While there are many factors that cause us to gain weight, one you would probably never suspect is how much bright light you are exposed to prior to bedtime. But as one research study shows, sustained exposure to bright light sources in the evening not only disrupts your ability to fall asleep, but also can lead to significant weight gain. 
How so, you ask?
Exposure to bright lights (indoor lighting sources, televisions, computer screens, tablet devices) disrupts our circadian clocks, which regulate, among other things, when we fall asleep and wake up. These light sources can interfere with the rhythm of our circadian clocks, and in so doing, disrupt our ability to fall asleep. At particular risk are those who work deep into the night, such as doctors, nurses, airline pilots and flight attendants, and those working night shifts or alternating day/night shifts in environments with bright light sources.
You see, deep inside our brains there is a small but vitally important gland called the hypothalamus, which houses our central circadian clock. This clock regulates the pattern or rhythm of our daily bodily functions including when we go to sleep and wake up. It is based on a 24-hour time period and has been programmed over time in each of us to signal at regular intervals when it is time to wake and sleep. This is known as our circadian rhythm.
When it comes to sleep, our bodies help the circadian clocks keep the rhythm going by sending messages from sensors throughout the body that tell the clock whether it is light or dark in our external surroundings. When the sensors detect low light or darkness, they send those messages to the circadian clock. Based on our previously established patterns/rhythms, the clocks react to these messages by sending out signals to other sensors in the brain and body that say, “OK, folks, time to start shutting down for the night.”
But when the sensors transmit messages to the circadian clock that indicate there are bright external lights in our environment at a time when the body’s rhythm expects darkness or dimmed lights, the circadian clock becomes confused. In effect, it says, “Hey, it’s 10 p.m., and normally I’m getting ready to call it a night about now but my scouts are telling me it is still light outside. I’m not exactly sure what to do now, but given that it’s still light out, I’m going to push back our shut down time.”
When this unexpected pattern repeats itself over a period of time, the circadian clock can’t handle the disparity of the incoming messages with the pre-established rhythm. The result is a clock that misfires, causing us to struggle to fall asleep and stay asleep.
The impact of a disruption of the circadian clock is profound and goes well beyond lost sleep. Research studies have shown it increases risk for obesity, diabetes, breast cancer and depression. In addition, it has been shown to inhibit mental acuity and the brain’s ability to adapt to new stimuli. In particular, disrupting the circadian rhythm has been shown to disrupt the portion of the brain that controls executive decision making. And it can also make you fat …
The Weight Connection
A study published in the journal Proceedings of the National Academy of Sciences, showed that laboratory mice whose daylight/darkness cycles were disrupted from 24 hours (12 light/12 dark) to 20 hours (10 light/10 dark) experienced dramatic weight gain compared to the normal-circadian clock mice even though both groups ate exactly the same amount of food each day. The researchers chose the 20-hour cycle in order to simulate a pattern where the mice were exposed to light sources at times when their circadian clocks expected darkness, and they noted a 38% jump in body weight compared to the control mice over a nine-week period.
In analyzing the reasons for the significant weight gain between the two mice groups (control group on a normal 24-hour cycle, and the disrupted group on a 20-hour cycle), the researchers found that the disrupted mice had higher levels of the hormones insulin and leptin in their blood plasma. And we’re not talking a small difference here — in both cases the amounts of insulin and leptin were substantially higher in the disrupted mice compared to the control group. This result was particularly unusual given that the blood glucose levels in both the control and disrupted mice groups were about the same. Generally, if there were extreme differences in insulin and leptin, one would anticipate finding corresponding extreme differences in blood glucose levels.
Normally, leptin acts as an appetite suppressant, and so one would think that high levels of leptin would mean weight loss because one is never hungry. Separately, insulin acts as a regulator of how much glucose (sugar) is stored in cells. Under the same diet conditions (as in this study), mice of the same beginning weight, eating exactly the same kind and amount of food, and otherwise living similar lives should have similar glucose levels, which they did, but also similar leptin and insulin levels. But they didn’t — in fact, the disrupted mice had insulin levels approximately 75% higher than the control mice, and leptin levels 50% higher than the control. The only lifestyle difference between the two sets of mice was the purposeful alteration of the body’s innate rhythm by introducing light at times the disrupted group expected darkness.
The researchers in this study speculated that disrupting the circadian clock by introducing bright lights at times when the mice were expecting dimmed lights or darkness caused the clock to alter the rodents’ metabolism with the result being a significant weight gain. Whether the substantial weight gain triggered the higher levels of insulin and leptin or vice versa is unknown.
So, if you are regularly exposed to bright lights in the evening hours and you’ve been struggling to maintain a healthy weight (as well as struggling to get a good night’s sleep), it is worth your while to try limiting your exposure to bright light sources a good hour or two before bedtime. Doing so will allow the sensors in your body to communicate with your circadian clock that it is getting darker and trigger the clock to begin the internal “power-down” messages throughout your body that prepare us for sleep. In addition, removing the “light shock” stimuli that is disrupting your body clock may dramatically lower your blood levels of insulin and leptin, and thereby contribute to inhibiting weight gain.
 Karatsoreos IN, et al. Disruption of circadian clocks has ramifications for metabolism, brain, and behavior. PNAS. 2011 Jan 10. [Epub ahead of print]