Do you really get people who are 'owls' and others who are 'larks'?
You will have heard the expression 'night owl or morning lark': 'night owls' being the kind of people who come to life late in the evening and into the small hours, often having energy and feeling alert at times when most of us are beginning to feel sleepy. By way of contrast, the 'lark' is someone who is at his or her best in the morning, preferring to be up early and to make the most of the early part of the day.
These 'chronotypes' as they are sometimes called, do exist. It turns out that this preference for sleep and waking has a genetic contribution, being an expression of our internally-driven circadian rhythms (or body clock). People who have one or other of these tendencies simply have a stable 'phase position' that is slightly different from the average. Usually people adapt to their body clock tendency, and often quite like it; they make it work for rather than against themselves in their choice of occupation for example.
However, in truth, most of us are neither extreme owls (night person) or larks (morning person), tending to be somewhere in the middle. Our body clock regulates many physiological processes, like hormone secretion and body temperature, as well as sleep timing. These rhythms, that are in part driven by the light/dark cycle, determine when we are alert and sleepy, as well as the optimal time for sleep.
Sometimes the process driving the need for sleep (sleep pressure built up over the course of the day) and the body clock which, in crude terms, controls the timing of sleep, may not be working together in harmony, leading to sleep disruption. Behavioral therapies may help 'jump-start' these two processes so they work together again in optimizing sleep quality and timing.