Can light in the bedroom at night drive weight gain?

I came across and interesting report today, published on the BBC website here. In essence, this study found that in a large group of women, the more light they tended to have in their bedrooms at night, the heavier they tended to be [1]. This is an ‘epidemiological’ evidence, and does not prove therefore that light exposure at night causes weight gain. However, is it possible that ‘light pollution’ at night may drive weight gain? Also, might ensuring genuine darkness at night assist weight control or even weight loss?

In a recent blog post I made the point that light exposure during the day generally helps to promote good sleep, but it has quite the opposite effect in the evening and at night. This is plenty of evidence now that shows that sleep disruption can have biochemical, physiological and metabolic effects that have the capacity to promote fat deposition in the body.

The effect of light exposure at night has been tested in animals. In one study, light exposure at night was found to suppress secretion of growth hormone – a hormone, which among other things, promotes ‘lipolysis’ (fat loss) [2].

In another study, exposing mice to light at night caused them to rapidly gain weight, even though light did not cause them to consume more food over the day [3]. A pdf of this study can be found here. In it, the authors reference much evidence regarding the impact of light at night on health in not just animals, but humans too.

The relationship between light, sleep, physiology and health is highly complex. However, as a general rule I think it’s fair to say we are probably helping ourselves by getting adequate light exposure during the day, but keeping our exposure to light exposure at night to a minimum. The first requirement here can often be met by being mindful of the importance of light and ‘getting out more’ when conditions allow.

For the second, some might consider investing in appropriate blinds or curtains. Another option, though, is to make use of eyeshades. I’ve found these vary enormously in terms of comfort and effectiveness. However, if you find a pair that work for you, this relatively small investment may pay back big in terms of your overall health in time.


1.    McFadden E, et al.The Relationship Between Obesity and Exposure to Light at Night: Cross-Sectional Analyses of Over 100,000 Women in the Breakthrough Generations Study Am. J. Epidemiol. first published online May 29, 2014

2.    Kasuya E, aet al. Light exposure during night suppresses nocturnal increase in growth hormone secretion in Holstein steers.vJ Anim Sci. 2008;86(8):1799-807

3.    Fonken LK, et al. Dim light at night disrupts molecular circadian rhythms and increases body weight. J Biol Rhythms. 2013;28(4):262-71.

7 Responses to Can light in the bedroom at night drive weight gain?

  1. Colin 30 May 2014 at 7:09 pm #

    Could it be more a case of: More Light pollution => Urban Dwelling => More Convenience => greater consumption of Processed food & more active socially than physically…?

    Just a thought.

    • Chris 31 May 2014 at 2:13 pm #

      Yours is an excellent point, Colin. many concomitant changes accompany urban dwelling including pressures that influence the kind of food that is offered for sale and the kind of food that is bought.

      However the way that alterations to diet have their effect is mediated via their capacity to induce changes in the balance of certain hormones.

      All creatures have aspects of their physiology that may be light sensitive in some way, and can detect seasonal variations. There has to be an explanation how turtles navigate vast oceans to return to a specific beach and at just the right time of year. They do not have sat-nav and no calendar. seasonal variation in day length, and polarisation of light might figure in how they pull off this stunt. Bears hibernate at the right time of year, and they spend the summer conspiring to be in the peak of condition needed to hibernate. Day length and variation in quantity/quality of carbs available raises their insulin seasonally and encourages overfeeding so they fatten up.

      It is not just food that influences the balance of certain hormones in a way that encourages over-feeding. Other things, light for one can influence hormonal balance and so can stress. It is worth taking up with the book that Iain Dobson recommends.

    • Christoph Dollis 1 June 2014 at 3:01 am #

      I doubt it because the same results were shown with experimental animals.

  2. Iain Dobson 30 May 2014 at 7:44 pm #

    If you want to read further, try “Lights Out” by T.S. Wiley [ISBN 067103868-0] available from all good book sellers. It is reasonable book but it has been written for an American audience, so perhaps is a little dumbed down at times. However, the message is clear and I am surprised it has taken the main stream media to cotton on the lack of sleep thing. One problem may be that you need to spend less time in front of the tv/computer/smart phone and that means also less exposure to the adverts – turkeys don’t vote for Christmas.

    • Chris 31 May 2014 at 2:00 pm #

      ‘Lights Out’ is an excellent book. Wiley has become a contentious figure, but that should not detract from the excellence of the book. It is a real eye-opener that explains why we need to close our eyes, and why its best that the room is dark when we do.

  3. Robert Park 2 June 2014 at 10:30 pm #

    If I could add something from past experience. At the conclusion to WW2 I served as a seaman gunner with the Royal Navy. Initially the navy remained under war time routine and ships spent most of the time at sea. Between preparation for action and sea-going duties the opportunity for sleep was restricted and was continually interrupted but none of this appeared to have any noticeable affect except if one was sleep deprived over a period of 2-3 days, that is, without sleep. All that was required was a short sleep, sometimes less than 4 hours. Onboard engine and machine noise was a constant feature as, indeed, so was stormy weather when food could not be cooked. Life onboard at sea was hard yet the onboard culture created an optimistic attitude where complaining was regarded as a weakness. The point here that no one was adversely affected by short sleep or disturbances which contradicts the findings of this research and provides scientific excuses on which people use to justify their need for more sleep or to squander money on purchasing blinds. The true results of this research may have a greater leaning towards the placebo-effect than it has about chemical bodily changes which may have little effect on our daily lives. In my naval career there were no fat seafarers.

