Humans are born with an internal biological timing system. At its core is a master clock in the brain, located in the hypothalamus, known as the suprachiasmatic nucleus (SCN). This clock coordinates rhythms throughout the body. While the biological mechanism is shared by all humans, its timing varies significantly between individuals.

 

These individual differences are known as chronotypes. Some people naturally feel alert early in the morning and become tired early in the evening, while others function best later in the day and reach peak alertness in the afternoon or evening. Chronotype is not a habit, preference, or matter of discipline — it is a biologically anchored trait influenced by genetics, age, and environment.

 

Chronotypes exist on a wide spectrum, from very early to very late, much like height varies from short to tall.

 

Modern societies, however, are largely organised around fixed schedules shaped by agricultural and industrial rhythms rather than by human biology. When one rhythm is treated as the norm, other biological rhythms become invisible — not because they are rare, but because systems are not designed to recognise them.

 

Today, around 80 percent of the population relies on an alarm clock on workdays in order to fit into inherited time structures. For many, this is not a matter of preference, but of biological misalignment.

 

Research in chronobiology shows that long-term misalignment between biological time and social time can have negative consequences for health and wellbeing. In 2018, a large population study from Northwestern University found that late chronotypes had a higher risk of premature mortality compared to early chronotypes.

 

Applied chronobiology focuses on what happens when knowledge about biological timing is brought into everyday life. It explores how differences in chronotypes influence learning, collaboration, leadership, and performance — and how schools, workplaces, and institutions can be designed to support human rhythms rather than suppress them.

 

Applied chronobiology is therefore not only about understanding rhythms, but about questioning the time norms that shape everyday life. When time is treated as neutral, biological differences are overlooked. When time is designed with biology in mind, fairness, inclusion, and wellbeing increase.

 

This perspective connects chronobiology with chronodiversity — the recognition of natural variation in human rhythms as a form of diversity — and with the concepts of chronoinclusion, chronoequity, and chronoleadership, developed by Camilla Kring. Together, these frameworks offer ways of understanding time as a biological, social, and ethical dimension.

 

We need more sustainable rhythms in our lives — rhythms that reflect human biology rather than inherited industrial time norms. Understanding biological time is a first step toward time justice: a society where people are not disadvantaged because of when they function best.