Biological rhythms, health, and gender-specific differences

Biological rhythms exist at any level in living organ-isms and, according to their cycle length, are classified into a) circadian (from the Latin circa dies, period of ~24 hours), b) ultradian (period <24 hours), and c) infradian (period >24 hours) rhythms. Circadian rhythms are the most widely stud-ied. The central circadian clock is located within the suprachi-asmatic nucleus of the hypothalamus, and is entrained by light-dark alternation. Moreover, peripheral circadian clocks are present in many other cells. Although circadian informa-tion is inherited with DNA, an individual circadian preference (chronotype) exists. Three main chronotypes can be identified: Morning-type, Evening-type, and Neither-type. A growing body of research indicates that Evening-type may be associated with a series of unfavorable conditions, also in the pres-ence of gender-specific differences. Moreover, organization of circadian rhythms may be disrupted by desynchronizing factors, such as exposure to light at night, jet lag, shiftwork, and daylight saving time. This article will review the available evidence of possible gender-specific differences related to the individual chronotype, as well as to the above indicated desynchronizing factors. Most studies on adolescents found an association between evening chronotype and unhealthy habits, such as reduced physical activity, higher consume of chocolate, soft drinks, smoking and alcohol. In adults, Evening-type was significantly associated with diabetes (in men), and metabolic syndrome (in women), and with sleep disturbanc-es and psychopathology, such as impulsivity, anger, depres-sion and anxiety disorders (the latter especially in women). A more-in-depth knowledge of individual circadian organization could also help in obtaining more effective patient care in the view of a personalized precision medicine.