Dim light at night Laura Fonken

Does Dim Light at Night Make You Fat?

Playing with your smartphone at night does more than just disrupt sleep.

Before electricity, humans got all of their light via exposure to the sun, fire, and the moon and stars. This meant that nights were spent in relative darkness. Today, our environment is quite different. Our cities and homes can now be brightly illuminated all the time, regardless of season or time of day. Also, our cities have bright LED street lamps that create “light pollution” filling outdoor city environments with much more light than is natural. 

Is light at night messing with your metabolism?

Researchers at Randy Nelson’s lab at The Ohio State University wanted to investigate how different patterns of light in the environment might affect body mass over time. To test this, they divided mice into different conditions based on daily light exposure:

  • 16 hours of daylight and 8 hours of dim light (approx. 5 lux)
  • 16 hours of normal indoor light (150 lux) and 8 hours of darkness

The researchers monitored the mice for eight weeks, measuring how much food they ate, how much physical activity they engaged in daily. The mice were also weighed weekly.

The researchers found that when mice were exposed to dim light at night over eight weeks, they experienced 50% greater weight gain than their counterparts living with a normal light-dark cycle. Differences were evident from the first week and persisted during the course of the study.

But here’s the crazy part. My first assumption was that the mice exposed to dim light…just ate more food. Makes sense, right? But that doesn’t appear to be the case.

The light-at-night mice didn’t consume more food than the other group – but they ate a significantly greater portion of their food when they would normally be asleep.

A second study replicated this study design, but instead restricted the rodents’ access to food to times when they would either be active or asleep. Sure enough, when the light-at-night group only ate at times when they would be awake and alert, they did not gain more body mass. So, as we’ve discussed before on this blog, it looks like when you eat – not just how much you eat – may play an important role in body mass and glucose metabolism.

Effects of circadian misalignment like this could be an important piece of the puzzle in our obesity epidemic – and something for all of us to consider in our own daily health practice.

Mice exposed to light at night gained 50% more weight than mice with a normal light-dark cycle Click To Tweet



Today on this episode of humanOS Radio, I talk to Dr. Laura Fonken. Dr. Fonken is a postdoctoral fellow in Steven Maier’s lab in the Department of Psychology and Neuroscience at the University of Colorado.

She was the lead author of the intriguing rodent study that I described above, but she has been involved with several other important studies in this area, which we discuss in detail in the interview. She is continuing to research how disruptions to circadian rhythms may affect physiological functions, and ultimately could make individuals more vulnerable to disease.

This affects all of us, to varying degrees, and perhaps in ways that we don’t even realize. Listen below to hear some more about this fascinating work.



