Refer to figure. 
It can be fully overridden by motivation and training --> INCORRECT. You cannot overcome the natural human body clock (circadian rhythm) other than by natural adaptation over a period of days.

It varies randomly from person to person and cannot be predicted --> INCORRECT. Whilst there are small individual variations, circadian rhythm is generally uniform and predictable across all humans.

It depends mainly on body mass and age rather than light–dark cycles --> INCORRECT. Light and dark are the key triggers for the circadian rhythm. Body mass and age are generally irrelevant. Although there may be small changes due to age, light and dark remain primary cues for our internal clock.

It is almost the same for all people living in the same area and is important when planning sleep and duty periods --> CORRECT. A population living in the same area will be subject to the same cues (light, dark, social activities, meal times and so on) and their circadian rhythms will on the whole be aligned to the same patterns. If you are operating from one time zone to another it is important that sleep/wake patterns (particularly the WOCL - window of circadian low) are considered when planning duty periods to minimise tiredness and error proneness during flight operations.

Circadian rhythm is the daily cycle which regulates many of our body’s physiological processes - often called the “body clock”. It controls and prompts digestive processes, hormone secretion and basic bodily functions such as sleep.

It averages 24 hours for all humans due to “Zeitgebers” - external cues which synchronise the circadian rhythm. The main cue is light and dark, but meal times, social activities and work can also influence your body's internal clock. With these cues removed, the natural, or “free running” circadian rhythm is actually closer to 25 hrs.

Regulation of body temperature is one of the most important functions of the Circadian Rhythm; sleep and mental performance is closely linked to body temperature. Sleep normally takes place when body temperature is falling. The natural time of worst performance and lowest vigilance is between approximately 2 and 5 am - the “Window Of Circadian Low", or WOCL. Pilots should be very cautious when operating during this period as the body will naturally want to be asleep. Mental alertness and vigilance will be poor and errors will be more likely.

If you fly to a new time zone your body clock will still be synchronised to your previous time zone, and until your body clock catches up, physiological functions and body temperature are out of sync with the new time zone. Your body will want to sleep during the day and stay awake at night for example, depending on the difference in time zones. Your body clock will synchronise to the new zone time at a rate of 1 - 1.5 hours per day and this cannot be sped up, but must occur naturally.