and then this back here this right here is  called the cerebellum. Now in the center of the brain is another region called the  diencephalon, and we can see that there is a portion that is highlighted, and we're  gonna come back to that in just a moment.

[00:06:20]But then you can see out here we have an eyeball.  So the eye, you can see we have it saying light and then less light. And so, obviously, the eyes  are for seeing. But what a lot of people don't understand is the eyes are not just for vision,  but also just detecting the presence of ambient light. And so you can see this is titled or  labeled one, and then it kinda comes here.

[00:06:42]You're seeing this cross section of the  photoreceptive layer of the eye called the retina, and there's a whole bunch  of cells in here that we're not gonna go over. You can see it's pointing to  this one purple one here. This is called the photosensitive retinal ganglion cell.  This one does not process vision. Right?

[00:06:58]This is not a rod. This is not a cone.  Well, not really processed anyways, but it's not detecting, right,  photons in terms for vision. Instead, this is just there to detect presence  of ambient light. And so what it's then gonna do is send a signal, and we can see  this coming up, and it's going up here.

[00:07:17]And then it's gonna go to this blue portion  that I told you. Right? So you can see all of it right here, but we've zoomed in on it in  this box. And so this blue portion is what we call the hypothalamus. So the hypothalamus  is really important for, like, homeostasis.

[00:07:35]Right? So, basically, it's not really balanced,  but it's kinda like your body's regulation. It's like a thermostat. It's helping to control  a whole wide variety of things. Inside of the hypothalamus are two structures  that we wanna concern ourselves with.

[00:07:49]There's this tiny one that you're not really  gonna be able to see all that well because it's very small called the suprachiasmatic  nucleus. Sometimes you'll just see it be called the SCN. And then we also over here  have the paraventricular nucleus. What's gonna happen is this. Your photo your  photosensitive retinal ganglion cell is going to detect whether there is a bunch  of light or there isn't a bunch of light, and it's gonna send a signal  to the suprachiasmatic nucleus.

[00:08:17]Well, that is then gonna send that and relay  it to the paraventricular nucleus. Now the goal here is actually gonna be you can't  really see it all that well in this to make sure because it's tiny. There's this little  green bean back here that we've kinda, like, highlighted. This is called the pineal gland. Now  the pineal gland is part of your endocrine system, and and this is gonna be secreting a hormone  called melatonin that I guarantee you've heard of.

[00:08:41]Now our goal is to get this signal to the pineal  gland. And so when you're looking at this, you're like, oh, that makes sense. K. So we'll  just go from that paraventricular nucleus, and we'll just come over here. Well, no.

[00:08:54]You are wrong. That is not allowed. We gotta take  a pretty intense detour, and that is because the pineal gland is regulated through the sympathetic  nervous system, and that means everything with the sympathetic nervous system has to go through  the spinal cord. And so what's gonna happen here is that the we're gonna send a signal from  that paraventricular nucleus down. Right?

[00:09:18]We're exiting, going through the midbrain, the  pons, the medulla oblongata, all three of these really just belong to the brain stem. And we're  gonna go all the way down to the lateral horn of the spinal cord, and that's where we're gonna  meet the intermediolateral nucleus. This is part of the sympathetic nervous system. And then what  it's gonna do is go, no. We're not going here.

[00:09:41]We need to go back to the pineal gland and  whoop. It goes right all the way back up, and it's gonna make its way to that pineal gland.  So that's what you're seeing here in three. So, again, light comes into the eye, goes to  the hypothalamus, goes all the way down to the spinal cord, and then goes all the way  back up, and we land at the pineal gland.

[00:10:01]And so the pineal gland has, you know,  been said to be like the third eye.

[00:10:07]That's actually more of a modern interpretation  of it. So from my understanding, and I'm not gonna sit here and pretend to be really  hyper knowledgeable around chakras, but the chakra third eye wasn't really correlated  with the pineal gland until very recently, within, like, the past one to two hundred years.  But this is still considered the third eye by a lot of people, and the reason they  think that is because you have one eye, two eyes, and then where their goal  here is to get it to this thing, and there's only one of them. Right? The  pineal gland is not nest it's not like lobed.

[00:10:41]You don't have, like, a right and a or a  right and a left lobe of the pineal gland.

[00:10:44]You're you're only gonna have one pineal gland,  so it's like the third eye. But what this thing does is it secretes a hormone called melatonin.  And melatonin is a hormone that is going to act on the brain and it basically just, like,  desensitize it or to an extent. Right?

[00:10:59]It's gonna lower brain activity to  the point where you are going to be compelled to sleep. So in the presence of  light, what's gonna happen is this is just getting a signal. It's like, hey. There's a  bunch of light outside. Don't go to sleep.

[00:11:13]But in the absence of light, that is  where it's gonna get the signal that it's time to go to sleep, and we're now  gonna produce melatonin. And this makes perfect sense if you think about it because  from an evolutionary perspective, you know, our ancient ancestors were hunter gatherers.  Right? It made sense to go to sleep at night.

[00:11:29]All the scary stuff that could eat  you or hurt you was active at night.

[00:11:34]So we are not nocturnal. We sleep during  the night. And so as the sun goes down, we start to feel the effects of melatonin  really start to take over. But you're also pairing that with the buildup of adenosine that  we we talked about from in this image. Right?

