Chronodisruption – Light exposure, health, why the dark nights make you feel terrible, and what you can do about it | by Dan Kirk | Dec, 2021

Dan Kirk

As my family and housemates can attest to, I am absolutely insistent on making the environment dark on an evening. Instead of having the bright overhead lights on in the kitchen or living room, I am insistent on having the dimmer ones on and turning TV brightness down as low as possible as it starts to get closer to bedtime.

I began doing this after hearing popular health scientist Dr Rhonda Patrick talk about how light exposure at night was a huge factor in influencing sleep onset. On a podcast (I can’t remember which — I listened to many of hers!) I remember her discussing a study in which participants first reported their usual bedtime before being maintained in a light-controlled environment and without access to clock, meaning they did not know the time. The result was that they slept much earlier than their reported bedtime if their environment was made dark on a night.

Other popular health scientists have also discussed this topic recently, one being Dr Andrew Huberman who compares checking your phone on a night as being the equivalent of waking up in the middle of the night in Abu Dhabi [1] (we will have to assume you’re in America for this analogy to work). Popular health and fitness podcast in the Spanish-speaking world Fitness Revolucionario recently hosted Prof Dr Álvaro Campillo who discussed the topic of light exposure on an evening and described it as the biggest factor on influencing sleep [2].

I’ve never been a good sleeper, so decided to try this out. Sure enough, I can concur that, at least for myself, making my environment dark on a night makes a huge difference to my night-time sleepiness and time taken to fall asleep. This is particularly obvious to me during the summer months, when the use of artificial light is less necessary and instead the setting sun creates a gradual dimming effect. In contrast, the winter months demand artificial light from early on in the evening, and these lights might remain on late in the night. This creates the unfortunate situation where tiredness onsets early on the evening — too early for sleep — and then the sustained, indoor, artificial light prevents sleepiness in the evening.

In fact, things are even worse in these winter months because it can still be dark on a morning. When not exposed to light (particularly sunlight) in the morning, the body does not receive the signals it requires that instruct it that it is time to be awake. This leads to a loss in the anchorage of circadian rhythm, which throws off sleep on the evening. For this reason, Huberman (amongst others) suggest bright light exposure, particularly that of sunlight, if possible, early in the morning.

Sleep, however, is not the only affected process. Disrupted light/dark exposure cycles actually have a large influence on many aspects of health, even if sleep is not perceivably affected. This spans from mood and energy to cancer, metabolism, and obesity. This is why so many people report feeling particularly bad as the dark nights draw in (November time in the Northern hemisphere). In fact, there is even a condition for this named “seasonal affective disorder” (with the ironic acronym “SAD”) which describes people who have their mood affected due to the altered light exposure during the winter months. Things can be so severe that suicide rates increase in the extreme latitudes during such times of the year [3].

Much of these alterations can be explained by changes in melatonin, which is famous for being “the sleep hormone” but also has roles to play in the body that make it more important to health than simply regulating sleep. It is released (mostly) by the pineal gland in response to evening darkness and its release is strongly inhibited by light exposure. In modern society, light pollution in urban areas and artificial light from screens like TVs, tablets and phones, computers, and lights around the house all inhibit melatonin release, and the consequence is a phase shift where melatonin is released later than desired sleep time [7]. This phase shift throws off melatonin secretion timing, which has knock-on effects on your sleep restfulness, morning wakefulness, daily energy, mood, blood pressure, body fat, and health as a whole, as Figure 1 demonstrates.

Figure 1: The effects of chronodisruption and aberrant timing in melatonin secretion on health

Being deep in the winter months, I’ve had discussions with many people recently on how their energy and mood has been impaired after making an Instagram post on this (see @trentiron) and sharing some tips on combating this. Because there was such an enthusiastic response to this, I thought I’d elaborate in a slightly more detailed post here.

