The behavioral trait of preference to schedule the daily activities for morning or evening hours forms a continuum, with the anchorage ends of “early birds” and “night owls,” and is called chronotype. Genetic effects contribute to the chronotype by about half and the other half is accounted for non-shared environmental effects. However, no “chronotype gene” has been identified yet. There is a growing body of literature on health hazards that has been attributed to the chronotype itself, being independent of a number of factors. So far, without any exception, of those health hazards that do differ between the chronotypes, all have been more common among the “night owls” than among the “early birds,” such as mood disorders, anxiety disorders, substance use disorders, personality disorders, insomnia, sleep apnea, arterial hypertension, bronchial asthma, type 2 diabetes, and infertility. Alarmingly, current data suggest that “night owls” tend to die younger than “early birds”.