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Laura Fonken: There’s really evidence in humans and other rodent models that eating at this time of day when you’d usually be resting is associated with increased weight gain.
Kendall Kendrick: HumanOS. Learn. Master. Achieve.
Dan Pardi: Laura Fonken, welcome [00:00:30] to the show.
Laura Fonken: Thank you for having me.
Dan Pardi: You do work in circadian biology, which is the timing of our biology. How did you get into this line of research?
Laura Fonken: When I started graduate school at Ohio State University, I joined a lab that was focused on seasonality, so Randy Nelson’s lab, which mainly worked on seasonal rhythms and animals. There’s definitely a big light component to seasonality.
Dan Pardi: Right.
Laura Fonken: Shortly after I joined the lab, the lab shifted focus into studying [00:01:00] more nighttime light exposure. What really led us to study light at night was we had a graduate student who ended up in the hospital for several nights with a staph infection. His biggest complaint staying in the hospital was that the lights were on all the time and he found it really disruptive, so we became interested in studying what is the association between these really disruptive light cycles that we generate in modern [00:01:30] times and human or animal health and physiology.
Dan Pardi: Oh yeah, the worst place to get a good night’s sleep is at the hospital. So, you’re work looks at light at night and things like immune functioning and mental functioning, but also metabolism. Tell us about your work in the effect of lights at night on metabolic functioning.
Laura Fonken: Sure. We actually ended up in the light at night and metabolism area a little bit serendipitously. Based off this graduate student’s experience in the hospital, we were actually [00:02:00] interested, initially, in how light at night may affect behavior and mood and parameters more like that. We started a study where we were exposing mice either to a standard light-dark cycle where they get regular lighting during the day, total darkness at night, or to a 24-hour light cycle where they get continuous lighting throughout the day and night. Then we were looking at a bunch of behavioral outcomes in the study, and one thing we noticed along the way was that the mice that were exposed to [00:02:30] the continuous light were getting fatter. It actually wasn’t initially what we went in to study, but then we kind of took a look at the literature and realized that there was a lot of evidence to support this association between disruptive light schedules and metabolism. So, we went back to the drawing board and designed our first experiment, where we looked at how different intensities of lighting at night can effect metabolism in mice.
Dan Pardi: Tell us a little bit more about the details of this study.
Laura Fonken: Sure, [00:03:00] so in our first experiments looking at exposure to light at night and effects on metabolism, we actually had three different groups. We either exposed mice to our standard light-dark cycle where they get total darkness during the night. We expose them to that continuous light schedule where they get 24-hours at the same lighting intensity. Then we included a third group and this group was exposed to standard levels of lighting during the day, so the same as our control group, but then instead of total darkness during the night, they were exposed [00:03:30] to five lux of dim light. Five lux of dim light is basically equivalent to having a TV screen on in an otherwise dark room. The reason we were interested in including this group is because we felt like it was more relevant to what you would experience in the real world. Continuous lighting doesn’t happen outside of really extreme latitudes and so it was more of an ecologically relevant condition to have a dim light at night group.
So, we put the mice in these [00:04:00] different lighting conditions over eight weeks and we tracked their body mass gain and also looked at things like glucose tolerance, which is a good measure of metabolic function, and we looked at some metabolic related hormones. Basically, what we found is that in both of our groups of mice that were exposed to light at night, so the dim light at night group and mice exposed to that continuous bright light, we saw increases in weight gain compared to the mice that were maintained in dark nights. Over the course of the study, [00:04:30] mice in dim light at night gained about 50% more weight.
Dan Pardi: Wow.
Laura Fonken: Yeah.
Dan Pardi: And did you monitor the amount of calorie intake in all groups?
Laura Fonken: We did, and that’s what we found really surprising at first, was that we didn’t actually see any differences in total caloric intake. The mice in light at night were gaining more weight even though they showed really comparable food intake, and they also showed similar levels of activity. We tracked their activity, as well. What we found was different was when [00:05:00] the mice were eating. Instead of eating most of their food during their active phase, which is actually the dark phase in nocturnal rodents, we found that they were eating basically equal amounts of food throughout their inactive and active phase. So, they’re doing more snacking during this time when they would usually be sleeping and this off, out-of-phase timing of food intake seems to be what was really driving this increase in weight gain.
So, we did another experiment where we food-restricted the mice so that they couldn’t [00:05:30] eat during their inactive phase, and we found that they no longer gained weight when they had night time light exposure.
Dan Pardi: Okay, let me provide a synopsis. Different groups of mice were subjected to different environmental light conditions. One group had normal light exposure over a 24-hour period. One group had constant light 24-hours a day. And another group had normal day light, but at night when it’s usually dark, then they had dim light. You found between the groups, you had comparable amounts of physical [00:06:00] activity and amount of calories that were taken in, so you might expect there to be no differences in weight gain, but you did find that. The groups that had light at night gained more weight. One important feature of the groups that had light at night is that they had more of their calories come at night. So, not more overall calories, but more calories in a 24-hour period happened during the period when they usually are resting. Is that accurate?