First, I will try to convince you that this is something worth caring about by showing you that melatonin has roles beyond simply regulating sleep. I feel this is important because I’ve found that, whilst discussing this with friends, people sometimes like to announce that they do not notice issues with sleep even when they’re in bright environments before sleeping. To this I always respond: that may be so, but that does not mean you’re exempt from the negative effect on your health. Next, I will share some tips on what you can do to combat this, and hopefully improve your sleep, energy, mood, and health. If you’re not super interested in the science parts but don’t want to feel like shit for 3 months out of the year, you can skip straight to this part and still benefit from the article. In any case, enjoy!

Light Exposure, Circadian Rhythm, and Health

Melatonin is primarily synthesised and released in the pineal gland and its release is regulated in accordance with circadian rhythm. Its secretion peaks in the evening and is inhibited by the bright light and UV rays of the sun, meaning its production ceases in the daytime [5]. At least, that was the case for the vast majority of evolutionary history. Now, however, artificial light has changed the game and delayed melatonin peaks to later in the night than would usually happen. This can throw off the whole secretion pattern of melatonin, and since melatonin interacts with so many pathways in the body, this can lead to a bit of a mess. For example, some melatonin receptors (M1) are seen in retina, ovary, testis, mammary gland, coronary arteries, gall bladder, aorta, liver, kidney, skin and the cardiovascular system. Let’s take a look at some of the areas of health melatonin has relation to.

Mood and Energy

I’ll start with this one since it is perceivable and therefore many people can relate it. Melatonin is involved in the coordinated, circadian secretion of factors that directly influence energy availability such as glucocorticoids and catecholamines (cortisol, for example) [7]. Management of the appropriate secretion of cortisol is crucial for health. Aberrant regulation of cortisol is a big contributor to chronic stress and altered energy levels. Cortisol is the final biological component of the stress response in humans, so its dysregulation can cause a lack of energy availability at times when you need it (i.e., in the morning to wake yourself up) and put you in a stressed state at other times of the day (i.e., the night, when it should be lowering to allow sleep onset). In fact, hypercholesterolemia is seen in a subset of depressive patients, and, fascinatingly, appropriately timed light exposure can improve depressive symptoms, mood, and energy, and these effects apply not only to depressed subjects but also those that do not have depression [3, 6]. Evening cortisol is also observed decreased in those receiving light therapy, which is a desirable health outcome [6].


Next, melatonin is a strong antioxidant. It scavenges radicals itself, but also causes the upregulation of antioxidant enzymes (such as glutathione and superoxide dismutase, two famous antioxidant enzymes) and downregulates prooxidant enzymes (such as nitric oxide synthase, which produces the prooxidant nitric oxide, and lipoxygenases) [5,7]. Since oxidative stress forms the basis for so many states of ill-health in the body, and many people are in a state of oxidative stress, there is a big emphasis in the health community for managing oxidative stress and promoting antioxidant intake.

Some examples where oxidative stress is thought to play a role are the neurodegenerative diseases Alzheimer’s and Parkinson’s, two conditions in which melatonin has been shown to be protective against. Melatonin helps in these diseases not only via reducing oxidative stress, but also by metal chelating and GSK-3 modulation (in the case of AD) and protection from excessive stimulation of glutamate receptors (in case of PD), as well as, of course, its role in sleep quality preservation [7]. While we’re here, other brain related disorders melatonin has demonstrated efficacy in include epilepsy, amyotrophic lateral sclerosis, AD, ischemic injury and head injury [7].


Another topic that is often discussed alongside oxidative stress is that of inflammation. There is a huge push in the health community on managing inflammation, with some claiming that it is the cause of all illness and aging in the body (and interestingly, melatonin declines with age [7]). Its role in health and disease can’t be denied, and having a chronic inflammatory state in the body is deadly. Many immune cells — key regulators of inflammation via the release of cytokine — have melatonin receptors, and their activity is influenced by melatonin.

