The process by which speciation occurs in a geographic region shared by both populations is called Sympatric Speciation. This is a $\text{"Real Thing"}^{TM}$.
How it happens
The above Wikipedia article is pretty dense. Here's a relevant snippet"
For example, micro-allopatry, also known as macro-sympatry, is a
condition where there are two populations whose ranges overlap
completely, but contact between the species is prevented because they
occupy completely different ecological niches (such as diurnal vs.
nocturnal). This can often be caused by host-specific parasitism [or predation],
which causes dispersal to look like a mosaic across the landscape.
Micro-allopatry is included as sympatry according to spatial
definitions, but, as it does not satisfy panmixia, it is not
considered sympatry according to population genetics definitions.
One example of how this might happen is you have a predator with peak activity during a specific time of day (e.g. a sight oriented predator that doesn't tolerate high temperatures might be most active in the morning). Populations of the prey species which are most active outside of the predator's peak activity (e.g. afternoon and dawn) would avoid the predator, have a better chance of survival, and be more likely to pass on their genes.
Since predators adapt to the behavior of prey and vice versa, eventually this will drive one population of the prey species to be active earlier and earlier with the other to be active later and later.
Eventually the two populations of the prey species will not share the same active period and not interact appreciably. Over time this will cause speciation.
Traits of the two groups
There is no certainty about what traits the two groups will develop. Obviously they'll develop traits that help them survive in the environment their population uses. For example, we don't know whether the nocturnal group will develop low light sensitive, echo location, or improved sense of smell. However, the trait will be consistent with neutral or positive chances of survival. Traits with negative survival potential are quickly weeded out.
To get the specific traits that you want really has little to do with the nocturnal/diurnal speciation and more to do with the specific evolutionary pressures those environments impose on those populations.
Some possibilities for your specific traits:
- short & heavy - is a good configuration for conserving heat, cold
environment? Also good for sprinters
- tall & wiry - is a good configuration to ridding the body of waste heat. Also a good shape for distance running
- keen senses - a requirement for spotting prey or avoid predators
- fleet footedness (over short or long distances?) good for catching prey by running it down or avoiding predators
Some possible traits for nocturnal population:
- sense tuned for the dark: echo location, low light vision, good
sense of smell, one, some, or all the above
- if, like humans, vitamin D is generated by exposure to the sun, then
very low melanin levels will be a requirement (pale skin).
- you could postulate a tougher skin to deal with brushing against
plants and rocks in the dark
- etc.
Some possible traits for diurnal population
- ability to see clearly at longer distances
- darker skin to deal with UV radiation
- brain better at ranged weaponry (e.g. bow)
Note your body configurations are good for the opposite of what you've proposed. Heavy, short people are better for the cold that we normally associate with night time. But if you could figure out a rationale for why it is cold during the day and warm at night, it still works.
