My aliens on the generation ship have a complex way of determining sleep schedule. For humans, it is as simple as adaptation + natural amount of sleep. With my aliens though, they reproduce differently than us and their genetics are different from ours(they use DNA but theirs is very different from ours(not surprising considering they have mostly reptilian traits, a few mammalian traits(lactation for one), and some unique traits(a second thumb on the opposite side of the hand for example)). There are lots of different factors into the sleep schedule of any 1 alien. First off is genetics. Here, 1 gene controls all the other genes that have to do with sleep and depending on the alleles, there are different results. Here are those alleles and their results: DD: Diurnal DN: Unknown NN: Nocturnal Or at least it would be if this was the only factor. But of course there are lots more. Genetics is the only factor when the eggs are being calcified in the abdomen(their eggs have an internal placenta(placenta within the amniotic sac instead of outside it), that sends villi through the amniotic membrane in between cells. The egg starts calcifying from the bottom and goes upwards. This allows for amniotic sac growth over 2 weeks. Once the calcification is sufficiently high up, the villi start to break, starting with the lowest ones and ending with the highest ones. This causes ischemia of the placenta which is broken down into proteins for the baby to grow.) Speaking of proteins, one of the proteins in the amniotic sac is a photosensitive protein. Daylight activates this protein into its metastable or m state. As twilight happens, depending on whether it is morning or evening twilight, the metastable protein either increases(morning) or decreases(evening). At night, all the photosensitive protein is in its stable or s state. This s state is also the state the protein is in during the calcification of the amniotic sack. This photosensitive protein triggers changes related to gene expression while keeping the actual genes the same. This means that hatching time is also an important factor. Given these 2 alone, there are 9 possible combinations: DD and Daylight: Diurnal DD and Twilight: Diurnal(the m protein there is is enough to continue expressing the genes) DD and Night: Crepuscular(1 allele of the gene is inactivated in all cells) DN and Daylight: Diurnal DN and Twilight: Crepuscular DN and Night: Nocturnal NN and Daylight: Crepuscular NN and Twilight: Nocturnal NN and Night: Nocturnal So approximately 1/3 of babies hatch out diurnal, approximately 1/3 of babies hatch out crepuscular and approximately 1/3 of babies hatch out nocturnal. Of course, since these aliens lactate, they have 15 areas of milk production in their abdomen. Each one can produce milk with only 1 type of milk protein or M protein. M_1 and M_2 are the different types. M_1 protein triggers the diurnal pathway and M_2 protein triggers the nocturnal pathway. Since it is inevitable that some babies will get the M_1 protein and some will get the M_2 protein, here are how those proteins change the sleep schedule: M_1 and Diurnal: No change M_1 and Crepuscular: More diurnal M_1 and Nocturnal: More crepuscular M_2 and Diurnal: More crepuscular M_2 and Crepuscular: More nocturnal M_2 and Nocturnal: No change So basically, M_1 protein moves the baby closer to diurnal and M_2 moves the baby closer to nocturnal in terms of sleep schedule. It is likely that different babies will be in different areas at each feeding so these changes are completely random at first(I'm saying at first because eventually it gets to the point where each baby has its designated feeding area just via development of the brain). Then after 2 years and 6 months of drinking milk(6 months in the pouch, 2 years out of the pouch), the sleeping schedule of the alien is pretty much established and if it is not fully crepuscular, it will change to being fully diurnal or fully nocturnal in most aliens. So by adulthood, very little of the population in terms of % is crepuscular and it is mostly a 50-50 split between diurnal and nocturnal. Of course there are some factors that even change this during adulthood such as frequency and length of power outages(the more frequent they are and the longer they are, the more tendency towards being nocturnal). But the first 2.5 years is when most of the change occurs. Is this complex system of determination of sleep schedule based on genetics, hatching time, milk proteins, and a few other environmental factors, reasonable for an intelligent species that has all 3 types of sleep schedules and has evolved this way to maximize defense against predators and overall the chance of survival?