Disruption of endogenous rhythms has been implicated in the pathogenesis of metabolic disorders and cardiovascular diseases.1, 2 There are links between metabolic cycles and circadian rhythms, as both oscillations are generated by the circadian clocks.3, 4 Recent findings have extended these links by suggesting that there may also be a relationship between metabolic cycles and seasonal fluctuations.5, 6 It seems that abnormalities in the circadian clock may predispose individuals not only to seasonal variations in mood and behavior but also to the metabolic syndrome. Seasonal variations in mood and behavior, that is, the global seasonality score, have a metabolic factor that is composed of weight and appetite. In particular, this score increases the risk of having the metabolic syndrome. The negative effect of seasonal variations is cancelled out by the protective effect of regular physical exercise in the struggle against the metabolic syndrome. Thus, it appears that the seasonal effect has a clear impact. In this issue of Hypertension Research, Kamezaki et al.7 show that serum levels of high-density lipoprotein cholesterol, fasting glucose and systolic and diastolic blood pressures are higher in winter than in summer. Furthermore, these seasonal fluctuations influence the assessment and prevalence of the metabolic syndrome. It is important that clinicians and research teams pay close attention to this finding in their future work.

Here one is left to ask what causes these metabolic variations to occur; this question is difficult to address. Both light/dark transitions and the photoperiod that guide the circadian clocks are likely to contribute to seasonal changes in these metabolic components.8, 9, 10 For example, if the lighting conditions at home are poor, it will negatively impact scores related to the metabolic component of the global seasonality score.6 Scheduled dynamics of lighting conditions at home or at the work place may therefore serve as a parameter on which intervention could be useful to influence seasonal metabolic signals and, ultimately, to prevent the metabolic syndrome.

In summary, the current evidence indicates that (1) seasonal variations of metabolic syndrome components and prevalence do exist, and (2) these variations must be considered at periodic health checkups. Furthermore, because of the ease of assessment, it might be worth determining and integrating the global seasonality score as a routine part of health status examination in persons at risk for, or who currently have, one or more components of the metabolic syndrome. If this change was implemented, effective treatment options such as scheduled exposures to light and regular physical exercise, both of which can help calibrate an individual's circadian clock, need to be considered for prevention and treatment of the metabolic syndrome.