Morning person or late riser

Biology can explain why some people prefer to wake up early whereas others only come alive at night.
Scientists nickname early risers “larks” and people who like to stay up late, “owls”. While about 80% of people fall into the middle of the spectrum, only slightly favouring the morning or the night, it is now believed that about 10% of the population are extreme larks and a further 10% are extreme owls. Larks are most alert around noon, function best in the late morning, and are talkative, friendly, and pleasant from around 9 am to 4 pm. Owls, on the other hand, are not really up and running until the afternoon, are at their best later in the day, and most alert around 6 pm.
The body clock
The preference for ‘morningness’ or ‘eveningness’ is a result of variations in circadian rhythms – the rough 24-hour cycle in the physiological processes of living organisms. Also known as the “body clock”, each individual has a unique profile, or chronotype, that describes their rhythmic behaviour over the course of a day, and which can vary significantly from person to person.
The body clock controls sleep-wake patterns by regulating body temperature and hormones such as melatonin and cortisol. A normal circadian rhythm sees melatonin rising just before bedtime and dropping just after waking. The stress hormone cortisol peaks moments before first consciousness, and core body temperature is at its lowest during the middle of the night. A person is therefore inclined to be a lark or an owl depending on whether these chemical changes happen earlier or later than the norm.
Our circadian clocks tend to tick slower or faster depending on age. On average, larks tend to be older, whereas younger university students and twenty-somethings tend to be owls. These age-related tendencies are part hormonal/biological and part social. Hormonal changes that occur as humans age reduce the need for the

longer periods of sleep required by the young. In addition, light exposure is constantly resetting our clocks: as we grow older our corneas and retinas cloud with age, reducing our light intake. Middle-aged adults saddled with many responsibilities also tend to forget their intrinsic biological clocks and reprogram their cycles to fit their demanding schedules. University students, on the other hand, are often notorious night owls due to academic and social pressures.
Till Roenneberg at the University of Munich is especially concerned with the sleep cycle of adolescents. In a landmark study of the sleeping habits of 25,000 people aged between 8 and 90, Roenneberg calculated the “mid-point” of each person’s sleep cycle – the time halfway between when they went to bed and when they woke up on days free of work obligations.
He found that children tend to sleep later and later in the morning until they reach about age 20. At that point, there is an abrupt change in sleeping habits, and the mid-point starts getting earlier and earlier again. Roenneberg believes such a sudden shift suggests a biological cause and serves as the first-ever maker for the end of adolescence. The study also reflects the trend for girls to mature faster than boys: the women in the study that slept the latest were 19.5 years of age compared to 20.9 years of age for the men. Roenneberg’s research involving teenagers has highlighted the unique sleep needs required by this age group: adolescents sleeping late should no longer be considered lazy, but as exhibiting normal biological traits for their age.