When do you feel sleepy?

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When do you feel sleepy?

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Alice and Neil discuss circadian rhythms - the so-called body clock that influences an organism's daily cycle of changes

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Hello and welcome to 6 Minute English. I’m Alice…

And I’m Neil. And I feel terrible!

You look terrible, Neil - if you don’t mind me saying so!

It’s because I had to get up really early this morning.

Oh dear! What time did you have to get up?

Eight o’clock.

Oh, Neil! That isn’t early! I get up at six every day. It’s so peaceful early in the morning.

Hmm. Well, some people are morning people and others… aren’t!

Yes. Well, today we’re talking about the biological reason for this - it’s all about circadian rhythms. They are produced by a so-called body clock in our brains that regulates our body functions: our body temperature, sleepiness and alertness, hunger, and hormone levels. Plants, animals, and many microbes have circadian rhythms.

You know a lot about circadian rhythms.

And I’ll ask you a question related to them. What does the word ‘circadian’ mean?

Is it…

a) around a day?

b) every day?

or c) twice a day?

Hmm. I’m going to say a) around a day.

Well, we’ll find out whether you got the answer right or not later on in the show. Now let’s talk about circadian rhythms and our internal clock.

Why do our bodies need an internal clock to tell us where we are in the day? Isn’t it obvious?

No, it isn’t - take jet lag, for example. We rely on the predictable cycle of light or dark in a 24-hour period to synchronise - or adjust - our body clocks to the environment - and if we mess about with the light and dark cycle by flying into a new time zone, it makes us feel really bad!

Good point - jet lag is the disruption of our circadian rhythms caused by high-speed travel across different time zones, which can cause tiredness and sleep problems. But Alice, if we rely on day turning to night to adjust our body clocks, what happens to blind people? - Because I assume their body clocks can’t do this.

Blind people who have some light perception are able to synchronise their circadian rhythms to the light-dark cycle. But those who have no light perception at all… well, let’s listen now to Debra Skene, Professor of Neuroendocrinology at the University of Surrey. She can explain what happens.

Debra Skene, Professor of Neuroendocrinology at the University of Surrey Totally blind people - they’ve lost that connection between the light-dark circle and the clock. So there isn’t anything wrong with the clock but the clock ticks and oscillates at its own endogenous period just the same as if I were to put you in a dark cave. Your biological internal clock would oscillate at your endogenous circadian period.

So if you’re totally blind - or able to see but living in a dark cave - you have a ticking clock but with no connection to the outside world. The clock oscillates at its own endogenous , or internal, period.

Oscillate means to move back and forth in a regular rhythm - like the pendulum on a clock.

Do you think my endogenous clock ticks faster than yours, Alice?

It isn’t a competition, Neil. And actually, mine probably ticks faster than yours since I’m a morning person. Anyway, the normal range in humans is between 23.8 to 24.8 hours. And this is also true for totally blind people.

But their clocks are free-running - they don’t get cues from the outside environment telling them when to wake up, when to eat, when to feel sleepy. So that means they might feel sleepy at the wrong time of day - for example, when they’re at work. Or alert in the middle of the night when they should be asleep.

It’s worth talking about people who do shift work too - which means work that takes place outside the traditional 9 to 5 day.

Shift workers may suffer similar problems to blind people because they are trying to sleep against the clock. They might sleep in the day and work at night for example - which goes against the light-dark pattern.

There are some long-term health problems associated with shift work - certain cancers, heart disease, and obesity.

So what can people do to help adapt their circadian rhythms to a night shift schedule?

Well, let’s hear what Professor Debra Skene has to say about it.

Debra Skene, Professor of Neuroendocrinology at the University of Surrey We do think that exercise and food, caffeine, may be able to modulate in some way, so has some influence on circadian timing, but not as strongly as the light-dark cycle.

So the strongest influence over our circadian rhythm is the light-dark cycle. We can’t alter night and day, after all!

Debra Skene says that other cues such as food and exercise will modulate - or adjust - the body clock. So eating three well-balanced meals at regular times each day can help your body clock adapt to an unusual schedule.

Taking naps - or short sleeps - just before you start a night shift can help you feel more alert. And keeping to the same sleep schedule every day will also help.

And don’t forget caffeine - my old friend! A cup of coffee works wonders for me in the morning. Now remember I asked: What does ‘circadian’ mean? Is it… a) around a day, b) every day or c) twice a day?

And I said around a day.

And you were… right! Well done, Neil. The term ‘circadian’ comes from the Latin circa , meaning ‘around’ (or ‘approximately’), and di ē m , meaning ‘day’. Now, let’s hear the words we learned today.

They are:

circadian rhythms


jet lag



shift work



That’s the end of today’s 6 Minute English. Don’t forget to join us again soon!


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