We are used to turning off the lights and closing the curtains before going to bed, and researchers have found more scientific evidence for this seemingly common sense approach. When many Americans sleep, there is some kind of artificial light in the room, such as TV, electronic products, or street lights outside the house. But a new study suggests that even a night's sleep in moderate light can have adverse effects on cardiovascular and metabolic health.
"To my surprise, even a small amount of light entering the brain through the eyelids can have a significant impact." Dr. Phyllis Zee, senior author of the study and director of the center for circadian rhythm and sleep medicine at Northwestern University, said.
In addition, there is increasing evidence that exposure to light at night can cause multiple injuries to the body or increase the risk of chronic diseases.
Physiological effects of light
Dr Zee and her team at Northwestern University conducted a small study of 20 subjects to analyze the effects of 100 lumens of artificial light on healthy adults during sleep.
"In this light, you can still see things around you, but you won't be comfortable reading." In this study, all subjects slept in an almost completely dark room for the first night. The next night, half the subjects fell asleep in a brighter room. (the light source is above the head.)
During their sleep, the researchers recorded their brain waves and heart rate, took blood every few hours, and carried out other tests. After getting up in the morning, both groups of subjects ate a lot of sugar to observe their body's response to the surge in sugar.
The results showed that there were significant differences between the two groups. Subjects who fell asleep in the light had a higher heart rate throughout the night and stronger insulin resistance in the morning, meaning that it was more difficult for them to control their blood sugar within the normal range.
Light interferes with metabolism
Dr Zee points out that sleeping in light at night can cause a variety of disturbances to metabolism.
Although some studies have pointed out that light can interfere with sleep quality, it is surprising that this study did not find this in the monitoring process. In fact, most of the subjects said they slept well.
The researchers also measured melatonin levels in the subjects. The hormone helps to regulate circadian rhythm and promote sleep. Generally, the level is low during the day and high at night. Studies have shown that exposure to artificial light at night can inhibit melatonin secretion. Scientists have also found that there is a correlation between melatonin interference and a variety of diseases, such as cancer, diabetes and so on. However, the study also found no reduction in melatonin levels in subjects who slept with light.
"This may mean that the light entering the eye is not bright enough to inhibit melatonin secretion."
However, Dr Zee and his team believe that such a small amount of light is enough to activate the sympathetic nervous system of the autonomic nervous system, which regulates the body's "fight or flight" response. During sleep, the system should have "cooled down" and switched to the parasympathetic nervous system, resulting in a decrease in heart rate and respiratory rate. However, the changes of cardiovascular function showed that a small amount of light was enough to switch the nervous system to a more active and alert state.
"Just as the brain and heart know the light is on."
Dr Chris Cowell, who studies circadian rhythms at UCLA, said the study showed that even relatively dark light interferes with our sleep wake cycle. He believes these findings make sense because the autonomic nervous system follows a stable circadian rhythm.
"In order for us to have a good night's sleep, the human body will carry out many coordinated actions, which are regulated by the balance of the autonomic nervous system."
Although the effect of light on the nervous system is not as significant as when awake, Dr. Cowell pointed out that the interference is still of concern. "If you want to have a good sleep, you can avoid this effect."
Increased risk of chronic diseases
The findings of this study are not big news. Cowell pointed out that many studies have shown that interference with circadian rhythm will affect the body's regulation of blood glucose levels.
In some of these human experiments, the light used in the study was much brighter, and the subjects were not really asleep. Although the study found that long-term effects could not be predicted, Cowell suspected that these harmful effects would accumulate over time. "It's only one night. Imagine what will happen in the long run?"
The body's "total clock", called the suprachiasmatic nucleus, is located in the brain, but the organs and tissues of the body have their own cellular "timers". Pancreatic cells, which secrete insulin, are an example. Interfering with the sleep wake cycle will affect their ability to secrete insulin normally, thus affecting the body's regulation of blood glucose.
"This increases the risk of chronic diseases such as insulin resistance, diabetes, and other cardiac metabolic problems." Dr. Charles chasler, a professor at Harvard Medical School, pointed out.
For example, a large-scale observational study of more than 40000 women found that during the five-year observation period, sleeping with the TV or light on had a 17% chance of gaining 11 pounds.
Zeisler himself has also carried out relevant research to investigate the longer-term effects of circadian rhythm interference on metabolism. In a recently published study, he and colleagues concluded that during the three week study period, the negative effects on subjects' metabolism were mainly caused by interference with circadian rhythm, not necessarily related to lack of sleep.
"Without increasing the brightness of artificial light at night, we have not found that long-term sleep deprivation will have an adverse effect on blood glucose metabolism."
Of course, this does not mean that lack of sleep is harmless to health, but just wants to emphasize the long-term effects of light exposure during night sleep.
"People may think that as long as they fall asleep, light has no physiological effect. But that's not the case."