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You wake up in the morning and go to sleep at night. But some animals do the opposite: wake at night and sleep during the day. What determines the time of day you’re active is your circadian rhythm, an internal clock that keeps daily time for you.

Internal Rhythms

Your body is a living, breathing clock.

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Everything about you runs on a constant schedule, not just when you’re awake. Your body temperature, cell division, blood pressure, urine composition, metabolic rate, and even your responsiveness to medication is all scheduled and tied to a specific time of day.The craziest part is that even if you locked yourself in a room that had no windows or clocks, your body would continue to run on this schedule! This 24-hour biological cycle is called a circadian rhythm. This comes from the Latin ‘circa’ for ‘about’ and ‘dies’ for ‘days.’Both plants and animals have circadian rhythms, though this internal rhythm, or biological clock, is not the same for everyone. Some clocks, like ours, have us up and active during the day, making us diurnal. Other animals, like many mammals, run at night and are nocturnal (‘nocte’ means ‘night’).

No matter when you are up and active, that internal clock keeps ticking. The daily rhythm of organisms continues even without any stimulus, such as daylight. Your bodily processes will run normally even when locked in that windowless, clockless room.

External Cues

However, in order to keep it calibrated to exactly 24 hours, the body does require signals from the outside world.

This is because our bodies are very closely tuned to 24 hours, but some are set for a little bit more or a little bit less. This may be one reason why you need a full nine hours of sleep to feel good in the morning while your friend does just fine with only seven.The changing of day to night and night to day is what keeps our internal clocks on track. But if you’ve ever flown to a different time zone or lived in an area that practices Daylight Savings Time, you know that these cues take some time to adjust to.When the time changes quickly, as is the case with the plane flight, our internal clocks have to play catch-up, causing us to have ‘jet lag.’ In this case, our internal biological clock is out of sync with the environment around us, so we don’t feel quite right.

Biological Clocks

The concept may be simple, but the biological clock itself is still somewhat mysterious.

We know that in mammals the biological clock is found within a small cluster of nerve cells in the hypothalamus. But we don’t know much about the biological clock of other animals and plants.One thing that makes biological clocks different from other metabolic processes is that they aren’t affected by temperature. This is a very good thing, too! As your body temperature decreases, your metabolic processes slow down, and as your temperature increases, metabolic processes speed up. But your clock doesn’t slow down or speed up – it keeps on ticking at the same rate.

Imagine if your clock did change speed with every little temperature change – you’d probably always feel like you had jet lag, even if you didn’t go anywhere!Biological clocks not only keep track of the time of day but also the time of year. This is especially important for plants, which grow at different rates during different seasons. Some plants flower in fall and winter when the days are shorter, while others flower in late spring and summer when the days are longer. These plants keep track of the time of year by how much sunlight they receive each day, and this helps them to know if it’s time to flower or not.

Lesson Summary

We sleep at night and awake in the morning because our biological clocks are programmed to do so. This internal rhythm is calibrated to a daily 24-hour cycle called a circadian rhythm. All living creatures have a biological clock, though the specifics of how that clock is calibrated differ.In mammals, the biological clock is found in the hypothalamus, but we don’t know much about how biological clocks work or where they are located in other animals and plants.What we do know is that while biological clocks require environmental cues such as daylight to be calibrated to an exact 24-hour cycle, they don’t require any stimulus to keep running on their normal, constant schedule. And, luckily for us, they run on their own pre-programmed schedule, ignoring variable environmental factors like temperature.

Learning Outcomes

Now that you are finished with this lesson you should be able to:

  • Define circadian rhythm
  • Describe the importance of the day-to-night transition for our biological clocks
  • Know which gland houses the biological clock in the body

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