    • on the other hand 4 June 2014 at 5:57 am #

      On the other hand Robert sleep deprivation is recognised technique used in the interrogation of POWs and those suspected of being spies, as is leaving a bright bulb burning in a cell, or shining a bright light in sleep deprived (and perhaps dilated) eyes.

      Accumulated fatigue has been one of my triggers for over-eating in the past and having read into the topic I have trended to the (evidence based) that the decision to eat and how much is not nearly so conscious a choice as my former view had it. There are hormones that govern when we feed, how much we feed, and to an extent the kind of food we crave. There are things that arise in species in the wild that cannot be explained without thought for hormones and certain cues capable of influencing hormonal balances that may arise with light and light cycles. We regularly forget it but we humans are a product of nature and evolution and our genes and hormones have evolved to respond to natural cues that modernity has scuppered.

      The invention of the light bulb has brought on a paradox. We could be exposed to light 24/7 if we chose, but working people may now work in places and at times that limits the time they spend outdoors in really bright light. This has consequences for vitamin D (the sunshine hormone) and consequences for cortisol.

      Cortisol and vitamin D get interesting. After the mapping of genome understanding did not trend in way people expected. Th genomes influence upon outcomes turned out to be more involved than the enthusiasts anticipated. The reasons are several. Key among then is gene switching.

      The simple addition of a chemical tag to gene can render it ineffective. The chemical tag is a methyl (CH3) group and the process of adding the tag is called methylation. Science doesn’t really know what drives methylated or what might drive demethylation (which is the removal of tag) but it is an area of intensive research now. Add the tag and the gene is turned off, remove the tag and the gene can express itself again. Some tags seem durable, some quite transient, and as yet nobody may know why.

      But cortisol is under the spotlight, it seems to have bearing upon gene switching. So does vitamin D maybe. Vitamin D seems to repair genes (or methyl switches) and just maybe cortisol encourages aberrance. Now think about this. At what times in the wild is increased mutation a good thing. The answer is when the species is under the greatest stress from the environment in which it lives. If the environment suits a state of adaptation why change the code or the responses to environment? On the other hand if a species is under stress of some kind because aspects of the environment no longer suit is it a good thing, systemically speaking, to hasten mutation?

      Of course genes may mutate. Genes may mutate at random or haphazardly and then selection pressures determine what should persist and what should not, but the responsive aspect arises in the epigenome which is the term for this switching layer of tags added or tags removed. The epigenome may switch in your lifetime, some switches might flick several times, and some switches only once, but the effect may be intergenerational too. The switched state may be passed from parent to offspring.

      Some switches flick quickly, some might need some chronic pressure to switch, we maybe don’t yet know. It is however an area of established research that light has consequences for cortisol and it is a growing fact that cortisol may have bearing for gene switching and gene expression. all you need do to be convinced is enter ‘cortisol’ (or glucocorticoid) and ‘methylation’ into a search engine. So it might be light stress takes time to have effect upon insulin and glucose metabolism.

      Many concomitant variables can have bearing upon hormones, and so isolating one set as being ‘causal’ can be tricky. There are several concomitant differences between the times you recount and those that this study explored.

      I liked your account Robert.

      My father served in the Navy and was posted to the eastern med. He spent a lot of time aboard a motor launch aiming a rifle at the mechanical bits of mines attempting to blow them up. I do not know how many successes he could claim. On one occasion the launch was blasted. He was in the water amidst a cauldron of burning marine diesel swimming for his life. He was lucky. He passed away in 2001. He had a haemorrhage, and was cabbaged, passing away several days later. Your account prompted fond memories. He’d be 93 now.

      The thing is, haemorrhage follows vascular weakness called an aneurysm. I cannot tell you what causes an aneurysm though I suspect it has causes in common with atheromas of CVD. Stress is a factor, as may be homocsyteine and cortisol. These may not be independent. Indeed I am increasingly of a mind they are linked. Chronically elevated cortisol, which can arise with light stress and from light pollution can raise homocysteine and it mat interfere with gene switching and genetic expression.

      Homocysteine is a powerful oxidant and through oxidising cholesterol homocsyteine (Hcy) can really damage or alter cell behaviour. Many life threatening degenerative diseases reveal a link with high Hcy. Oxidised cholesterol is truly quite toxic to a many a cell, and Hcy is now strongly implicated in its capacity to convert healthy cholesterol to oxycholesterol.

      Truthfully, if something alters the balance of certain hormones, and if the altered balance persist over time, there is very likely to be physiological consequence that will manifest itself in a physical change of symptom in time. These people haven’t discovered anything new, it has been subjected to research before, but what they point to isn’t common knowledge even amongst scientists.

      Now here’s something I learned in the last 24 hours.

      Oxycholesterol can be terminal for cells. In coronary arteries it will provoke weak spots called atheromas that can cause heart attack or stroke. Can anything fix oxidised cholesterol? Yes it can. Sulphur ions can be added to oxidised cholesterol and that detoxifies them. Better still sulphated oxycholerols have been said to be protective or healing. We get sulphur from onions and garlic in the diet. If you eat a lot of fat and garlic and onions then you get your fix of sulphur, while the fat helps make optimal use of it. What this amounts to is that the French maybe had a traditional diet which supplied enough sulphur to help counter oxidative stress, lower Hcy, and even ‘repair’ oxidised cholesterol. This might explain the French paradox.

      It pays to know your onions, Robert, and old dogs can learn new tricks. The knack is never thinking that the stuff we don’t know isn’t worthy of learning.

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