Melatonin can be immunostimulatory or anti-inflammatory, depending on the context in which it is present. In this way, melatonin regulates a healthy immune response, promoting immunity when necessary and managing inflammation when it is not necessary [7]. A downregulation of serum proinflammatory cytokines such as IL-6, TNFa, IL-1B, CRP, and IFN-y, whereas a promotion of anti-inflammatory cytokines such as IL-10, are actions of melatonin [8]. Elevations of the former list of notorious cytokines are observed in a whole array of health conditions, including (but not limited to) autoimmune diseases, neurodegenerative diseases, cancers, and metabolic conditions and obesity [9].


On that note, melatonin is involved in all of these disorders, and often in a protective manner. Melatonin is protective against obesity, in that rats supplemented with melatonin are protected from high-fat diet-induced obesity [7] or show a reversal of hyperadiposity [8]. Removal of the pineal gland in rats causes weight gain, and this state can be rescued via melatonin supplementation [4]. Interestingly, these effects are despite food intake remaining unchanged. This is thought to occur due to the action melatonin has on increasing energy expenditure by the browning of adipose tissue (converting white fat to the metabolically favourable brown fat) [7, 4].


Melatonin has an important influence in insulin sensitivity through various distinct mechanisms. Firstly, visceral fat (a type of fatthat is particularly metabolically unfavourable) is reduced [4]. Melatonin receptors are found in the islet cells of the pancreas and regulate insulin secretion. Removal of melatonin receptors or the pineal gland itself both result in impaired glucose regulation [7]. There is a plethora of other ways that melatonin influences glucose metabolism, such as:

  • M1-mediated phosphorylation of tyrosine kinase on the insulin receptor (reducing GLUT4 expression);
  • impacting adipokine (leptin and adiponectin release), which in turn influences insulin sensitivity;
  • directly acting on beta cells;
  • and impacting adipocyte differentiation [4].

Thus, is it not surprising that nightshift workers and those exposed to evening light show poorer insulin sensitivity and suffer health consequences because of this [7].

Cardiovascular disease, hypertension & cancer

Melatonin is also protective against CVD by improving fatty acid profile (reducing LDL cholesterol, total cholesterol, and CRP) and inflammatory profile, and by preventing leukocyte rolling and adhesion to the endothelial wall [4, 7, 8]; hypertension, by reducing arterial blood pressure, heart rate and baroreflex responses, and stimulating nitric oxide production (leading to vasodilation) [4, 7, 8]; certain cancer types, particularly breast cancer [7, 10] and reproductive disorders [7].

What You Can Do About It

Hopefully you’re convinced that — even if your sleep is fine — it would be wise to avoid a state of dysregulated circadian rhythm and melatonin. Now, the question is: what can you do about it? If you live in a country where the days get shorter during the winter, you’re going to be vulnerable to some of these effects. Fortunately, there are steps you can take to help yourself under these circumstances. Below, I list a few of the ones I’ve found most helpful (or have read as being most effective) that you can try out.

Make the environment dark on a night

Big surprise, right? But like I state at the beginning of the article, I have found this to have the largest effect for me personally. Starting at around 2h before your intended sleep time, reduce the light you’re exposed to in your environment. Turn off particularly bright and fluorescent lights, in favour of dim, orangey one. Don’t forget to also turn the brightness down on your devices like phones and laptops. Light from such devices is thought to be particularly harmful for melatonin secretion due to the blue-light. Thus, I highly recommend using blue-light blockers to control this exposure. (However, I feel compelled to leave a note here that the additional stimulatory effect on the brain that comes with the interactive component of using these devices is also not good for sleep onset, so try to avoid them at all if possible). If you’re in an environment where you cannot control your light on a night, you could consider investing in some blue-light/light dimming glasses. It’s not certain these will work (and is probably also dependent on the type you get) but you could try it out — most of them are cheap.

Expose yourself to light soon after waking

I feel more people are probably aware that keeping light low on an evening is a good idea but less are aware of the importance of the opposite — light exposure — on a morning. This is especially relevant in these winter months where it can be dark for house after waking up. As soon as possible after waking, make your environment light by switching multiple lights on, turning the brightness high on phones and tablets, open the curtains, etc. If possible, get outside and exposure yourself to sunlight. This is important not only because it will be much brighter (in term of total lux [11]) but also because the effect of the UV rays on the skin also influences circadian rhythm. I get that it might be cold, dark, and raining or snowing outside, so this isn’t always feasible, but if you can manage it, then do so.

