Does exposure to light at night and circadian disruption alter LH secretion and reproductive hormone regulation?

Light at Night and Reproductive Hormone Regulation

Exposure to artificial light at night (ALAN) and the resulting circadian misalignment significantly impact the female reproductive axis by altering both central hormonal triggers and local ovarian function. For women around the age of 45, who may already be experiencing perimenopausal shifts, these disruptions can further complicate hormonal stability.

Clinical evidence and reproductive outcomes

Chronic circadian disruption, particularly in the context of night-shift work, is strongly associated with reproductive dysfunction.

• Hormonal shifts: Large-scale epidemiological data, including the Nurses’ Health Study, show that women with over 15 years of night-shift exposure have significantly higher plasma estradiol levels compared to those who never worked nights.
• Cycle irregularity: Circadian misalignment is linked to irregular menstrual cycles, reduced fertility, and an increased risk of anovulation. This is largely attributed to the desynchronization between the master clock in the brain and the peripheral clocks in the reproductive organs.
• Menopausal transition: In women aged 45 and older, circadian disruption can exacerbate the fluctuations in luteinizing hormone (LH) and estrogen that characterize the menopausal transition, potentially intensifying symptoms.

Mechanistic explanations

The disruption of reproductive hormones by light at night occurs through multiple interconnected pathways:

• Melatonin suppression: ALAN activates the retinohypothalamic tract, leading to acute suppression of melatonin. Melatonin normally provides a circadian signal that helps regulate the hypothalamic-pituitary-gonadal (HPG) axis; its suppression can lead to the disinhibition of estrogen production.
• SCN-GnRH-LH pathway: The suprachiasmatic nucleus (SCN), the body's master clock, provides essential gating signals to kisspeptin and GnRH neurons. This "permissive window" is required for the timing and magnitude of the preovulatory LH surge. Circadian misalignment disrupts these signals, leading to irregular LH surges and impaired ovulation.
• Ovarian clock disruption: Peripheral clocks within the ovaries (in granulosa and theca cells) regulate genes responsible for steroidogenesis, such as StAR, CYP19A1, and CYP17A1. Disruption of these local clocks directly alters the production of estradiol and progesterone, regardless of central signals.
• Substrate availability: Clock genes also regulate the expression of HMG-CoA reductase and other enzymes that control the availability of cholesterol, the primary building block for all reproductive steroids.

Bottom line

Strong evidence demonstrates that exposure to light at night causes circadian misalignment that significantly alters LH pulsatility and elevates estradiol levels. This disruption occurs through both central nervous system signaling (SCN/GnRH) and direct effects on the ovarian steroidogenic machinery. Avoiding light at night is critical for maintaining the hormonal coordination necessary for reproductive health.