Diurnal vs Nocturnal

These aliens have genes that influence whether the alien will be diurnal or nocturnal. Hatching time also influences this.


DD - Diurnal

NN - Nocturnal

DN or ND - Unknown until hatching time

Hatching time:

Hatch during daytime - Diurnal

Hatch during nighttime - Nocturnal

Hatch during twilight - Unknown

Time of day

If a baby alien has both the diurnal and nocturnal alleles and hatches during either twilight(morning twilight also known as the blue hour or evening twilight with all those rainbow colors(except green but I do see purple and pink in an evening twilight. In fact especially in the summer, I can see at just the right moment and in just the right place sunlight splitting up into separate beams of red and yellow light as it goes through the windows of my house)) then it is completely unknown if the alien will be diurnal or nocturnal. Same goes for if hatching time and genetics are opposite. If however the hatching time is in twilight but the alien has just diurnal or just nocturnal alleles or if the alien has both alleles but hatches in either the day or the night it is determined by the one that doesn't give an unknown result.

Note: Ignore those Watches and just look at the regions of daytime, nighttime, and twilight.

Is it plausible to have nocturnal and diurnal creatures of the same species?

  • $\begingroup$ So uhm ... what's your question? $\endgroup$
    – AndreiROM
    Commented Nov 25, 2016 at 21:16
  • $\begingroup$ Do diurnal parents have problems raising nocturnal babies? How does being diurnal or nocturnal actually manifest in your aliens? $\endgroup$
    – DrBob
    Commented Nov 27, 2016 at 15:49
  • $\begingroup$ @DrBob I'm guessing it would be a lot like the experience of humans raising children with Smith-Magenis Syndrome. $\endgroup$ Commented Apr 4, 2020 at 12:13
  • $\begingroup$ @EttinaKitten Or like the sleep deprived parents of a baby who won't sleep through the night! :-) $\endgroup$
    – DrBob
    Commented Apr 4, 2020 at 15:02

2 Answers 2


I'm assuming the word "hatching" in your post means that your alien embryos develop inside eggs which are themselves exposed to the environment, like non-mammals on Earth.

The egg shell could be transparent or translucent and lined with photosensitive biomolecules on the inner side. These molecules have one stable configuration and one metastable configuration with a lifetime of a few minutes. During the day, incident photons cause a transition from the stable state to the metastable state. At the end of the day, the amount of light received by these molecules drops to nearly zero and the molecules return to the stable state.

The embryos feed from the photosensitive molecules, and which metabolic pathway the molecules follow inside the embryos depends on state the molecules are in. The end-product of the sable-state molecule's metabolism is molecule s, and the end-produce of the metastable-state molecule's metabolism is molecule m. Thus, during the day the embryo has a lot more m and at night it has a lot more s.

Now, the embryos may have allele N or allele D (or both) of a certain gene which is responsible for producing a certain protein with two variants, S and M. These proteins are both useless during the embryo stage. However, the activator for N turns out to be s and the activator for D turns out to be m. Thus, during the day the embryo has more M and at night it has more S. By a remarkable evolutionary coincidence, these proteins have a lifetime of T/2 (i.e. the probability that they have been decomposed after a time T/2 is 50%), where T is however long twilight lasts in your aliens' homeworld. Thus, during twilight the amount of one protein slowly diminishes while the amount of the other protein increases: both proteins are simultaneously present inside the embryo.

At birth, a baby alien must start feeding off something else (other aliens, mother's milk, whatever), and it is born hungry, so it must take its first meal within minutes of being born. The new diet is compatible with both metabolic pathways but changes the baby alien's metabolism: the s metabolic pathway now relies on protein S and produces a molecule called s2, while the m pathway now relies on M and produces molecule m2. The molecules s2 and m2 are powerful neurotransmitters which determine whether the alien is nocturnal or diurnal (respectively). It also turns out that s2 is an inhibitor of the m pathway and m2 is an inhibitor of the s pathway, so once the alien has had its first meal its metabolism stops for a while. s2 and m2 are also highly reactive with each other, forming a useless compound which is simply excreted by the baby alien. Eventually, whichever molecule (s2 or m2) is present at a higher concentration in the body is the last one standing, so the other metabolic pathway is permanently shut off, at which point the alien is defined as either diurnal or nocturnal. If it has roughly equal amounts of s2 and m2 (because it was born at dusk or at dawn), it is impossible to tell a priori which molecule will win (and the "battle of the molecules" takes a little longer than usual to settle, but it always does eventually because the probability that the alien has exactly the same number of m2 and s2 molecules in its body is nearly zero).

