Some people habitually sleep with a bedside lamp on for a sense of security or convenience for getting up at night. This seemingly harmless habit may actually be a "hidden killer" to health. A young woman in her early twenties recently struggled with obesity, weighing a staggering 220 lbs, and was diagnosed with high blood sugar and hypertension—conditions not typically seen at her age. The underlying cause was traced to her long-standing habit of sleeping with the lights on.
According to Ningbo Evening News, Dr. Zheng Tian-ming, Deputy Director of the Sleep Medicine Department at Ningbo University Affiliated Kangning Hospital, shared a case of a woman in her twenties weighing over 220 lbs who also had high fasting blood sugar and hypertension. Her fasting blood sugar level reached 14 mmol/L. According to World Health Organization criteria, a level ≥7 mmol/L is diagnostic for diabetes.
When Zheng inquired about her lifestyle habits, he discovered that she had always preferred sleeping with the lights on, which was precisely what caused her unnoticed weight gain. He explained that even weak nighttime light can penetrate the eyelids, stimulating the photoreceptor cells on the retina. This misleads the brain, disrupting the circadian rhythm and suppressing the secretion of melatonin, a hormone that promotes sleep. This seemingly minor interference can, day after day, damage metabolic function.
A 2019 study published in the prestigious international journal JAMA Internal Medicine had already highlighted the health risks of sleeping with lights on. Scientists from the U.S. National Institutes of Health tracked over 43,000 women for five years. Results showed that compared to women who slept in complete darkness, those who habitually slept with a nightlight or television on had:
- A 17% higher risk of gaining 5 kg (about 11 lbs) or more.
- Increased risks of becoming overweight and obese by 22% and 33%, respectively.
Two Mechanisms Explain How "Night Light" Leads to Weight Gain
Why does sleeping with a light on lead to weight gain and even obesity? There are two primary reasons:
1. Induces Insulin Resistance
Researchers suggest that artificial light at night can cause insulin resistance, reducing the body's efficiency in lowering blood sugar. To maintain blood sugar stability, the pancreas is forced to "work overtime," secreting more insulin. High levels of insulin are the primary driver that promotes the synthesis and storage of fat in the body.
2. Disrupts Hormonal Balance, Increasing Appetite
Zheng adds that nighttime light disrupts the balance of two key appetite-regulating hormones: leptin and ghrelin. Leptin signals fullness to the brain, while ghrelin stimulates hunger. The "sleep fragmentation" caused by nighttime light leads to shallow, easily disrupted sleep. Even if it doesn't cause waking up to eat midnight snacks, the body may perceive an energy deficit. This can lead to increased appetite the next day, with a particular craving for high-calorie, high-sugar "sinful foods."
Therefore, he cautions against underestimating the small nightlight in the bedroom. It may be a hidden driver behind obesity, diabetes, and cardiovascular diseases, quietly undermining your health.
