نطق اول

Lecture 1:

Listen to part of a lecture in a biology class.

Okay.Today I want to talk about sleep.We all sleep.Humans sleep.Many animals sleep.When we sleep,we aren’t actually unconscious,but in a state of reduced awareness of our surroundings.Now,what does that mean for animals in the wild if they are in a state like that unable to monitor their environment?

Well,they are helpless,vulnerable to predators maybe.Right,they are vulnerable to predators,yet they sleep,so let’s talk about why,the biology of sleep.So the first thing I want you to understand is that sleeps are very risky behavior.And risky behaviors don’t evolve unless they bring the animal some kind of benefit that outweighs that risk.Sleep must have a really important function.

And I want to emphasize that there is a difference between sleep and other forms of inactivity,like regular rest.Insects,for instance,rest,but they don’t sleep.What’s specific to sleep is that the brain alters many ofits usual activities including its connection to sensory and motor organs.It shuts those connections down,so a sleeping animal can usually neither sense nor move.I say usually because,well,we’ll get to that in a minute.

So basically all mammals and birds sleep,but there are some unusual ways of sleeping.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,so their brain can’t shut down completely,but dolphins get around this.How?

Well,they sleep one brain hemisphere at a time.Sleeping dolphins actually look like they are just resting awake and occasionally swim up for air.So how can we even tell they are sleeping? Well,we measure their brain activity,which showed that one half was active while the other was sleeping.That’s some adaptation,huh?

So what is the function of sleep?As I said,it must be important,but why? Okay.I know you are all tempted to say that sleep’s when the whole body rests,that sleep is our mechanism to recover from physical activity.And that’s true.When you sleep,your muscles lose their tone,they relax.The body saves energy.

But this could happen during regular waking rest,I mean,that’s probably what happens when insects rest.You don’t need sleep for that.You could get the same benefit by resting awake.So this doesn’t explain why sleep evolved.To explain the real function,the most important benefit of sleep,you’ll need to focus on the brain,not the muscles.Remember how we can tell that a dolphin’s sleeping,not just resting?

It’s because of a unique pattern of brain activity,which is a clue that sleep’s function has to do specifically with the brain.And that’s the general consensus ‘cause that’s what could explain why it evolved.It also explains why sleep is a characteristic of mammals and birds because they have the most complex brains of all animals.The simpler brain,like a reptile’s,gets by with little sleep,or even without sleep.

Okay.So we know that sleep benefits the brain,but in what way?We’re not sure yet.Okay.So one hypothesis is that during sleep the brain synthesizes molecules that it needs for proper functioning when awake.The longer we stay awake the more those molecules get depleted,so the brain needs to replenish them,and this supposedly happens during sleep.

I’m talking about energy sources like glycogen and some types of neurotransmitters that are needed to transmit signals between nerve cells.So these get replenished during sleep and the next morning our brain is working with a full supply of energy and neurotransmitters again. That explains why we feel so alert and mentally refreshed after a night’s sleep. Yes.Jim?

Well,you know,I don’t.I don’t work well in the morning at all.

Yeah,I do my best work at night too. Not everyone is at their best in the morning.I’ll grant you that,but that doesn’t rule out the replenishment hypothesis.

It’s just a bit more complicated than I said. We are fairly sure that there is this other rhythm of mental activity going on that’s independent of sleep.It’s like a preprogrammed 24-hour cycle.Your mental activity peaks at a certain hour every day,like at night,for you too,then it goes down at some other point.

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,so the replenishment may be needed to keep the mental peaks high.Does that make sense?

The two mechanisms are not mutually exclusive.That’s what I’m trying to say.

by accident.