I'd like to keep this as science based as possible. I'm not even sure if this sort of question fits this exchange. Is there a possible world wide affect that when triggered (by human/cosmological initiative) would make most if not all humans go nocturnal?

The effect on other creatures, plants and species preferred to be as minimal as possible. A few hundred species and some crops going extinct wouldn't matter. But if mass extinction can't be helped, well, we can explore how humans can adapt to that too.The technological level before this sudden change would be as of present.

If there could be some cultural/sociological/political change that can bring about the transfer from a diurnal to a nocturnal life, the incentives for such a change can be explored too. This would give the advantage of staying away from damaging existing global ecosystems and substance cycles.

I'm hoping that proposed changes would have humans and the new global ecosystem survive for eons rather than being on a timer till doom (like the case of our sun going missing), and something permanent, something we cannot fix within at least a century.

  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Commented Mar 16, 2018 at 8:44
  • $\begingroup$ The plot of the book I am legend revolves around the idea of humans becoming nocturnal. The book was adapted to several movies. $\endgroup$ Commented Mar 16, 2018 at 11:52
  • $\begingroup$ Why on Earth would the "global ecosystem" care whether humans are diurnal, crepuscular, or nocturnal? And even now there are many humans who work night shifts, so it's not as if all humans are active during day time and sleep during night time. $\endgroup$
    – AlexP
    Commented Mar 16, 2018 at 16:24
  • $\begingroup$ Dear AlexP, i was mentioning "global ecosystem" in the context that the proposed phenomena which would make humans nocturnal should not interfere with the global ecosystem. $\endgroup$ Commented Mar 16, 2018 at 17:31

3 Answers 3


It is too hot during the day.

This already happens in tropical countries where it is unpleasantly hot in daytime. People come out at night.

Singapore night market


This does not require a lot of imagination. Even in temperate countries with hot summers, people hide inside in the heat of the day then come out to socialize in the cool night.

The thing which could make this happen everywhere is a dearth of power. You can stay diurnal in Phoenix because of air conditioning. If there is not air conditioning and there is sweltering heat, people would sleep in cool basements in the day and come out at night. Plants that could handle the heat would do fine.

  • $\begingroup$ That won't work on most of humanity though, many places will still be cool enough, simple shade will do a lot as well. Making even hte poles this hot will completely change the earths surface, for one the equator would be over boiling. $\endgroup$
    – John
    Commented Feb 7, 2019 at 22:17

For some reason all humans turn allergic to direct sunlight. Only a few humans have this now, but you could research what makes it happen and turn it into something wide spread.

Religion/ Cultural reasons. Wether it is a new prophet or fake news being widespread. something is saying sunlight is bad and we should avoid it at all costs. When enough people change to nocturnal, others will start to change to it as well out of neccesity. (Opening hours of work, shops, social events, ...) but this would not happen overnight.

A new species comes to existance. (We made it, crashed alien zoo ship with space lions on board, ...). This is a highly efficient predator and very dangerous and hard to kill. However, it is only active in direct sunlight. You could tweak this to your needs until it turns out that it is safer and more cost efficient to change to a nocturnal schedule than to try and combat this species. Although I think humans would rather fight it.

Another thing I thought of, but is probably not very scientific, is a sort of epidemic that makes almost all humans sick. There does not need to be a high death toll or anything, but those who got ill and recovered, ended up with very sensitive eyes. When light hurts the majority of the population, such a switch would happen natural.


A massive infection which has the effect of leaving the infected humans with porphyria like symptoms.

Some kinds of porphyria make the skin extremely sensitive to sun light, with exposure resulting in pain, swelling and bleeding. More than enough to avoid exposure to daylight and seek refuge in the night.

  • $\begingroup$ I am not sure, and i'm interested if such a case exists, if any species has in the past got infected with something which severly hampered its normal way of life and decided to live with it. Unless a symbiosis of sorts develops, the body would gear to eliminate the infection. And in the case of humans, if the body can't the scientists will. Plus, quarintine is very affective nowadays, scientists will hunt for cures for this too and return human life to normal. I'll give this 30-60 years tops, before a cure is found and life returns to normal. $\endgroup$ Commented Mar 16, 2018 at 9:39
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    $\begingroup$ @User3.14159265, herpes zoster is what remains in one's body after healing from chickenpox. So not always the body succeeds in eliminating an infection $\endgroup$
    – L.Dutch
    Commented Mar 16, 2018 at 9:45

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