[00:00.00]Professor: OK, today, I want to talk about sleep. We all sleep. Human sleep, many animals sleep. [00:11.92]When we sleep, we aren't actually unconscious, but in a state of reduced awareness of our surroundings. [00:19.22]Now, what does that mean for animals in the wild, if they are in a state like that? Unable to monitor their environment?
[00:27.05]Male Student: Well, they're helpless, vulnerable to predators, maybe.
[00:31.00]Professor: Right. They are vulnerable to predators. Yeah, they sleep. [00:35.41]So let's talk about why, the biology of sleep. [00:40.00]So the first thing l want you to understand is that sleep's a very risky behavior. [00:45.38]And risky don't evolve unless they bring the animals some kind of benefit that outweighs that risk. [00:52.41]So sleep must have a really important function. [00:55.86]And I want to emphasize that there is a difference between sleep and other forms of inactivity, like regular rest.
[01:03.42]The insects, for instance, rest. But they don't sleep. [01:07.61]What's specific to sleep is that the brain alters many of its usual activities, including its connection to sensory and motor organs, it shuts those connections down. [01:19.71]So a sleeping animal can usually neither sense nor move. And I say usually because…well, we will get to that in a minute. [01:28.99]So, basically all mammals and birds sleep. But there are some unusual ways of sleeping.
[01:37.82]Take marine mammals. Like dolphins, dolphins need to swim up for air once in a while, so they can't completely shut off all movement and sensation. [01:48.56]So their brain can't shut down completely. But dolphins get around this. How? [01:55.12]won't they sleep one brain hemisphere at a time? [01:59.41]Sleeping dolphins actually look like they're just resting, awake and occasionally swim up for air.
[02:05.45]So how can we even tell their sleeping? Well, we measured their brain activity which showed that 1/2 was active while the other was sleeping. That's some adaptation, uh?
[02:18.18]So, what is the function of sleep? As I said, it must be important, but why? [02:25.07]Okay. I know you're all tempted to say, uh, that sleeps when the whole body rests that sleep is our mechanism to recover from physical activity. [02:37.40]And it's true. When you sleep, your muscles lose their tone, they relax, the body saves energy. [02:44.13]But this could happen even during regular waking rest. I mean, that's probably what happens when insects rest. [02:51.00]You don't need sleep for that. You can get the same benefit by resting awake. So this doesn't explain why sleep evolved.
[02:59.14]To explain the real function, the most important benefit of sleep. You'll need to focus on the brain, not the muscles. [03:07.46]Remember how we can tell that a dolphin's sleeping not just resting, is because of a unique pattern of brain activity, which is a clue that sleeps function has to do specifically with the brain. [03:19.92]And that's the general consensus cause, that's what could explain by it evolved. [03:25.80]It also explains why sleep is a characteristic of mammals and birds, because they have the most complex brains of all animals. [03:33.87]The simpler brain uh like a reptile gets by with little sleep or even without sleep. [03:40.71]OK, so we know that sleep benefits the brain but in what way? We are not sure yet.
[03:46.68]Okay. So one hypothesis is that during sleep, the brain's synthesize molecules that it needs for proper functioning when awake. [03:57.53]The longer we stay awake, the more those molecules get depleted. [04:01.99]So the brain needs to replenish them, and this supposedly happens during sleep. [04:06.99]I'm talking about energy sources like glycogen and some types of neuro-transmitter that are needed to transmit signals between nerve cells. [04:15.76]So these get replenish during sleep and the next morning our brain is working with a full supply of energy and neuro-transmitters again. [04:24.47]With that explains why we feel so alert and mentally refreshed after night sleep. Yes, Jim?
[04:30.92]Male Student: Well, you know, I don't work well in the morning at all.
[04:35.57]Female Student: Yeah, I do my best work at night too.
[04:38.38]Professor: Not everyone's at their best in the morning. I grant you that. [04:42.31]But that doesn't rule out the replenishment hypothesis. It's just a bit more complicated than I said. [04:48.64]They were fairly sure that there's this other rhythm of mental activity going on that independent of sleep. [04:55.64]It's like a pre-programmed 24 hours cycle. Your mental activity peaks at a certain hour every day, like at night, for you two. [05:04.35]And then it goes down at some other point. [05:07.55]But the point is that after a sleepless night, that peak is not as high as it would normally be, and the longer you go without, the lower those peaks get. [05:17.55]So the replenishment may be needed to keep the metal high. Does that make sense? [05:23.43]The two mechanisms are not mutually exclusive. That's what I'm trying to say.
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