Introduction {#s1} ============ The relationship between personality and health is a well-established phenomenon ([@B1], [@B2]). According to the most common model, extraversion and neuroticism are personality traits that have a direct and indirect effect on the prognosis of disease. The main hypothesis relates to social interactions in this relationship, while a new hypothesis has been suggested based on neural processes---the biological sensitivity hypothesis ([@B3]). This hypothesis states that individuals with a high level of sensitivity (neuroticism) experience an inadequate response of their nervous system in relation to their physiological demands, that is, the nervous system perceives a stressful situation as threatening and prepares the organism for adaptive responses. This hypothesis was proposed on the basis of a number of studies showing an association between the genetic polymorphism of the neurotrophin gene (*BDNF*) ([@B4]) and anxiety disorder ([@B5]) and depression ([@B6]). The biological sensitivity hypothesis suggests that extraversion is associated with health in a different way. Extraversion is linked to the level of arousal and energetic demands, which in turn is manifested in the number of biological processes taking place in the body. Extraverts typically have high metabolic rates, the amount of oxygen consumed and blood circulation in the body ([@B7]). Extraversion is also linked to a higher reactivity of the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system. In addition, as these systems are activated when there is a threat or danger, in extraverts it takes a larger psychological or emotional input for these systems to be activated ([@B8]). Therefore, since people with a high level of extraversion have a large energetic load, it is not surprising that they can tolerate large loads of negative emotions without a health risk, while at the same time being more responsive to positive stimuli in the world and to positive emotions in themselves. Moreover, they are more sensitive to positive feedback and can cope more effectively with the emotional consequences of negative events. That is, they are better at self-regulating and have a lower risk of depression or anxiety ([@B9]). This relationship can be generalized to the functioning of human systems in general ([@B10]). It should be taken into account that personality is linked to the functioning of different physiological systems, which can be evaluated indirectly from neuroendocrine indicators. Extraversion, in this sense, is related to the number of different neuropeptides present in the blood, the activity of the HPA axis, and neurotransmitters such as serotonin and dopamine. It has been confirmed in research that the greater extraversion of a person, the more neuropeptides present in the blood ([@B11]). Neuroticism is also connected with changes in the activity of the HPA axis: due to lower serotonin and dopamine in the nervous system, people with higher levels of neuroticism experience a higher level of arousal in response to stress ([@B12]). It should also be taken into account that a growing body of research has demonstrated that neuropeptides, including neuropeptide Y (NPY), and their genes are associated with various human disorders ([@B13]). So, based on the above, the physiological model of the relationship between personality and health may be assumed as follows: higher levels of extraversion in humans is associated with a lower incidence of diseases, and higher levels of neuroticism is associated with increased incidence of diseases ([@B14]). It is also assumed that these relationships are mediated by processes in the human nervous system. In recent years, the studies of the relationship between extraversion and health have been concentrated on two aspects: extraversion is connected with a low risk of common diseases, such as diabetes, and a higher risk of mental disorders ([@B9]). Most studies have shown that extraversion is a factor for good health ([@B15], [@B16]), though the results are inconsistent ([@B14]). This inconsistency can be explained in part by different methods used to operationalize extraversion, as well as a large number of health indicators ([@B17]). Another difficulty in research on extraversion and health can arise from its conceptualization in different ways, for example, in social and psychological aspects ([@B16], [@B18]), in relation to personality traits or in connection with the functioning of neural mechanisms ([@B19]). Although some studies have shown that extraversion is linked to health by an indirect way---through biological indicators ([@B20]), this relationship is not well-researched in the context of an evolutionary theory of personality ([@B16], [@B21]). It can be argued that when psychological, physiological, and genetic aspects of the neuroticism--health relationship are considered, the