Laura Fonken: Yeah, exactly. There’s really evidence in humans and other rodent models [00:06:30] that eating at this time of day when you’d usually be resting is associated with increased weight gain.
Dan Pardi: Historically, so much of the discussion in health sciences revolves around the idea that weight is all about calories in and calories out. This work shows that the timing of calorie intake really matters. What do you think is going on in the body to explain this?
Laura Fonken: That’s a really interesting question. Basically, what it seems like is that your circadian system primes your metabolism for food intake at certain times of day. [00:07:00] We know that the circadian system regulates a whole host of homeostatic functions in mammals, and one of the most important things it regulates is metabolism. As I mentioned, it’s not just something that’s been shown in rodents, but there’s also been work in humans looking at different time of food intake and how this can affect calories are processed, how it can affect your glucose responses to food. For example, I know Ken Wright has a number of studies looking at forced circadian [00:07:30] desynchrony or different timing of food in humans and shows that humans can have very different glucose responses depending on time of day of food intake and things like that.
Dan Pardi: One study you published in 2014 in the Journal of Physiology and Behavior looked at the effects of exercise on the metabolic effects of dim light at night. Tell us about that study.
Laura Fonken: In that study we were interested in exercise for two different reasons. First, we know that exercise is important for combating obesity, [00:08:00] so exercise reduces weight gain. But, secondly, exercise also has important feedback effects on the circadian clock. Especially in rodents, wheel running at certain times of day can actually strengthen the circadian system. We had this two-fold hypothesis that exercise may diminish the effects of light at night because it may work to both strengthen circadian rhythms, which we see become dampened with exposure to light at night, and it also may just reverse the effects directly [00:08:30] of this additional weight gain that you see in light at night. That’s, actually, exactly what we found. When mice that were housed in dim light conditions, so mice with exposure to light at night, when they had access to a running wheel, we no longer saw increases in weight gain. It suggests that there really are interventions that you can do to try to prevent weight gain with exposure to light at night.
Dan Pardi: Light at night appears to be a risk factor for weight gain, but it’s not necessarily determinant. For instance, [00:09:00] even if you get light at night, but you’re keeping the timing of your eating normal and you don’t let the timing of your eating drift, that doesn’t necessarily lead to weight gain. Secondarily, if you are exercising, that, too, helps to prevent weight gain from exposure to light at night. Translating this to humans, when would humans need to get exercised to mitigate the effects of night time light exposure.
Laura Fonken: In our study, we didn’t have the exercise at a specific time of day. They had access to the running wheel throughout the [00:09:30] day and night. Mostly, the tend to run on the running wheel during their active phase. In humans, though, I think that there is work suggesting that being active earlier in the day can actually be beneficial for … in training the circadian system. Much like bright light therapy, how people will use bright light therapy specifically early in the morning, exercise can also be used as a training signal for the circadian system. Having exercise towards the morning can be a little more [00:10:00] beneficial than exercising late at night, which may actually keep you awake longer.
Dan Pardi: Were you able to ferret out anything related to exercise intensity to be critical here in this equation? For instance, does the intensity of exercise need to be sufficient enough to activate adrenaline from the sympathetic nervous system in order to provide the timing signal to the circadian system?
Laura Fonken: Mm-hmm (affirmative), that’s really interesting. We didn’t actually evaluate that in our study. It was just the availability or non-availability. It would be interesting to look back and try to see if we noticed differences [00:10:30] between animals that ran different levels and depending on the timing that they tended to run more, but we just didn’t look at those sorts of variables in that study.
Dan Pardi: Okay, so we know a healthy amount of exercise is good for the circadian system to prevent against the ill effects of dim light at night. There seems to be some suggestion that earlier timing of exercise could be good, but we don’t know, necessarily, if a modest walk is going to be better or as good or not as good as, let’s say, something that’s more intense.
Laura Fonken: Yeah, an ultra-marathon or something like that.
Dan Pardi: Yeah, or like [00:11:00] a crossfit workout.
Laura Fonken: Yeah.
Dan Pardi: My suspicion is that you would definitely see the effects from higher intensity exercise, but it’s to be determined whether low intensity exercise is beneficial. Who knows, maybe the effect is related to calorie expenditure independent of intensity. Anyway, some of your recent work intersects with the field of aging research, which is a huge interest of my own. Tell us about your work in this area.
Laura Fonken: Sure. I finished my PhD at Ohio State in 2013 and I ended up moving to [00:11:30] the University of Colorado in Boulder. My more recent work has been in Steve Maier’s lab at the University of Colorado. Steve is really well-known for his work on stress and neuroimmunology, studying how immune system activation in the brain can affect behavior. In his lab I’ve become more interested in projects focusing on neuroimmune regulation, with a specific interest, [00:12:00] of course, still in circadian rhythms.
