D&D 5E Circadian Rhythms of the Underdark

[BookTenTiger]

What marks the cycle of days in the Underdark?

On the surface, the rising and setting of the sun help establish when the day starts and ends. However, there are many other natural circadian cycles that mark periods of the day. Birds start chirping in the morning. Nocturnal animals awaken at night. Flowers open in the morning, close in the evening. The temperature gets warmer throughout the day, colder throughout the night.

What would be equivalent circadian rhythms in the Underdark?

I like the idea of fungus that glows during "daytime" and fades at "night."

Tides that still feel the pull of the moon.

Flocks of bats or other flying predators that awaken at the same time every day, travel en masse to their hunting grounds, then return 12 hours later.

What are some other ideas?

 

[J.Quondam]

Periodic seismic activity, like geysers.
Feeding cycle of underground beasties, like cave crickets, darkmantles, or fire beetles or whatnot.
Changes in air currents in response to day/night/seasons on the surface.

Over very long terms, underdark denizens might mark the growth of stalagmites or the fading of glowing (radioactive) mineral veins.

 

[MarkB]

Sufficiently large seas in the underdark would experience tides. I tied that in as a plot point in the Kuo-Toa section of Out of the Abyss.

 

[Ralif Redhammer]

Menzoberranzan uses Narbondel, a massive pillar on which magic is used to create light (fire in past editions) that would traverse the pillar to mark the passing of time and the end of a day.

 

[Faolyn]

I'm (slowly) working on a "Demiplane of Dungeon" setting where mysterious gongs peal out every sixish hours. First one gong, then two, then three, then four, then back to one.

 

[Yora]

Do cave animals have sleep patterns?

 

[BookTenTiger]

Do cave animals have sleep patterns?

I believe they have found evidence of circadian rhythms even in blind cave fish.

Here's an article I failed to comprehend:

Circadian rhythms in Mexican blind cavefish Astyanax mexicanus in the lab and in the field - Nature Communications

Body clocks modulate physiological processes to follow a day–night cycle, but whether animals exposed to constant darkness express circadian rhythms is unknown. Here the authors examine the expression of circadian genes in Mexican cavefish, and find that these resemble a pattern expected from...

www.nature.com

 

[AdmundfortGeographer]

I liked the idea that some foundational life form, such as a phosphorescent powder-like fungus has some biorhythm tied to time, such as day and night. A fungus which was a food source for species who themselves were food sources, etc.

In caverns where it’s in large quantities, maybe the cycles of the fungus establish daily rhythms to match.

Downcrawl essentially offers this which I adopted for my underground regions.

 

[Mind of tempest]

temperature changes would likely be important.