association between the biological sensitivity hypothesis and personality are discussed. There are a number of studies on neuropeptides and the association between personality and health, for example, an association between *BDNF* gene polymorphism and health has been demonstrated ([@B22]). Other studies are also dedicated to the role of neuropeptides in the processes of social functioning ([@B23]). In particular, *BDNF* plays an important role in the functioning of the nervous system. It belongs to the neurotrophins, a class of neuropeptides, which are synthesized primarily in neurons and glia of the central nervous system. The effect of *BDNF* is connected with cell growth, differentiation, and survival of neurons in the central nervous system. Moreover, it plays an important role in the functioning of the nervous system: neurons can release *BDNF* in response to nerve impulses and after its release, *BDNF* activates specific receptors, which are present on the membrane surface of neurons and glial cells. Their effects are connected with long-term structural changes in neurons ([@B24]). As a result of the above, *BDNF* plays a major role in the process of social interactions by participating in the development of the nervous system and maintaining its functioning, as well as its structure ([@B25]). The study presented here focuses on the relationship between personality traits and a molecular genetic polymorphism in *BDNF* gene and serum concentration of peptide YY (PYY), a biomarker for activity of the hypothalamus--pituitary--adrenal (HPA) axis and the functioning of the neural mechanism ([@B26]). The polymorphism in the *BDNF* gene consists of a single-nucleotide polymorphism (SNP), which is present in exon IX of this gene. There are four alleles in *BDNF*: C, T, A, and G. The polymorphism T-allele leads to an alteration in the gene\'s structure of the region with a Trp-Val substitution ([@B27]). Many studies have been conducted to determine the relationship between the *BDNF* gene polymorphism and various functions in the organism, including behavior and affective state ([@B28], [@B29]), anxiety ([@B30]), smoking ([@B31]), obesity ([@B32]), and some personality traits, in particular extraversion ([@B33], [@B34]). The importance of extraversion for health and longevity was demonstrated in many studies that showed that *BDNF* polymorphism has an impact on many psychological aspects of life. Extraversion is one of the components of the Big Five personality traits, which was determined in a number of studies in which it was shown that extraversion can be modified by the *BDNF* gene ([@B34]). Another genetic factor, which is associated with an increased risk of diseases, such as diabetes, obesity, and cardiovascular diseases ([@B14]), and is therefore important in studies of personality and health, is leptin. Leptin is produced by adipocytes in the body and functions as a satiety signal and regulator of the body weight. It is released by the body through a hormonal pathway to control the levels of glucose and lipids and also exerts a function as a neurohormone ([@B35]). The relationship between leptin and neuropeptide Y (NPY) has been found in research, which demonstrated that both of these factors are involved in the regulation of eating behavior, reproduction, and cardiovascular, respiratory, gastrointestinal, and immune function. Moreover, NPY has been found to be related to both the anxiety response and stress. Leptin is involved in the brain and has a neuroprotective effect, which explains the relationship between leptin and many diseases ([@B36], [@B37]). Studies show that the level of stress is linked to the polymorphism of the *BDNF* gene. For example, in one study on patients with major depression, it was shown that patients with this disease had high levels of leptin and lower levels of BDNF ([@B38]). On the other hand, the levels of hormones connected with stress and the HPA axis are connected with the polymorphisms of the *BDNF* gene. For example, in a study by Gómez-Merino et al. ([@B39]), the *BDNF* Val/Val genotype had significantly higher levels of cortisol compared to the C allele in response to acute stress. The aim of the presented study is to investigate the relationship between personality and *BDNF* gene polymorphism and BDNF plasma levels. It is believed that extraversion is connected with biological markers in the context of the neurotrophin theory ([@B39]). Thus, it is hypothesized that the *BDNF* polymorphism is linked to psychological indicators, such as extraversion, and with biological indicators of the *BDNF* gene, such as leptin, and cortisol, and to the neuroendocrine systems\' functioning. Therefore, in this study, we have determined the levels of the different components of personality (extraversion, neuroticism, depression, neuroticism, anxiety, openness) and anxiety (measured by