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SummaryCircadian rhythms, among other factors, have been shown to regulate key physiological processes involved in athletic performance [1–7]. Personal best performance of athletes in the evening was confirmed across different sports [8–12]. Contrary to this view, we identified peak performance times in athletes to be different between human “larks” and “owls” (also called “morningness/eveningness types” [13] or “chronotypes” [14] and referred to as circadian phenotypes in this paper), i.e., individuals with well-documented genetic [15–20] and physiological [21–24] differences that result in disparities between their biological clocks and how they entrain to exogenous cues, such as the environmental light/dark cycle and social factors. We found time since entrained awakening to be the major predictor of peak performance times, rather than time of day, as well as significant individual performance variations as large as 26% in the course of a day. Our novel approach combining the use of an athlete-specific chronometric test, longitudinal circadian analysis, and physical performance tests to characterize relevant sleep/wake and performance parameters in athletes allows a comprehensive analysis of the link between the circadian system and diurnal performance variation. We establish that the evaluation of an athlete’s personal best performance requires consideration of circadian phenotype, performance evaluation at different times of day, and analysis of performance as a function of time since entrained awakening.Video Abstract

Team performance is a complex phenomenon involving numerous influencing factors including physiology, psychology, and management. Biological rhythms and the impact of circadian phenotype have not been studied for their contribution to this array of factors so far despite our knowledge of the circadian regulation of key physiological processes involved in physical and mental performance. This study involved 216 individuals from 12 different teams who were categorized into circadian phenotypes using the novel RBUB chronometric test. The composition of circadian phenotypes within each team was used to model predicted daily team performance profiles based on physical performance tests. Our results show that the composition of circadian phenotypes within teams is variable and unpredictable. Predicted physical peak performance ranged from 1:52 to 8:59 p.m. with performance levels fluctuating by up to 14.88% over the course of the day. The major predictor for peak performance time in the course of a day in a team is the occurrence of late circadian phenotypes. We conclude that circadian phenotype is a performance indicator in teams that allows new insight and a better understanding of team performance variation in the course of a day as often observed in different groupings of individuals.

Birds are equipped with a complex circadian pacemaking system that regulates the rhythmicity of physiology and behavior. As with all organisms, transcriptional and translational feedback loops of clock genes represent the basic molecular mechanism of rhythm generation in birds. To investigate avian clock gene expression, partial cDNA sequences of six mammalian clock gene homologs (Bmal1, Clock, Per2, Per3, Cry1, and Cry2) and a novel avian cryptochrome gene (Cry4) were cloned from the house sparrow, a model system in circadian research. Expression patterns were analyzed by semi-quantitative RT-PCR and RNase protection assays using total RNA extracted from adult male house sparrow brains. With the exception of Cry4, pronounced rhythmic mRNA expression of all the clock genes analyzed was encountered, with mRNA levels varying considerably between the various genes. Although some basic features of the molecular circadian feedback loop appear to be similar between mammals and birds, the precise phase relationships of the clock gene mRNA rhythms relative to each other and to the light zeitgeber differ significantly between the house sparrow and mammals. Our results point to the existence of differences in the organization of avian and mammalian circadian clock mechanisms.

Seasonal variations can be found in almost any parameter of an organism's biochemistry, physiology, endocrinology, and behaviour. This phenomenon, generally called photoperiodism, results from one of the major functions of the circadian system, i.e. the translation of environmental information into rhythmic intraorganismic signals, which then regulate or influence physiology and pathology. We induced melanoma in three groups of syngeneic C57BL/6 mice synchronised to different photoperiods (8, 12, or 18 h of light within 24-h days) by subcutaneous injections of HFH18 melanoma cell suspensions. All animals from all three photoperiodic groups developed exponentially growing tumors. The average tumor volume on day 31 post injection was significantly smaller in animals exposed to light/dark conditions (LD) 8 : 16 h as compared with animals held in LD 18 : 6 h and intermediate in animals from the equinox group. These results indicate that C57BL/6 mice react to photoperiod, which can exert a significant effect on tumor growth.

Mammalian circadian rhythms are regulated by the suprachiasmatic nucleus (SCN), and current dogma holds that the SCN is required for the expression of circadian rhythms in peripheral tissues. Using a PERIOD2::LUCIFERASE fusion protein as a real-time reporter of circadian dynamics in mice, we report that, contrary to previous work, peripheral tissues are capable of self-sustained circadian oscillations for >20 cycles in isolation. In addition, peripheral organs expressed tissue-specific differences in circadian period and phase. Surprisingly, lesions of the SCN in mPer2Luciferase knockin mice did not abolish circadian rhythms in peripheral tissues, but instead caused phase desynchrony among the tissues of individual animals and from animal to animal. These results demonstrate that peripheral tissues express self-sustained, rather than damped, circadian oscillations and suggest the existence of organ-specific synchronizers of circadian rhythms at the cell and tissue level.

Seasonal variations can be found in almost any parameter of an organism's biochemistry, physiology, endocrinology, and behaviour. This phenomenon, generally called photoperiodism, results from one of the major functions of the circadian system, i.e. the translation of environmental information into rhythmic intraorganismic signals, which then regulate or influence physiology and pathology. We induced melanoma in three groups of syngeneic C57BL/6 mice synchronised to different photoperiods (8, 12, or 18 h of light within 24-h days) by subcutaneous injections of HFH18 melanoma cell suspensions. All animals from all three photoperiodic groups developed exponentially growing tumors. The average tumor volume on day 31 post injection was significantly smaller in animals exposed to light/dark conditions (LD) 8 : 16 h as compared with animals held in LD 18 : 6 h and intermediate in animals from the equinox group. These results indicate that C57BL/6 mice react to photoperiod, which can exert a significant effect on tumor growth.