Circadian rhythms are physical, mental, and behavioral changes that follow approximately a 24-hour cycle. Chronotype is the natural inclination a person has to sleep and perform activities at a certain time of day. If you know someone who chooses to wake up very early, they are an early chronotype (aka a lark)
What determines our chronotype?
The physiology behind our circadian rhythms and chronotype is very complex and not fully understood. Research indicates that chronotype is determined by the central circadian 'clock' located within the brain. Specifically, the suprachiasmatic nuclei (SCN) of the hypothalamus - think of this as the conductor in an orchestra. Interestingly, circadian oscillators (clocks) are also present in cells around the body. These clocks are synchronized by SCN (the conductor) and can generate their own circadian rhythms.
How do you know what chronotype you are?
Most studies measure chronotype as a preference for morning/evening activities, using some form of questionnaire. Chronotype can also be based on actual sleep behaviour (using a device to measure sleep) or biological markers of sleep timing (e.g., melatonin).