[00:11:52]From that that homeostatic sleep drive. So as the  adenosine is building up, we also are gonna start to have melatonin be produced, and that combined  effect is what's going to kick us into sleep, especially as our that circadian wake drive  is going to drop. So all of this is happening, but that's not it. Because let's also look here  because this is gonna show us just throughout the course of the day and night these two  competing hormones. Because so, basically, we have we just talked about melatonin,  but we also have to talk about cortisol.

[00:12:27]Now cortisol, everyone knows as the stress  hormone. So this is going to be produced, by the adrenal glands or your suprarenal  gland. And cortisol is very important.

[00:12:39]It's not just about stress as in, like,  I'm stressed. I got a test coming up.

[00:12:43]Cortisol is very important to help just  kinda activate you. So if we look at here, you can see 6AM, 12PM, 6PM, and then  it kinda repeats. So or 12AM, then 6AM, so on and so forth. So you see here, let's  let's assume you're waking up at 6AM. Right?

[00:13:00]So if you wake up at 6AM, then what's  gonna happen is your cortisol is gonna start to increase. Now you notice it is  not peaking right at 6AM. Right? It's not like you wake up just like, you know,  that'd be that'd be intense. Instead, you'd you know, it takes a little bit of time to  get going, and then all of a sudden, like, yeah.

[00:13:17]I feel good. But then what's gonna happen  is throughout the course of the day, it's just gonna be in this slow decline.  Now there's other things that are gonna help keep you awake, but cortisol is really  there to just kind of whoop, help bring you.

[00:13:29]Right? Cortisol is a hormone of alertness,  and so it's gonna drop and drop and drop, and then you're gonna see it's going to hit  its very lowest point at around midnight or so, assuming you're on a normal sleep pattern,  and then it's gonna start to rise.

[00:13:44]And, eventually, it's gonna get to  a point where you're gonna wake up, and then it's gonna do the exact same thing.  Well, at the same time, melatonin when you wake is going to decrease, and then that is just gonna  crater. Right? You don't need to be awake. Right?

[00:13:57]You don't wanna be awake at I mean, sorry. You  don't wanna be asleep. You do wanna be awake.

[00:14:01]You don't wanna be asleep at noon, And then what's  gonna happen is it's gonna come all the way up, and then it's gonna start to peak. And  then you can see it's really peaking here.

[00:14:10]This is about, like, 1AM, 2AM, 3AM, around  that time frame. And then all of a sudden, it's gonna start just dropping. And so what we  see is there's these crisscross patterns. Right?

[00:14:21]So at around 6PM, this is where you're really  gonna start feeling just the lag of the day.

[00:14:26]Right? Thing like, the wearing, the tearing down  of the day, just everything kinda, you know, wearing on your on your brain. But at the same  time, over here, this is where you're like your body is just starting to wake up. Right? And so  I don't know if any of you ever track your sleep.

[00:14:38]You know, I'm a big fan of tracking my  sleep. I've been doing it for probably, like, fifteen years at this point. The the the  methods for tracking it have gotten profoundly better. But it's really interesting to be  looking at this to see kinda just like the cycles you're going into between deep sleep  and that and that's a whole other level.

[00:14:55]That's a whole other thing where we're talking  about different, you know, brainwave states too.

[00:15:00]But at the same time here, you have to  understand, it's like right before you wake up, it's like you you're jostling. You're moving  around in your bed, and that's because cortisol is increasing and melatonin is decreasing. And  eventually, that crisscrosses, and sure enough, there you are. That's exactly what you're  doing. So if we go back to this first image, you know, again, you can see here  that, like, melatonin is really high.

[00:15:21]Right? Cortisol is really low at this point,  but adenosine is really high as well. And so that all of that kinda adds together and compels  you to sleep. And then as you clean up right? So you're gonna clean up all that adenosine and all  the other metabolites that have been accumulated in your brain throughout the day, and then your  cortisol is gonna slowly start to rise along with other things that are kinda compelling you to wake  up, and all of this is happening in about a day.

[00:15:47]Right? I mean, again, this isn't this  isn't a perfect twenty four hour cycle, but it works pretty consistently. And this  is why, again, if you're in that, you know, darkened windowless I don't know what situation  you're in to be here. I don't know why you're here and you need to leave. But if you're in this  horrifying scenario, you would have a rough idea.

[00:16:05]You know, you'd be like, you know,  it feels like midday. You know, it feels like we're in the middle of the  night, and that is just gonna be based around all of these things working together for your  biological clock or your circadian rhythm. So, again, this is part of a massive lesson all about  sleep and wake cycles. This is a premium feature, but remember there are other free features  for you to go ahead and check out. So, like, if we click on this article  here, this is 100% free.

[00:16:34]It's all about the suprachiasmatic  nucleus. Again, the videos, those are gonna be a premium feature as are  our quizzes, but you get to see all of these amazing images for 100% free. We're gonna go  ahead and leave a link down the description below to a separate but equally awesome premium  feature. Right? It's gonna be on the reflex arcs.

[00:16:52]It says study unit. But, you  know, while we're down there, we'll also link to one of these free articles.  Let's do the suprachiasmatic nucleus. I'm feeling adventurous today. So you're gonna  see two links down in the description.

[00:17:02]Go ahead and check those out. You're not  you're not going to regret it. But hopefully, made a lot of sense and you enjoyed  today's video. If you did, you know, do what YouTubers say. Smash that like button,  hit subscribe, comment, all that kind of stuff.

[00:17:19]But either way, I'm very appreciative of you hanging out with me, and I  will see you in the next video.