Chronic exposure to artificial light at night alters endocrine signaling in ways that can raise or reshape stress-hormone activity, but the strength of evidence differs between animal experiments and human studies. Experimental work in rodents establishes a plausible causal chain: dim or nocturnal light suppresses melatonin, shifts circadian timing, and changes activity of the hypothalamic-pituitary-adrenal axis, producing higher nocturnal glucocorticoid levels. Jennifer M. Fonken at Ohio State University and Randy J. Nelson at Ohio State University reported that mice exposed to dim light at night showed altered circadian behavior, increased body mass and elevated corticosterone, the rodent analogue of human cortisol, illustrating a mechanistic route from light exposure to stress-hormone change. George C. Brainard at Thomas Jefferson University has demonstrated how short-wavelength light strongly suppresses melatonin, a hormone that normally signals night to the brain and interacts with stress systems.
Mechanisms linking light and stress hormones
Artificial light, especially blue-rich spectra from LEDs and screens, reaches the retina and activates intrinsically photosensitive retinal ganglion cells. That signal modulates the central circadian pacemaker in the suprachiasmatic nucleus, which coordinates peripheral clocks and the timing of cortisol secretion. Suppression of melatonin and shifted circadian phase can lead to a flattened or phase-shifted daily cortisol rhythm—for example, higher nighttime cortisol or reduced morning surge—which is a feature associated with chronic stress, sleep disruption, and metabolic dysregulation in other contexts.
Population-level evidence and consequences
Human observational and laboratory studies show consistent melatonin suppression by nocturnal light and altered sleep, but direct, consistent increases in daytime cortisol attributable solely to ambient nighttime lighting are less uniform. The International Agency for Research on Cancer at the World Health Organization has classified circadian disruption related to shift work as probably carcinogenic, highlighting broader health consequences of chronic circadian disruption that can involve altered stress-hormone regulation. Elevated or dysregulated cortisol patterns are linked to increased risk of mood disorders, cardiometabolic disease, impaired immunity, and impaired cognitive function—outcomes that are magnified in urbanized or shift-working populations with high exposure to artificial light. Cultural and socioeconomic factors determine exposure: densely lit cities, 24-hour economies, and unequal housing produce uneven health burden, and ecological impacts extend to nocturnal wildlife whose stress physiology and behavior are affected by light pollution.
Taken together, animal experiments provide strong mechanistic evidence that chronic artificial light at night can increase or dysregulate stress hormones, while human data indicate plausible risk mediated through circadian and sleep disruption; more well-controlled longitudinal human studies are needed to quantify effect sizes and inform mitigation.