If you have a habit of falling asleep with the TV on, it could be playing a role in your ability to maintain a healthy weight. The link between sleep and obesity is strong, with those who sleep fewer hours having an increased risk of obesity.1
However, in recent research published in JAMA Internal Medicine, it wasn’t sleep duration and quality that appeared to influence weight gain and obesity, but rather exposure to artificial light that was the problem.2
You needn’t have significant light exposure to potentially affect your weight, either. In the study, the weight gain association was strongest for sleeping with a light or the television on in the room.
Exposure to artificial light at night increases obesity risk
Compared with women who had no exposure to artificial light at night, sleeping with a television or light on in the room was associated with a greater risk of gaining about 11 pounds (5 kilograms) or more, a body mass index increase of 10% or more, incident overweight and incident obesity over the course of the follow-up period.
“Exposure to artificial light at night while sleeping appears to be associated with increased weight, which suggests that artificial light exposure at night should be addressed in obesity prevention discussions,” the researchers noted.3
As noted, there were differences in weight gain depending on the type of light exposure. While sleeping with a small nightlight was not associated with weight gain, sleeping in a room with light coming from outside the room was associated with a modest weight increase. However, women who slept with a light or television on were 17% more likely to have gained about 11 pounds or more.
Study co-author Chandra Jackson, Ph.D., head of the National Institute of Environmental Health Sciences Social and Environmental Determinants of Health Equity Group, explained that the study could have implications for people living in urban areas, where source of artificial light like streetlights and neon signs could interfere with the sleep hormone melatonin.
“Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night,” she said. “Exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity.”4
Why light exposure at night is dangerous
Exposure to light at night, even if it’s dim, leads to disruptions in natural light and dark cycles that can have far-reaching effects on health.5 In a study by researchers at Ohio State university, light exposure at night was even linked to disruptions in immune and endocrine function.
The study involved hamsters, which normally sleep during the day. For nine weeks, one group was exposed to dim light at night while a control group was exposed to typical light during the day and darkness at night. Dim light at night not only led to increased body mass in the hamsters but also affected the hamsters’ offspring.
Although the offspring were raised in normal lighting conditions, with light during the day and darkness at night, parental history of light exposure prior to conception led to offspring with impaired immune response and decreased endocrine activity.
These health conditions were passed down through either parent’s genetic material, meaning it didn’t matter whether it was the mother or father that was exposed to dim light at night; the effect could be traced to either parent.
The impaired adaptive immune function noted in the hamster offspring illustrates a transgenerational effect of light at night, and while it didn’t influence DNA sequence, it did affect the DNA’s epigenetic expression.
“Together, these data suggest that exposure to dLAN [dim light at night] has transgenerational effects on endocrine-immune function that may be mediated by global alterations in the epigenetic landscape of immune tissues,” the researchers explained.6
Nighttime light exposure alters melatonin
Melatonin is an important hormone produced by your body’s pineal gland. One of its primary roles is regulating your body’s circadian rhythm. When it gets dark, your brain starts secreting melatonin (typically around 9 p.m. or 10 p.m.), which makes you sleepy.
Levels typically stay elevated for about 12 hours, then, as the sun rises, your pineal gland reduces your production, and the levels in your blood decrease until they’re hardly measurable at all. When your circadian rhythms are disrupted, such as from shift work, jet lag or nighttime light exposure, your body produces less melatonin.
“ … [R]oom light exerts a profound suppressive effect on melatonin levels and shortens the body’s internal representation of night duration,” researchers wrote in The Journal of Clinical Endocrinology & Metabolism. “Hence, chronically exposing oneself to electrical lighting in the late evening disrupts melatonin signaling and could therefore potentially impact sleep, thermoregulation, blood pressure and glucose homeostasis.”7
Aside from helping you sleep, melatonin may help prevent cancer, acting as a “full-service anticancer agent,” inhibiting the initiation, progression and metastasis of cancer.8 In one clinical trial, patients with glioblastoma were given either radiation and melatonin, or radiation alone. While six of the 14 patients receiving melatonin were alive one year later, only one of the 16 who received radiation alone was still alive.9
Another study found that melatonin reduced the growth of prostate cancer,10 and it also has a strong correlation with breast cancer. For instance, women who live in neighborhoods with large amounts of nighttime illumination are more likely to get breast cancer than those who live in areas where it’s dark at night.11
In fact, there was a 30% to 50% higher risk of breast cancer in the countries with the highest levels of nighttime light compared to the countries with the lowest nighttime light exposure.
