Many women have this habit: when they are alone, they like to sleep with the light on, which makes them feel more secure. There are all sorts of myths about sleeping with the lights on. The most horrifying is that exposure to artificial light at night can cause disruption of circadian rhythms, inhibit the production of melatonin in the pineal gland, and lead to higher levels of estrogen, which in turn can cause breast cancer. Researchers in Spain have published a study confirming that artificial lighting at night can increase the risk of cancer, especially prostate and breast cancer. There have also been geo-ecological studies that compared satellite pictures of the earth and found that South Korea and Israel, which have stronger lights at night, have a 30-70% increased risk of breast cancer. Therefore, many experts have cited sleeping with lights on at night as one of the reasons for the increased incidence of breast cancer. However, studies to date have been limited by the number of cases and study design, and the results have been inconsistent. Today, I’m compiling a recent study that included more than 100,000 women, followed for a total of 6.1 years, and the data speaks for itself. The study was conducted in the United Kingdom, where researchers recruited 105,866 women in their 20s through a questionnaire to find out how bright the lights in their bedrooms were when they slept and how well they slept, such as whether they woke up often during the night. The mean age of the subjects was 46.5 years (16 to 102 years), and 97% of the women reported nighttime sleep light exposure in their 20s, with a number of women reporting that they woke up during the night and turned on the lights or went into a brightly lit room. 17% of the subjects had experience of working night shifts within the 10 years of the study. The study had a follow-up period of 6.1 years, during which time 1,775 of these more than 100,000 young women were diagnosed with breast cancer. A study of the bedroom lighting and nighttime sleep patterns of these women found that: 1) there was no association between nighttime sleep lighting and overall risk of breast cancer; 2) there was no association between nighttime sleep lighting and whether the breast cancer was invasive or in situ; 3) there was no association between nighttime sleep lighting and whether the breast cancer was estrogen-receptor (ER)-positive or ER-negative; 4) there was no association between nighttime sleep lighting and whether the breast cancer was ER-positive or ER-negative; 4, adjusting for the duration of sleep, as well as factors such as sleeping at unusual times (off-peak hour sleep) and working at night also did not affect the results; 5, the risk of developing breast cancer before menopause was lower if one woke up frequently during the night in one’s 20s. Everyone in this world wants to live simply, but sometimes they have no choice. For example, someone may turn on the lights frequently because they have trouble sleeping at night. Similarly one may have to work night shifts because of livelihood reasons, which leads to falling asleep at times that are not peak sleep times, which in turn increases exposure to artificial light at night. There is no way around any of this. If one really wants to follow the claim that these affect the risk of cancer, that’s just unacceptable. In addition, the study showed that exposure to lights at night did not increase the risk of breast cancer if the overall sleep schedule was adjusted, i.e., if the lost sleep at night was added back. This is despite the fact that sufficiently strong lights can reduce melatonin levels. However, night shifts and night light exposure are not so strong as to increase the risk of breast cancer, as shown through this UK study. Although women in their 20s who regularly wake up at night have a lower risk of developing breast cancer before menopause, this requires further research. Although the conclusion of this study is one that everyone is willing to accept, light does not cause breast cancer, an irregular lifestyle does. Humans have evolved through long periods of darkness without permanent lighting, so the human body is adapted to natural circadian rhythms. When it gets dark, our bodies automatically enter a state of preparation for sleep, melatonin, leptin, etc. are released and metabolism slows down……. However, the fast-paced life nowadays completely breaks this circadian rhythm, and that’s why it’s causing so many problems.