Light around sunset

In the winter months where thes sun is setting around 6 o’clock, you may notice you get a wave of tiredness around this time due to the decreased light entering your eyes here. This can confuse your body into thinking its time to sleep, although in reality it is far too early. On this occasion, we can use light as a tool to help us combat this. Maintain bright exposure around this time to prevent this lethargy and only start reducing this light in the hours preceding your intended bedtime. Try to avoid only using tableside or ambient lighting at this time because it is too early to make use of this type of light. Instead, stick to brighter and harsher lights, like fluorescent or office lights. Just make sure not to forget to turn them off as it gets a little later…

Physical activity

Light is the biggest regulator of circadian rhythm, but it is not the only one. Physical activity also plays a role. Physical activity (regardless of when performed helps with regulating metabolic hormones and hormones involved in energy availability, thus influencing sleep quality and daytime tiredness. Exercise can alter the timing of key components of circadian rhythm such as peak heart rate, blood pressure and body temperature. Importantly, though, this exercise must be chronic — single and irregular bouts are not enough [12].

Whilst the timing of physical is not of paramount importance (within reason), it’s also not irrelevant. To align with circadian rhythm, activity should be higher through the day when the body is designed to be awake and active (i.e., central body temperature is higher, cortisol is higher), and then reduced in the evening to allow the onset of sleep (with an accompanying drop in central body temperature and cortisol). As one might expect, physical activity too close to desired sleep time could lead to a phase-shift by delaying circadian rhythm. Exercise causes a rise in body temperature and cortisol, which is antithetical to what is required on an evening for sleep onset.

Now, obviously these tips should be considered with practicalities taken into account, too. If, on a given day, you have no choice other than to exercise later on the evening, you may have to go for it. However, if this becomes a regular occurrence, you may encounter some chronodisruption in these circumstances, which as we’ve seen above could lead to health consequences. So, if you do find yourself in the situation, it may be worth reviewing your timetable to see if you can do this exercise at an earlier point in the day.

Eating schedule

You may have heard recently that it’s not just important “what” you eat but also “when” you eat. This is indeed true, and this is something that can be ascribed to circadian rhythm. Looking at scientific studies and the hormonal fluctuations we observe across the day, we can conclude that insulin sensitivity is highest in the morning and decreases across the day, making us more adept at partitioning nutrients like carbohydrates through the daytime hours. Conversely, on the evening, we should consider reducing our carbohydrate and overall caloric intake, since our body will make poorer use of these nutrients then [4].

However, it is not just circadian rhythm that influences food, but also food influences circadian rhythm. The body is expecting that we are awake and consuming calories across the day, and then fasting and resting through the night. By eating large meals on an evening and having breakfast too early on the morning after waking (especially when it’s still dark outside) we run the risk of interfering with our circadian rhythm once more. In this way, however, we can use the timing of our food to our advantage to help us anchor our circadian rhythm. Biasing more of your calories through the daytime and less on a night contributes to a well-timed circadian rhythm and prevents phase-shift. Conversely, eating large meals late on an evening (which can be quite commonplace in certain parts of the world, for example those in the Mediterranean region) could have the opposite effect.

Again, though, we should be sure to integrate such tips with our personal lifestyle. For example, I often find myself training on an evening. For bodybuilding purposes, placing more of your carbohydrates directly after the workout is beneficial. Thus, for me in my life, following this tip too strictly would be counterproductive to my goals. However, since learning more about this topic, I’ve definitely reduced the caloric content of my evening meals, and especially on rest days I make sure to bias more of my calories through the day, eating mostly proteins and fats on the evening.

