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This month's theme is rhythms, so let's explore the living and non-living cycles that allow life on Earth to live, adapt, and thrive. [CHEERY INTRO MUSIC]. A rhythm is any kind of regular and repeated pattern.

Repeated patterns often occur in cycles of some sort, where there's a process that goes to completion then starts over. Biological rhythms are cyclic patterns that occur in organisms. These patterns are so important that there's actually an entire field of research dedicated to studying them called chronobiology.

Chrono- means time, bio- means life, and -ology means study of, so this entire field is about studying the periodic, or cyclical, processes of life on Earth. It's hard to miss rhythms and cycles living here on Earth. For starters, there's the whole light and dark thing happening outside on certain intervals, and depending on where you live you might get regular patterns of warm or cold weather.

The majority of rhythms and cycles on Earth are a result of the Earth itself- specifically, the fact that it's a huge ball of mass rotating and orbiting another even bigger ball of mass in space. Even if the Earth was completely barren of life, the rocks, minerals, water, and atmosphere itself would all be affected by the rhythms and cycles happening throughout the galaxy. For starters, the Earth rotates on its own axis- this spinning results in what we call day and night.

And while it's spinning, the Earth is also rotating in an orbit around the sun. Orbiting, in addition to the Earth's axis being tilted, results in what we call seasons. We've only touched on two types of cycles by discussing the Earth's spin and orbit, and yet these rhythms have a /major/ effect upon life on Earth.

Predator and prey dynamics are different during the night than during the day, and the changing availability of sunlight across the seasons affects ecosystems from the bottom up. And we haven't even talked about the moon yet! The orbiting of the moon around the Earth and its relative position to the Sun results in the lunar cycle- that constant change we see in the night sky from new moon to full moon back to new moon.

And while some of us certainly enjoy just watching those changes in the night sky, the lunar cycle has a big impact on life of Earth- specifically, marine organisms. High tides are actually higher during the full moon, and these tidal rhythms result in and serve as environmental cues for behavioral changes. So yeah, in terms of rhythms and cycles there's a /lot/ going on with just Earth and its location in space.

But the field of chronobiology isn't just about what's happening outside- it also considers what's happening /inside/ an organism. Internal, or endogenous, biological rhythms are controlled by signals /inside/ an organism. And some of the most important endogenous rhythms on Earth are called circadian rhythms- these repeat on a cycle of approximately 24 hours.

That's right- the whole Earth spinning around its axis thing has lead to some of the most widespread and critical biological rhythms on the planet. Circadian rhythms are present in animals, plants, and even in bacteria! What's really cool is that while circadian rhythms line up with this external process, circadian rhythms are regulated by processes inside the body, and they happen even without environmental cues like light and temperature.

And this isn't just in theory. When organisms are placed in continuously dark regions underground, or even into space, their circadian rhythm still exists- it just gets slightly off, or desynchronizes. This is possible because circadian rhythms are controlled by biological clocks.

While you might keep track of time on your cell phone's clock, almost all of the tissue and organs in your body keep time by using biological clocks, which are made up of special proteins. Biological clocks direct the expression of specific genes as well as cellular activity. But here's the thing.

There are so many cells and biological clocks throughout an organism that there has to be some kind of main synchronizer. Imagine if you have a power outage in your house, and when the power comes back on you have to reset all of the clocks in the kitchen. There's the one on the stove...microwave...coffee maker...etc.

And you want them to all be in synch, so you probably set them all to your phone's time, or one on a laptop. Bye bye biological clocks! But what does this in organisms?

In mammals, there's a master clock inside the brain that coordinates the biological clocks throughout the body. It's inside the hypothalamus and made up of about 20,000 neurons, and yes, it has a long and complicated name. The master clock structure of nerve cells is called the suprachiasmatic nucleus, but luckily we can go ahead and shorten that to the SCN.

While environmental cues like light can reset or calibrate circadian rhythms by stimulating the SCN (think jet lag), genes themselves are responsible for several components of biological clocks and feedback loops. These are called clock genes, and they've been discovered in humans, fruit flies, mice, fungi, and several other organisms. Scientists have found that clock genes can influence all kinds of things, including sleeping during the night, hibernation during the winter, and potentially even behavior. 24ish hour circadian rhythms aren't the only periodic biological rhythms we see on Earth.

Some rhythms are shorter than 24 hours and are called ultradian rhythms. These include things like 90-minute REM cycles during sleep and cyclical fluctuations of hormones throughout the day. And some biological rhythms are much longer than circadian rhythms.

These are called infradian rhythms and include seasonal patterns like migration, hibernation, and reproductive cycles. Life on Earth seems really unpredictable at times, but even in all of its chaos, life has some patterns that are deep set across species, time, and space. We're only recently discovering the genetic mechanisms underlying the mysteries of these rhythms, and who knows what we'll find out next about the cycles of life on Earth.

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