Of course, if the alien only has one allele, only one of the proteins will be produced and only one of the modified metabolic end-products (s2 or m2) will be produced regardless of whether the embryo is born during the day or during the night.

(Note that it is only the photosensitive biomolecule in the inner lining of the egg that has a metastable state; m, m2, M, s, s2 and S are all stable.)


  • DD: only protein M is produced => only molecule m2 is produced => diurnal
  • NN: only protein S is produced => only molecule s2 is produced => nocturnal
  • DN, born during the day: M is present in higher quantities than S => more m2 is produced => only m2 is left in the end => diurnal
  • DN, born at night: S is present in higher quantities than M => more s2 is produced => only s2 is left in the end => nocturnal
  • DN, born at twilight: both M and S present in roughly equal quantities => can be diurnal or nocturnal

Sorry for the weird nomenclature. Hopefully that wasn't too confusing. :)

Some interesting scenarios which come to mind if you go for this explanation:

  • A DN alien born at twilight who is for any reason unable to consume the appropriate food immediately but somehow doesn't starve to death would be neither diurnal nor nocturnal (it would either be awake all day or sleep all day, depending on whether m2 and s2 are promoters or inhibitors of neural activity, respectively) until its first "proper" meal, and the time of said meal (as opposed to the time of birth) would determine whether it is diurnal or nocturnal.
  • An egg which has been inside a cave, a very dense forest, a building or similar during the last few hours before hatching is equivalent to the egg hatching at night. If said building has artificial lighting, then it could be equivalent to the egg hatching during the day (if the light which causes the photosensitive biomolecules to switch to the metastable state is visible light) or still equivalent to it hatching at night (if it is UV light or something else not available via artificial lighting).
  • Following the previous idea, in a society with enough scientific knowledge concerning what determines whether an individual is diurnal or nocturnal, parents (or malevolent individuals) could force an alien to be diurnal or nocturnal (whichever suits their needs) by placing the egg in an appropriate place before it hatches.
  • An alien who is diurnal could switch to nocturnal by injecting (or consuming) lots and lots and lots of s2 (or s), and an alien who is nocturnal could switch to diurnal in a similar fashion. If you want, you can make this highly dangerous due to reactions between m2 and s2 (causing mutual annihilation and thus a sharp decrease in neural activity while both neurotransmitters are present), due to reactions of s2/m2 with other neurotransmitters which also differ between diurnal and nocturnal individuals (causing whatever symptoms you can think of), or simply due to psychological factors.

Hope this helps!


A species could have a number of specializations built in, so to speak. Which one expresses itself could be a matter of which hormones the embryo is exposed to over various stages of its development. Consider that bees and ants spawn specialized individuals in this way.

These traits could, alternatively, be decided randomly based on the parent's DNA, and the "luck of the draw", the same way that hair or eye color work for us.

Unfortunately I can't see the time of day that the individual is born affecting their DNA. You can't mix scientific facts with pseudo-magic like the power of the stars, or the time of day. Or rather you can, but it's no longer realistic.

  • $\begingroup$ Lots of things can affect DNA including UV light which varies with time of day. UV can get through an egg shell so time of day and thus UV exposure at time of hatching can affect the DNA. It also is an environmental factor in the sense that if they are born at night, it is easier for them to be nocturnal and if they are born at daytime, it is easier for them to be diurnal. If born at twilight(either one) it is equally easy and is where DNA becomes super important in determination. $\endgroup$
    – Caters
    Commented Nov 25, 2016 at 21:33
  • 1
    $\begingroup$ @caters - sure, UV can affect DNA - over time. How the egg is stored might be a significant factor to the outcome. The time of day you're born won't change the specialization because it's already hard coded in your DNA. Also, if UV affects the DNA of these creatures to such a high degree, you're probably looking at a lot of mutations occurring. A lot of these mutations would be very dangerous, and would result in still births, cancer, etc. DNA is too complex to allow such wanton mutation to take place and still have a reliable outcome. $\endgroup$
    – AndreiROM
    Commented Nov 25, 2016 at 22:06
  • $\begingroup$ It doesn't need to affect DNA in order to affect their phenotype. $\endgroup$ Commented Apr 4, 2020 at 12:15

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