As China has been going more and more industrialized, sleep loss has become increasingly serious in China. According to a survey about sleep in 2002, an estimated 45.4% people in China were suffering from sleeping problems and an estimated 28% of them had insomnia. Based upon the latest survey published by the Chinese Sleep Research Association in 2010, the estimated incidence of insomnia in China has reached as high as 38.2%. Hence, insufficient sleep duration has become a prevalent issue in our society, tending towards worse. Sleep is commonly viewed as a restorative process that influences homeostatic regulation of the autonomic, neuroendocrine, and immune system (Dinges et al., 1995 and Krueger and Toth, 1994) and maintains normal physical functioning for all humans regardless of sex, age or ethnic origin (RECHTSCHAFFEN and BERGMANN, 2002). Since sleep plays an important role in learning, memory processing, cellular repair and brain development (Dinges, 2006, Graven, 2006, Stickgold and Walker, 2007 and Tononi and Cirelli, 2006), sleep deprivation as a stressor could exert great influence upon cognitive function, motor performance and emotion (Chee and Chuah, 2007 and Kahn-Greene et al., 2007), affecting the neuroendocrine stress systems and aggravating depression, anxiety and other mental disorders in the long term (Meerlo et al., 2008). Circadian rhythmicity and sleep play important roles in the regulation of plasma cortisol concentration by the hypothalamo-pituitary-adrenal (HPA) axis (Guyon et al., 2014). Experimental evidence indicates that sleep deprivation may affect cortisol levels and its circadian rhythm (Thorsley et al., 2012). Numerous studies have found links between sleep and changes in cortisol concentration, but the implications of these results have remained largely qualitative. A couple of studies showed that sleep deprivation or sleep loss had demonstrated to result in a significant elevation of cortisol levels (Lac and Chamoux, 2003 and Omisade et al., 2010). This negative relationship has been advocated by several correlational studies (Balbo et al., 2010), as well as by experimental studies on total or partial sleep deprivation (Rodenbeck et al., 2002 and Vgontzas et al., 2003). On the other hand, sleep deprivation is considered to be able to affect various affective disorders, depression in particular, positively (Voderholzer et al., 2004) or negatively (Rao et al., 2009). A study on chronically restricted sleep inducing depression-like changes in neurotransmitter receptor sensitivity and neuroendocrine stress reactivity in rats indicated that disrupted and/or restricted sleep might contribute to the symptomatology of mood disorders (Novati et al., 2008), though the possibility of other factors influence such as forced physical activity exists. Hypothalamo-pituitary-adrenal (HPA) axis hyperactivity, higher cortisol levels, a decrease in glucocorticoid receptors and lower negative feedback in bipolar disorder episodes were found during euthymia, depression and mania (Daban et al., 2005 and Fries et al., 2009). The study of acutely hospitalized manic patients reports elevated plasma cortisol levels (Cookson, 1985). According to recent studies, sleep deprivation could affect neuroendocrinology and affective disorder at the same time, and neuroendocrine maladjustment maybe the reasons of affective disorder. We hypothesize that neuroendocrine maladjustment is closely linked with mental disorders during sleep deprivation.
Previously, some sleep deprivation studies were based upon animal subjects, the conclusion of which could hardly apply onto human body. Small sample size has also been one of the shortcomings among the few sleep deprivation studies based on human subjects. Particularly, studies on endocrine hormone changes effects induced by sleep deprivation on affective disorder with a sample size more than 30 are scarcely seen in general population groups, let alone special population groups, such as the military personnel. In military settings, where irregular sleep schedules are common due to operational constraints, a research about U.S. Army officers showed that soldiers slept only 3.0 h during the exercise and some of them were active throughout (Lieberman et al., 2005). To the best of our knowledge, up to now, there has been no commonly accepted conclusion concerning the mechanism by which sleep deprivation induces affective disorders, such as mania and depression. In our study, based upon our previous researches, we found out that serum renin, angiotensin II, and cortisol, serum dopamine, epinephrine and norepinephrine increase induced by sleep deprivation (Sun et al., 2013) and serum renin, cortisol were closely linked with mental resilience (Sun et al., 2014). Therefore, the aim of this study was to investigate effects of cortisol level change induced by sleep deprivation upon mental status and to determine the optimum cut-off value of serum cortisol as a biochemical indicator to predict mental status change in servicemen.
