Does late-night light exposure blunt cortisol timing and reduce adrenal DHEA secretion?

Late-night light exposure significantly impacts the hypothalamic-pituitary-adrenal (HPA) axis, the primary system responsible for the body’s hormonal stress response and daily rhythmicity.

Clinical and mechanistic evidence

The biological master clock, the suprachiasmatic nucleus (SCN), is highly sensitive to light signals. When light—particularly blue-spectrum light from screens or overhead sources—is detected by the retina late at night, it sends immediate signals to the SCN. This activation disrupts the SCN’s normal nocturnal signaling, leading to two primary endocrine effects:

• Alteration of cortisol timing: Late-night light exposure can acutely increase cortisol levels when they should naturally be at their lowest. This leads to a "blunting" or shifting of the normal diurnal cortisol rhythm. Studies show that bright light exposure during the biological night can delay the circadian phase, subsequently altering the cortisol awakening response and the magnitude of the morning peak.
• Dysregulation of DHEA secretion: Dehydroepiandrosterone (DHEA) and its sulfate form (DHEA-S) are primarily produced in the adrenal zona reticularis under the stimulation of adrenocorticotropic hormone (ACTH). Because cortisol and DHEA share this same upstream HPA regulator, the circadian disruption that alters cortisol also impacts DHEA. Research suggests that circadian misalignment—like that caused by artificial light at night—can flatten the diurnal slopes of these hormones, potentially reducing the overall output of DHEA.

Adrenal-level mechanisms

Emerging research highlights the role of "peripheral clocks" located directly within the adrenal glands.

• Local clock genes: Genes such as BMAL1 and PER within the adrenal tissue help regulate the gland's sensitivity to ACTH.
• Steroidogenic proteins: Circadian disruption dysregulates key enzymes and trafficking proteins, such as the steroidogenic acute regulatory protein (StAR), which is essential for transporting cholesterol into the mitochondria to begin hormone production.
• Melatonin suppression: Late-night light consistently suppresses melatonin. Melatonin normally acts as a modulator of the HPA axis; its absence removes a natural "brake" on nocturnal adrenal activity, further contributing to the blunting of normal hormonal rhythms.

Bottom line

Late-night light exposure disrupts the SCN and adrenal clock genes, leading to mistimed cortisol secretion and a plausible reduction in DHEA output by altering the HPA axis's daily rhythm. Practicing strict light hygiene after dark—such as using dim, warm-toned lighting and avoiding screens—is a validated strategy to maintain normal adrenal hormone timing and secretion.