Abstract
Many animal studies have reported an association between melatonin suppression and the disturbance of metabolic responses; yet, few human studies have investigated bright light effects on metabolic and hormonal responses at night. This study investigated the impact of light on plasma hormones and metabolites prior to, and after, an evening meal in healthy participants. Seventeen healthy participants, 8 females (22.2 ± 2.59 years, mean ± s.d.) and 9 males (22.8 ± 3.5 years) were randomised to a two-way cross-over design protocol; dim light (DL) (<5 lux) and bright light (BL) (>500 lux) sessions, separated by at least seven days. Saliva and plasma samples were collected prior to and after a standard evening meal at specific intervals. Plasma non-esterified fatty acid (NEFA) levels were significantly higher pre-meal in DL compared to BL (P < 0.01). Plasma glucose and insulin levels were significantly greater post-meal in the BL compared to DL session (P = 0.02, P = 0.001), respectively. Salivary melatonin levels were significantly higher in the DL compared to those in BL session (P = 0.005). BL at night was associated with significant increases in plasma glucose and insulin suggestive of glucose intolerance and insulin insensitivity. Raised pre-prandial NEFA levels may be due to changes in insulin sensitivity or the presence of melatonin and/or light at night. Plasma triglyceride (TAG) levels were the same in both sessions. These results may explain some of the health issues reported in shift workers; however, further studies are needed to elucidate the cause of these metabolic changes.
Highlights
Artificial light exposure at night has become commonplace throughout the developed world [1, 2]
To the author’s knowledge, this is the first study to investigate bright light exposure at night on hormonal and metabolic responses prior to and after a standard evening meal in healthy young individuals by exposing them to two light sessions: bright (>500 lux), equivalent to light the intensity in the workplace, and dim light (
The salivary melatonin profile was significantly reduced by bright light exposure at night
Summary
Artificial light exposure at night has become commonplace throughout the developed world [1, 2]. Circadian rhythms are seen in any biological processes that display an endogenous oscillation of about 24 h They are generated by the suprachiasmatic nuclei (SCN) located in the anterior hypothalamus and influenced by external cues called zeitgebers (commonly daylight). Disruption of circadian coordination may be manifested by endocrine imbalances [9], incidence of obesity [10] and type 2 diabetes [11, 12]. This raises a controversial issue as to whether aberrant light exposure may influence metabolism by changing the time of the circadian system [13]
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