One of the first studies that I did when I joined the Maier lab was, I was interested in looking at whether immune cells in the brain are regulated by the circadian system and I was specifically focused on microglia, which are the primary innate immune cell in the brain. Basically, in my first experiment, what I did was I looked for the presence of circadian clock teams, so these teams that are involved [00:12:30] in regulating the circadian system, in microglia cells. I also looked at whether there rhythms in immune activation in these cells. I found that there was indeed functional circadian mechanisms in microglia and that the circadian system seemed to regulate the inflammatory capacity of these cells throughout the day and night.
What was really interesting to us was that we found that when you look at microglia during the active phase of animals, that they’re much [00:13:00] less inflammatory than when you look during the inactive phase. Then, we kind of further took that into the context of aging. We know that with aging there tends to be somewhat of a breakdown in circadian rhythms at both the behavioral and the molecular level. We looked at circadian clock function in aged microglia compared to microglia from adult animals, and we found that circadian rhythms and microglia function were broken down with aging, so the microglia no longer had this fluctuation between [00:13:30] period of heightened activation and then a period where they were less active. In the old animals, microglia just seemed to be active all the time, so more inflammatory, basically.
Dan Pardi: Have you found anything to combat that in your aging mice?
Laura Fonken: Another researcher in the lab, Ruth Barrientos, has shown that aging can actually rescue a lot of the neuroinflammatory effects that happen with aging, so we, in general in aged animals, get this more active neuroinflammatory [00:14:00] profile. She’s found that when animals have access to a running wheel, even though the aged animals tend to run very little, the presence of a running wheel is still able to reverse a lot of the increases in neuroinflammation that we see. So I would be interested in looking at whether this exercise effect is partially tied to rescuing some of these circadian parameters that we see that become disregulated in aged animals.
Dan Pardi: That is very interesting. A little bit of exercise can go a long way, [00:14:30] it seems.
Laura Fonken: Yeah, exactly. That was the title of her paper, actually.
Dan Pardi: Ah. Oh, okay.
Laura Fonken: Little Exercise, Big Effect.
Dan Pardi: That was totally lucky on my part.
Laura Fonken: It’s pretty remarkable, just because the difference between the amount the young animals and the aged animals run at night is huge. The aged animals are really barely moving in comparison, but you still get these real, big protective effects of even just this minimal level of activity.
Dan Pardi: I gave a presentation at a conference a few years back on why we get fat [00:15:00] and part of the presentation, I discussed the effects of exercise on brain inflammation, particularly in areas of the hypothalamus that are involved in the regulation of body fatness. Essentially, diet-induced obesity can cause inflammation of these centers, but when you exercise, especially when your muscles are working with adequate intensity, you produce what are called myokines, things like interleukin 6. Down stream of that, you produce other immune chemicals like interleukin 10 and these substances will reduce inflammation. You see this positive effect [00:15:30] in the brain, not only on markers of inflammation and stress, but also in restored sensitivity to hormones like leptin and insulin that play key roles in body fatness regulation. We want to be sensitive to these hormones and the downstream effect of exercise helps promote a more sensitive state. The endocrine effects of exercise has really broad, positive effects on the brain and, just as you spoke of, here’s another one. Exercise keeps the time keeping system of the body stronger as we age.
Laura Fonken: In science, [00:16:00] we try to, obviously, find ways to improve health and when you come back to it, exercise has just such a positive effect on so many conditions.
Dan Pardi: So true. What are some other topics you’re interested in researching?
Laura Fonken: I’m interested in looking at how and when exactly circadian rhythms in immune responses develop. I actually wrote a review paper last year with my old supervisor, Randy Nelson, on development of the circadian system and how disruptive lighting [00:16:30] during development can throw off the development of the circadian system. When we were writing the review, I noticed that there actually wasn’t a whole lot on that topic and I think it’s really important to look at because we’re exposed to these disruptive light schedules throughout life, so not just in adulthood but we also see kids staying up later, we have nightlights on, different lighting environments in the home setting. I think it’s really important to look at how the circadian system develops and whether light during [00:17:00] development can have negative effects on circadian rhythms, particular in immune cells, is the context I’d be looking at.
Dan Pardi: That’s such important work. It wasn’t that long ago that premature babies were kept in 24-hours of light in the NICU so that the staff could monitor them. We know now that the circadian system isn’t fully developed in newborns and that exposure to natural light rhythms, darkness at night, light during the day, is really important for a fully functioning circadian [00:17:30] system throughout life, and for long-term health outcomes. So, this is great work, Laura and thank you so much for the work you do and for taking the time to come talk with us.
Laura Fonken: Yeah, it’s been great talking with you, too. Thank you so much for having me on your show.
Dan Pardi: Absolutely. Have a great one.
Kendall Kendrick: Thanks for listening and come visit us soon at humanOS.me.

