tolerance, and plasma leptin levels. Thus, it seems possible that other metabolism-related functions are also subject to regulation by the SCN or peripheral clocks [62]. Fluctuations in body weight have been associated with changes in day length in various species such as hamsters and sheep, suggesting a central role for the circadian clock in regulating body weight [62]. In humans, studies have demonstrated an increased incidence of obesity among shift workers [66]. Circadian clock plays a major role in determining body weight by influencing the expression and secretion of hormones. Leptin 24-hour levels were significantly lower in obese compared with nonobese adolescent girls [67], suggesting that blunted circadian variation may play a role in leptin resistance and obesity. Similarly to leptin, the rhythmic expression of resistin and adiponectin was greatly blunted in obese mice [67, 68]. In obese subjects, adiponectin levels were significantly lower than lean controls [69]. In rats, melatonin, a synchronizer of the SCN clock, decreased weight gain in response to high-fat diet and decreased plasma leptin levels. These effects were independent of total food consumption [70].
