In September 2024, a 38-year-old woman consulted Narendra Kotwal, director of endocrinology at Paras Health, Panchkula. She reported persistent fatigue, difficulty falling asleep, irregular menstrual cycles, mood disturbances such as irritability and low mood and had gained 5 kg over six months. The woman works nights at a call centre and lives close to a brightly lit digital billboard.

Kotwal, a retired lieutenant general in the Indian Army and president of the Endocrine Society of India, noted that the patient was overweight and exhibited features suggestive of insulin resistance – skin tags, a high waist-to-hip ratio and a large neck circumference – indicating a pre-diabetic state. That is not odd: A 2023 study showed that 136 million Indians are pre-diabetic, as we in August that year.

The patient also had pronounced dark circles under her eyes. Laboratory investigations revealed subclinical hypothyroidism and elevated evening cortisol levels, which disrupt the body's natural stress response and sleep cycle.

Kotwal attributed her condition primarily to light pollution or photo pollution – chronic exposure to artificial light at night during biologically-intended sleep hours.

Streetlights, illuminated billboards, neon signage, industrial and office lighting during night shifts, household lighting, and external security lights are the common sources of light pollution.

Kotwal’s prescription started with a series...