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The most interesting thing that came to my mind reading this was how different life would be without any distinction between night and day. If the amount of sunlight reaching the planet were consistent at all times, would life (assuming it followed an evolutionary path like earth's) still evolve to have frequent sleep or rest periods?

This is so intriguing indeed.

Maybe most creatures on such planets would evolve towards unihemispheric sleep, as some Earth species did (eg. dolphins).

Unihemispheric slow-wave sleep (USWS) is sleep with one half of the brain while the other half remains alert.


I was wondering if such creatures would sleep at all with 24 hour daylight. They might evolve other mechanisms to get the same benefits.

Lions sleep 22 hours/day, even though it is light during many of those hours.

Light/dark is only one factor in sleep, there are others.

Evolution on earth is heavily influenced on dark/light cycles through its entire history. No evolutionary path that has received this influence can tell us anything about what would happen, on things that are highly correlated to it, for one that has never received this influence.

of course, but the reasons lions sleep far more than the day/night cycle requires are a factor that applies elsewhere, and thus might result in something similar.

It's tough to say, there is more than one purpose for sleep, and it may have served other purposes in the evolutionary history that no longer benefit all organisms. For example, humans no longer need to be safely holed up while predators roam the Savannah at night.

It's also hard to know of the infinitely many evolutionary twists and turns lifeforms in the Universe may take, how many of those pathways involve something like sleep - especially on planets without a diurnal cycle.

Anything and everything is a wild guess based on a sample of one where extraterrestrial life is concerned.

Why would you assume there'd be animals at all?

Rogue planets [0] are also very intriguing. Planets drifting through interstellar space without any star or neighbors.

Although they may never get sunlight, I suspect the view in their sky would be spectacular indeed (like ours when away from cities and light pollution, but even better.)

Maybe all that unfiltered starlight would be enough to coat the planet in a unique twilight all around, or maybe life there would specialize in other senses besides sight.

The planet may generate sufficient heat from its core and most of the life there might live underground, or there might be some other heat-providing geochemical processes on the surface.

[0] https://en.wikipedia.org/wiki/Rogue_planet

[1] http://www.bing.com/images/search?q=Milky+Way+from+Earth

One of the Star Treks visited an inhabited rogue planet. If life could find a way in such circumstances, I’m sure it would be very interesting. I imagine it would also be uniquely likely to visit and leave star systems.

I love this imagination!

There are a ton of questions to be raised about how life on such a planet would work. Life on earth is driven by rhythms: a circadian rhythm, tidal rhythms, monthly rhythms, seasonal rhythms - it begs the question whether life is even possible without these built-in cycles.

My initial hunch is yes, and perhaps we can look for evidence of such in caves, deep ocean, and single-cellular life.

We have very similar conditions just a few dozens meters under the sea, where sun light doesn't reach. If I remember correctly, creatures that live there still sleep, although their cycles are not linked to 24 hours.

I wonder if there are still some kind of seasons down there or if the tides have an impact.

I suspect tides have an impact until you leave the continental shelf and get into deep, deep water.

The thermal vents from volcanic activity seem to be reasonably well insulated from cyclic activities at the surface. They would likely only see variation from cycles with much longer scales, such as the 26 ky axial precession cycle, the 41 ky axial tilt cycle, the 100 ky orbital eccentricity cycle, the 112 ky apsidal precession cycle, the 300-500 My tectonic supercontinent cycle, etc--the ones that can change climate and geology, rather than just weather.

The vent dwellers might get an inkling if an ice age has been going on the surface for a while, or if runaway greenhouse effect is boiling off the ocean surface.

Sleep is the natural state of life, it uses far less energy and we only wake up as a means to procure more energy. On a planet where plants have a constant source of energy to grow the animals may sleep more.

Sleep is the natural state of life on this planet. Other planets may have evolved different biology where the concept of sleep is unknown, unneeded or impossible.

Your metabolism does not slow down while sleeping so it does not really save that much energy.

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