Humans are diurnal. We have chemical signals the control our waking and sleeping cycles, we are evolved with good day and color vision, poor night vision and smell, and an excellent ability to shed heat in the mid-day sun.

That being said, there exists a group of humans in a rainforest environment where the best time to be awake is twilight. For these people, the primary food source varies by wet and dry season, but either way it is fish. The old fisherman's adage is that the fish are most active at dawn and dusk. In the dry season, these people use twilight to fish in oxbow ponds and shallows on the edges of the great rivers of the forest. In the wet season, the rivers flood their banks and become long lakes. The people fish just before daybreak and after sunset, attracting small schooling fish by lantern-light.

Nighttime picture of a lake in a rainforest

There are other agricultural activities, which primarily revolve around gathering fruits and nuts from semi-wild orchards planted in the forest, and growing vegetables in floating farms (detailed in the linked question).

Overall, would it be beneficial for these people to adopt a fully crepuscular lifestyle? A crepuscular chronotype would be one where there are two periods of sleep per day, one during mid-day and the other mid-night. There would be two ~six hour periods of activity around dawn and dusk, with two ~six hour periods of rest in between.

Are there chemical or biological reasons why this would not work? Does the advantage of being awake when your primary food source is available overcome the poor nighttime senses for an agricultural society?


  • As this is a tropical rainforest, day length does not vary over the year, so no need to worry about long days or nights.

  • Technology level is Bronze Age. The crepuscular lifestyle would have developed after these people became primary fishermen, perhaps a few thousand years before.

  • The humans are biologically identical to us. The environment is comparable to what one might find in the Amazon or Congo basin.

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    $\begingroup$ You've probably seen this, but this reddit post claims primates are likely to become crepscular if they face predators during the day and night. So removing humans from the top of the food chain might be a decent place to start. $\endgroup$
    – Random
    Commented Feb 2, 2018 at 5:11
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    $\begingroup$ We sleep 8 hours out of 24 (on average), so your crepscular humans would probably sleep 2x4 hours or 6 hours at night with 2-3 hours during the day (siesta!) $\endgroup$ Commented Feb 2, 2018 at 8:21
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    $\begingroup$ The humans are biologically identical to us The human circadian rhythm is hardwired in your design. Humans who have to try and live a different pattern (even a regular night shift) are known to suffer negative health consequences. So you'll need to make them somewhat different biologically. $\endgroup$ Commented Feb 2, 2018 at 10:55
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    $\begingroup$ @Hankrecords Jet lag is simply an adjustment of your circadian rhythm from one time zone to another, but the OP is talking about breaking the fundamental basis of the diurnal circadian rhythm. In a human that would almost certainly lead to psychological problem like depression. Also note that the day-night cycle matters a lot - it's another way to totally mess up your brain. Note that depression and the day-night cycle and a normal cycle cycle are closely linked and it's hence no accident that psychologists will treat sleep problems as a major issue. $\endgroup$ Commented Feb 2, 2018 at 11:26
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    $\begingroup$ I do not think he said it is impossible. You can sleep your night normally and then also rest during the day. Lots of people and animals in the tropics do that I think. I think Stephens issue was that if you cut down the amount of sleep during the night to 4 hours you would be messing with the circadian clock and that would not be "biologically identical". $\endgroup$ Commented Feb 2, 2018 at 13:47

4 Answers 4


A six-hour sleep in the middle of the day is quite unusual, but 2-3 hours? The Spanish call it a siesta.

The advent of air-conditioning has made them less common, but the principle was that from about 11:00 to 14:00 it was just too hot to do anything. So, the working day started earlier, in the cool twilight of dawn, and ended later, in the cool twilight of dusk.

So, a crepuscular lifestyle has been proven over thousands of years to be perfectly viable - the only real question is about making it 6 hours of sleep. Although, looking at Mediterranean countries, I suspect a 4:8:4:8 pattern of sleep/awake is more likely than 6:6:6:6


I am from India and I can say that already there is some change in chronotype of our IT professionals in the past decade or so. They have to work when their clients are active say in US and other western nations. So the Multi National Companies here in India (like TCS, Wipro, Infosys,..) have a workforce available during our night time. I think economic reasons will be the basis on which human chronotype would change.

  • $\begingroup$ What are MNC's? Could you edit that in to your answer? $\endgroup$
    – kingledion
    Commented Feb 6, 2018 at 18:36
  • $\begingroup$ That's not really changing the chronotype, as the OP suggests. That is, having two periods of activity, around dawn & dusk, and sleeping in the middle of the day and night. $\endgroup$
    – jamesqf
    Commented Feb 6, 2018 at 20:20
  • $\begingroup$ @jamesqf Technically, noctural is a different chronotype from diurnal. However, you are right that the two separate sleeping time periods is a bigger deal. $\endgroup$
    – kingledion
    Commented Feb 6, 2018 at 21:21
  • $\begingroup$ @kingledion: Even now, we have people who, by choice, shift their working schedules. For instance, most of the year I tend to work a few hours in the morning, do outdoor recreation in the middle of the day, then work from evening until around midnight. But in the summer I shift this so I'm doing the outdoor stuff in the cooler morning & evening hours, working inside while it's hot. Add an afternoon siesta, and I'm not that far from your crepuscular humans. (One of the benefits of self-employment :-)) $\endgroup$
    – jamesqf
    Commented Feb 7, 2018 at 2:50

To avoid the heat
Since tropical rain forest are extremely hot and humid, people would have a harder time cooling themselves. In order to avoid overheating, they would have to avoid the daytime, where the temperature is highest. That way, the humans would adopt a crepuscular lifestyle in order to avoid the heat of the day, but also the darkness of the night where they can't see.


Historically this has already occurred in multiple recorded instances... our circadian rhythm is malleable.

Removing any time devices from a person, no clocks, no day/night cycle, and their clocks will change. Longer I believe, something like 30 hours... there have been studies, I'll see about finding citations.

In the pre-industrial era, middle ages possibly, in northern regions we operated with two waking cycles per day... one during daylight, one at night.

Optometry is telling us all now to stay off electronics at night because the blue light disrupts or circadian rhythm and prevents us from falling naturally to sleep...

I've spent years on swing shifts where I got to a point where predawn light triggers a sleep response (though I avoided actual dawn light desperately because the wakefullness brought on by a flood of natural light is really hard to overcome)

...also consider the Arctic, at certain times of the year "day" and "night" are fundamentally arbitrary. I was up there in the summer one year and was surprised how comfortable doing whatever, whenever became.

This is a very strongly supported logic in both biology and sociology... being diurnal is an easy fit for civilized man, so we think of it as "natural"... but I wouldn't say it is intrinsic to the human condition by any stretch.

  • $\begingroup$ "Non-24-hour sleep–wake disorder" affects mostly blind people, it is a "disorder" version of the experiment I mentioned... $\endgroup$
    – OhkaBaka
    Commented Feb 7, 2018 at 21:36

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