Chronotype is an established concept designed to identify distinct phase relationships between the expression of circadian rhythms and external synchronizers in humans. Although it has been widely accepted that chronotype is subjected to ontogenetic modulation, there is no consensus on the interaction between age and gender. This study aimed to determine the relationship between age- and gender-related changes in the morningness-eveningness character in a large sample of people. A total of 14,650 volunteers were asked to complete the Brazilian version of the Horne and Östberg chronotype questionnaire. The data demonstrated that, on average, women were more morning-oriented than men until the age of 30 and there were no significant differences between men and women from 30 to 45 years of age. In contrast to the situation observed until the age of 30, women older than 45 years were more evening-oriented than men. These results suggest that the ontogenetic development of the circadian timekeeping system is more plastic in men, as represented by the larger amplitude of chronotype changes throughout their aging process. The phase delay of adolescence and phase advance of the elderly seem to be phenomena that are more markedly present in men than in women. Thus, our data, for the first time, provide support that sharply opposes the view that there is a single path toward morningness as a function of age, regardless of gender.

The rotation of the Earth around its own axis and around the sun determines the characteristics of the light/dark cycle, the most stable and ancient 24 h temporal cue for all organisms. Due to the tilt in the earth’s axis in relation to the plane of the earth’s orbit around the sun, sunlight reaches the Earth differentially depending on the latitude. The timing of circadian rhythms varies among individuals of a given population and biological and environmental factors underlie this variability. In the present study, we tested the hypothesis that latitude is associated to the regulation of circadian rhythm in humans. We have studied chronotype profiles across latitudinal cline from around 0° to 32° South in Brazil in a sample of 12,884 volunteers living in the same time zone. The analysis of the results revealed that humans are sensitive to the different sunlight signals tied to differences in latitude, resulting in a morning to evening latitudinal cline of chronotypes towards higher latitudes.

Chronotype is a circadian phenotype expressed in the preference of individuals to perform their activities and sleep in specific phases along the day. The objective of the study was to identify anxiety levels, quality of sleep and different chronotypes of university students and investigate their possible relationships. This is a cross-sectional study with a quantitative approach, in which 103 undergraduate students answered the Morningness-Eveningness Questionnaire (MEQ) the State and Trait Anxiety Inventory (STAI) and the Pittsburgh Sleep Quality Index (PSQI). There is a relationship between chronotype, quality of sleep and anxiety in the investigated population. Evening chronotype students showed higher anxiety status and have poor sleep quality when compared with morning chronotype students. The high occurrence of anxiety levels and poor sleep quality in evening students may be a consequence of high academic demand in a shift incompatible with the phase delay of the circadian timing system of these individuals.

In this paper we compare rhythmic parameters of human body temperature simultaneously measured with digital thermometers and thermistors with memory (Thermochron iButtons®). Thirteen healthy male and female volunteers (mean age of 25) measured oral and axillary temperatures with digital thermometers every hour during wakefulness for two days and every three hours for three days, totalling five consecutive days. Concomitantly, they wore badges with thermistors in the thorax and the wrist. Temporal series circadian rhythmicity was evaluated with the Cosinor technique. Acrophases average were 17:32 h±2:02 h for axillary temperature, 10:12 h±7:26 h for thoracic temperature, 17:18 h±00:50 h for oral temperature and 4:15 h±1:55 h for wrist temperature. The rhythmic parameters of the wrist are more robust than those of the thorax. The collection of data in humans, generally performed with digital thermometers, is restricted to the waking phase and depends on the discipline of the volunteers. We suggest that the use of the thermistor with memory in the wrist can be adopted as alternative methodology to studies of human body temperature rhythmicity, especially recommended for long temporal series.

In this paper we report findings on the presence of circadian rhythms in six species of cave-dwelling fishes from Brazil. Locomotor activity of islolated individuals was automatically recorded for 10 consecutive days under constant darkness. The species tested show varied degrees of specialization to subterranean life and we found varying degrees of the circadian components of locomotor activity as measured by the periodogram algorhythm of Lomb–Scargle. Both the presence and robustness of the circadian components seem to vary according to the degree of specialization to subterranean life, the more specialized, the less circadian rhythmicity was detected.

There are several determinants of mental health symptoms, ranging from individual characteristics to social factors. Consistent with patterns in the general population, students with evening characteristics tend to exhibit more anxiety symptoms and poorer sleep quality compared to morning students. Meal timing also appears to affect sleep and may be associated with mental health symptoms. In this context, the aim of the present study was to investigate the association of the timing of the main and last meals of the day with sleep quality and anxiety levels, according to the chronotype of university students. This study was conducted in colleges in São Paulo, Brazil, and involved application of a questionnaire to 162 university students. The questionnaire collected sociodemographic information meal and study times, and included scales assessing eveningness and morningness, sleep quality, and anxiety. Students demonstrating a phase delay in both chronotype and dinner timing exhibited higher levels of anxiety compared to morning-type students. Although no associations were observed between meal timing and sleep quality, sleeping later was associated with poorer sleep quality. The study suggests that evening students and those who eat late at night are more prone to presenting mental health symptoms. More studies are needed to further investigate this association.