Abstract
We studied the dynamics of melatonin suppression and changes in cortisol levels in humans in response to light exposure at night using high-frequency blood sampling. Twenty-one young healthy participants were randomized to receive either intermittent bright (~9,500 lux) light (IBL), continuous bright light (CBL) or continuous dim (~1 lux) light (VDL) for 6.5 h during the biological night (n = 7 per condition). Melatonin suppression occurred rapidly within the first 5 min and continued until the end of each IBL stimuli (t1/2 = ~13 min). Melatonin recovery occurred more slowly between IBL stimuli (half-maximal recovery rate of ~46 min). Mean melatonin suppression (~40%) and recovery (~50%) were similar across IBL stimuli. Suppression dynamics under CBL were also rapid (t1/2 = ~18 min), with no recovery until the light exposure ended. There was a significant linear increase of cortisol levels between the start and end of each IBL stimulus. Under CBL conditions cortisol showed trimodal changes with an initial linear activating phase, followed by an exponential inhibitory phase, and a final exponential recovery phase. These results show that light exposure at night affects circadian driven hormones differently and that outcomes are influenced by the duration and pattern of light exposure.
Highlights
Unlike birds and reptiles[18,19,20], the mammalian pineal gland is not directly light sensitive
Melatonin and cortisol time course data were analyzed using both between-condition [data compared between individuals exposed to light exposure (IBL and continuous bright light (CBL)) and dim light conditions] and within-participant by condition analysis [data compared between light exposure (IBL and CBL) and dim light conditions under constant routine (CR) 24 h earlier] to ensure analytic robustness
Using a between-condition analysis [data compared between individuals exposed to light exposure (IBL and CBL) and dim light conditions], we found that the melatonin levels were significantly different between dim light and continuous bright light (CBL) or intermittent bright light conditions (p < 0.01)
Summary
Unlike birds and reptiles[18,19,20], the mammalian pineal gland is not directly light sensitive. SCN lesion or transection of the multisynaptic pathway abolishes light-induced suppression of melatonin[22,23]. When humans were kept on a 3-h day, with 1 h of sleep in the dark and 2 h of wakefulness in the light in each 3-h interval, mean cortisol secretion was highest during the first hour of waking following www.nature.com/scientificreports the sleep interval[28]. Scheer et al.[31], reported an increase in cortisol during the morning, within one hour after wake, but no change during an evening exposure using the same intensity (800 lux) and duration (1 hour) of light exposure. The aim of the current study was to investigate the precise temporal dynamics of changes in melatonin and cortisol levels induced by nocturnal bright light exposure
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