Make your sleeping environment cold

The evening rise of melatonin, dip in cortisol, and onset of sleep go hand-in-hand with a decrease in central body temperature and an increase in peripheral temperature (as heat is expelled, thus moving from the centre of the body to the surface). The failure of body temperature to fall can interfere with the onset of sleep, which is why it’s so difficult to fall asleep in the summer months when the temperature is too high. However, it can also happen that central heating in houses during the winter months means the internal environment in homes can also be pretty warm, which could interfere with sleep onset. Thus, one way you can help with maintaining a healthy circadian rhythm is by making your environment on a night cooler as you get closer to body time, helping your body with decreasing its central temperature and improving sleep onset and quality.


As a stimulant and inducer of cortisol, caffeine has a negative effect on sleep. As the sun sets earlier in the day and you begin to get drowsy in the late afternoon, it can be tempting to reach for a coffee to get you through to the end of the workday. This is a bad move. Caffeine stays active in the body for a long time [13]. Although differences in caffeine metabolism exists, it is still likely that caffeine consumption in the late afternoon ist still active in your system when it’s time to go to sleep. Even if you do not notice an interference with sleep following caffeine consumption later in the day, that does not mean it’s not having an effect on the circadian parameters discussed in this article, and therefore still negatively impacting your health. I think coffee is amazing and caffeine is a very useful and healthy drug, but it must be used appropriately.

Wrapping Up

Many people struggle during this time of the year and feel so much more energized during the summer — so if you fall into this category, take comfort in knowing you’re not alone. Despite how many people are affected by these issues, however, I was so surprised to notice how few people were aware of what they can do to minimise the harms — or that there even were any harms! Hopefully you’ve been able to learn something from this article and take away some practical tips so that you can better manage your health, energy, and mood in these times of the year for the rest of your life.


[1] Erickson, M. (2020, 3 junio). Setting your biological clock, reducing stress while sheltering in place. Scope.

[2] 286: Horarios de Comida y Ayuno para optimizar la Cronobiología, con el Dr. Álvaro Campillo ⋆. (2021, 7 octubre). Fitness Revolucionario.

[3] Bedrosian, T. A., & Nelson, R. J. (2017). Timing of light exposure affects mood and brain circuits. Translational Psychiatry, 7(1), e1017.

[4] Tähkämö, L., Partonen, T., & Pesonen, A. K. (2018). Systematic review of light exposure impact on human circadian rhythm. Chronobiology International, 36(2), 151–170.

[5] Hardeland, R., Pandi-Perumal, S., & Cardinali, D. P. (2006). Melatonin. The International Journal of Biochemistry & Cell Biology, 38(3), 313–316.


[7] Singh, M., & Jadhav, H. R. (2014). Melatonin: functions and ligands. Drug Discovery Today, 19(9), 1410–1418.

[8] Cardinali, D. P., & Hardeland, R. (2016). Inflammaging, Metabolic Syndrome and Melatonin: A Call for Treatment Studies. Neuroendocrinology, 104(4), 382–397.

[9] Furman, D., Campisi, J., Verdin, E., Carrera-Bastos, P., Targ, S., Franceschi, C., Ferrucci, L., Gilroy, D. W., Fasano, A., Miller, G. W., Miller, A. H., Mantovani, A., Weyand, C. M., Barzilai, N., Goronzy, J. J., Rando, T. A., Effros, R. B., Lucia, A., Kleinstreuer, N., & Slavich, G. M. (2019). Chronic inflammation in the etiology of disease across the life span. Nature Medicine, 25(12), 1822–1832.

[10] Chepesiuk, R. (2009). Missing the Dark: Health Effects of Light Pollution. Environmental Health Perspectives, 117(1).


[12] Vitale, J. A., Lombardi, G., Weydahl, A., & Banfi, G. (2018). Biological rhythms, chronodisruption and chrono-enhancement: The role of physical activity as synchronizer in correcting steroids circadian rhythm in metabolic dysfunctions and cancer. Chronobiology International, 35(9), 1185–1197.

[13] Institute Of Medicine. (2002). Caffeine for the Sustainment of Mental Task Performance: Formulations for Military Operations. Nutrition Today, 37(1), 26–27.

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