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#bioPGH Blog: Circadian Rhythms
Oct 19

#bioPGH Blog: Circadian Rhythms

By Dr. Maria Wheeler-Dubas, Research and Science Education Outreach Coordinator

Biophilia NetworkA resource of Biophilia: Pittsburgh, #bioPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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Every autumn, some of the biggest science news is the announcement of the Nobel Prize winners. This year, the Prize in Medicine or Physiology went to three researchers “for their discoveries of molecular mechanisms controlling the circadian rhythm.”  These three men, along with their respective research teams and collaborators, laid out the groundwork for how we understand circadian rhythms—the collective term for daily cycles of activity in living things. Some form of a 24-hour rhythm has been observed in a wide variety of organisms including plants, microbes, fungi, and vertebrates, like ourselves. Since circadian rhythms have been making the news lately with the Nobel announcements, let’s look a little closer at what a circadian rhythm really is!

For humans (as mammals), our daily lives include behavioral, physiological, and cognitive patterns that occur over the course of a day. For example, most of us notice that we tend to feel sleepy or awake at certain times, and there are times of day when our reaction times are fastest or our blood pressure is the highest. That groggy feeling of jetlag is our body having to reset the daily “clock” of our circadian rhythms. As primarily diurnal (awake during the day) creatures, human activity is largely based on day time versus night time, which are the same cues for a nocturnal species, only they use those signals differently. That leads us to an important point, though: circadian rhythms are largely based on sunlight.

How are our daily rhythms based on sunlight? This is where things get a bit complicated. Your brain has cluster of specialized nerve cells (neurons) called the suprachiasmatic nucleus, or the SCN. The retinas of our eyes pass information about the amount of daylight to this cluster, and the SCN then directs the production of chemicals in the body that regulate daily functions and activities. (And in case the question arose in your mind, this does mean that visually-impaired individuals can be susceptible to circadian rhythm disorders.)

Well, if humans and other animals use our retinas to relay information about our 24-hour daily cycles, how do organisms without eyes do the same thing, and what are they doing differently at different times of day? You may remember from an earlier post that may flowers open and close at different times of the day. In addition to that, other plants may change the positions of their leaves throughout the day in addition to undergoing different kinds of metabolic processes at varying times of day. Plants determine the presence or absence of light through an assortment of phytochromes and cryptochromes—complex pigments and proteins that allow plants to detect red or blue lights, respectively. Once these compounds detect different types of light, they activate the associated functions within the plants by regulating different activities within plant cells. Plants can also use temperature as a cue for daily activities as well, though, light often “overshadows” this cue. Similarly, fungi can use both light and temperature to regulate daily changes in activity. For fungi, the suite of daily events can range from metabolic activities to bioluminescence (lighting up in the dark) and beyond, depending on the species.

So no matter if you’re a human, or a cat, or a daisy, or a mushroom—the time of day affects you! If you’re reading this late in the evening or in mid-afternoon, perhaps you’re feeling a bit sleepy. Or maybe it’s mid-morning and you’re still ready for the day to keep rolling. Either way, isn’t it amazing how nature keeps track of time, even without a watch!

Connecting to the Outdoors Tip: On your next venture into nature, especially if you have small children, make a list of the different animals you see or expect to see. Are those animals nocturnal (active at night) or diurnal (active during the day)? Or are they perhaps crepuscular, active during twilight? If you have plants near your home, check out the position of the leaves at midday and in the evening. Depending on the species, the leaf positions might differ throughout the day.

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Science Direct: Suprachiasmatic Nucleus

UCLA Health: Circadian Rhythms

Nobel Prize Winners 2017

NIH Circadian Rhythms Fact Sheet

Plant Cell: Plant Circadian Rhythms

Oliviera et al. 2015: Circadian Control Sheds Light on Fungal Bioluminescence

Photo Credits: Jacqui Barker and Peter Trimming CC-BY-2.0