  • Peter Watson

    Dan, thanks for this interview and for all the info you provide on this site, and a huge “Thank you!” to Dr. Laura Fonken for sharing this research. Very interesting and reinforces the reality that exercise, exercise, exercise and consistently good sleep are among the most beneficial things we can do for ourselves, at any age, but especially as we grow older.

    (Note: It’s pretty clear that Laura meant “exercise” instead of “aging” when she said “Another researcher in the lab, Ruth Barrientos, has shown that aging can actually rescue…”, right before the 14:00 mark.)

    Laura’s research on circadian rhythms is fascinating! We are a completely sleep deprived, light polluted society and can only hope that bad sleep becomes less a symbol of status, and more one of stupidity. Think of medical interns and the traditionally brutal shifts they pull. It’s completely unnecessary and is basically a hazing ritual. “I went through it, and so must you.” Rites of passage are important, but do you really want medical care from someone who hasn’t slept for 20 hours?

    I will confess that on the home front, it’s all too easy to be exposed to light before bed, especially the blue light from the multitude of screens we all stare at all day. I’ve gotten amber computer glasses and use the amber “night” settings on devices to mitigate that, but based on this interview, avoiding light of any kind will be better for my sleep. I will also confess that unnecessary eating in the evening is a struggle for me, and if turning off the screens & lights helps with that, I’d be thrilled. Looks like it’s early bed in a dark room for me tonight.
    Thank you!

    • danpardi

      @ptwatson:disqus, thank you for the nice compliments and for the careful listening. Yes, I do believe she meant exercise not ‘aging’ at that time point. I also agree with you about the rights of passage medical training system we have set up. I understand doctors need to have grit because need to medical attention happens at inconvenient times, yet I too want the most refreshed doctor possible in my time of need.

      You wrote:
      “I will confess that on the home front, it’s all too easy to be exposed to light before bed, especially the blue light from the multitude of screens we all stare at all day.”

      It’s a constant battle, isn’t it? My goal is not to let perfect be the enemy of good. Also, in case you haven’t seen it, check out my conversation with Prof. Jamie Zeitzer on the subject → http://blog.dansplan.com/a-new-product-to-significantly-reduce-jet-lag-interview-with-stanford-professor-jamie-zeitzer/

      Also, in the blog, I describe how to modify your phone screen colors. It’s not ideal but it’s pretty darn good for reading at night (but not for video).