Light at night may even affect your brain health
Another reason to pay careful attention to the light in your bedroom at night is to protect the health of your brain and even your mood. Twenty men slept in a laboratory under varying lighting conditions — no light exposure for two nights, then a dim light of five or 10 lux on the third night (for comparison, twilight is measured at 10.8 lux12).
After the second and third nights, the participants performed working memory tests while undergoing functional magnetic resonance imaging (fMRI). The goal was to evaluate the effects of dim light exposure during sleep on functional brain activation during a working memory task the next day.13
When sleeping under 10 lux light conditions, there was decreased activation in the right inferior frontal gyrus, an area of your brain involved in response inhibition, attentional control and the detection of relevant cues when performing a task.
Exposure to 5-lux light had no statistically significant effect on the participants’ brain activity, which suggests that while your brain may tolerate the extremely dim 5 lux, at just 10 lux your cognition and working memory may suffer.
That being said, in a study on hamsters, exposure to even 5-lux light at night for four weeks led to symptoms of depression — and the symptoms disappeared after two weeks when the dim light at night was removed.14
Screens at night are a bad idea
Keep in mind that it’s not only the light from televisions that is problematic. Exposure to LED-backlit computer screens, tablets and cellphones at night also significantly suppresses melatonin production and feelings of sleepiness. When your brain “sees” blue light at night, the mixed message can add up to serious health issues.
In 2011, for instance, researchers found that evening exposure to LED-backlit computer screens affect circadian physiology. Among 13 young men, exposure to five hours of an LED-lit screen at night significantly suppressed melatonin production along with sleepiness.15
The issue extends far beyond sleep, however. LEDs have virtually no beneficial infrared light and an excess of blue light that generates reactive oxygen species (ROS), harming your vision and possibly leading to age-related macular degeneration (AMD), which is the leading cause of blindness among the elderly in the U.S.
LED lights may also exacerbate mitochondrial dysfunction leading to chronic conditions ranging from metabolic disorders to cancer. If you view screens at night, it’s therefore essential to block your exposure to blue light while doing so. In the case of your computer, you can install a program to automatically lower the color temperature of your screen.
Many use f.lux to do this, but I prefer Iris software for this purpose. In addition, when watching TV or other screens, be sure to wear blue-blocking glasses after sundown, and turn them off well before you plan to go to sleep.
For optimal health, you need bright light during the day and darkness at night
Sleeping in a completely darkened room, without lights from alarm clocks, televisions or other sources will improve your sleep quality, help you maintain a healthy weight and improve your overall health. If you get up during the night to use the bathroom, it’s important to keep the lights off so you don’t shut off your production of melatonin.
Also, remove any light-producing alarm clocks and install blackout drapes to keep light from creeping in your windows. You can also wear an eye mask to keep out light pollution. Even before you go to bed, I recommend wearing blue light-blocking glasses after sunset to avoid blue-light exposure.
Equally important to sleeping in pitch darkness is getting sunshine in the morning. Melatonin is affected by your exposure to both light and dark. When it is light, production of melatonin naturally drops. Getting at least 15 minutes of sunlight in the morning hours helps to regulate the production of melatonin, dropping it to normal daytime levels, so you feel awake during the day and sleep better at night.
The good news is that if you’ve been sleeping with the TV on, this is a simple and straightforward fix that can significantly improve your health. If you have a tendency to fall asleep with the TV on, make a point to watch television somewhere other than your bed, then get up to go to sleep in your dark bedroom as soon as you feel tired.