What is Your “Circadian Rhythm”?

Humans (and all organisms) have an organization to physiological processes that are primarily anchored around two environmental contrasts: the 24-hour cycle of light and darkness. The cycle of light and darkness corresponds to behavioral changes: the light period is also the waking, active, and food foraging phase. The dark period is also the sleeping, inactive, and fasting phase. This physiological and behavioral architecture is known as circadian rhythm, from the Latin ‘circa’ (around), and ‘dies’ (day).

The negative effects of sleep curtailment on appetite, hunger, metabolism, and food intake are well documented in the literature on sleep.

Even around 5.5hrs sleep per night – what some might consider a good sleep – stimulates the craving for high-carbohydrate, high-calorie foods, disinhibits food control and promotes overeating (29,8, 39, 24). If you don’t have a macro intake geared towards your variables to help subside cravings, refer to the IIFYM macro calculator.

The ‘Body Clock’

Colloquially known as your ‘body clock’, the most potent synchronizer (known as “zeitgebers”, or “time-givers”) of circadian rhythm is light – information communicated through your eyes to the brain which indicates whether it is night or day, and the physiological fluctuations that accompany either period.

The brain then syncs up with the rest of the body’s systems to allow us to anticipate changes in the environment, e.g. sleep to wake, and fasting to feeding. In fact, while light entrains the central circadian timer in the brain, feeding entrains the circadian timers in the digestive system (31).

Thus, synchronization of the circadian system is vital to metabolic processes from blood sugar management, to carbohydrate and fat metabolism (Ibid.).

You’ve likely experienced circadian dysregulation before, aka jet lag. The digestive difficulties on the first day or two after a long-haul flight are a result of eating in a new time zone, when it could be 4 am in your body’s internal clocks. It’s also why it takes a couple of days to get over jet-lag – you need exposure to daylight in your new time zone, coupled with new food timing patterns, to reset the system.


Circadian Rhythm Light Therapy

In industrialized societies, we spend around 88% of the time in enclosed buildings and are exposed to 4 times less natural light during the day (22,43). During the day, natural light can be anywhere from 2,000 to 100,000lux; average office lighting can often be less than 500lux, which is too low for circadian entrainment and disturbs circadian rhythms (4).

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In the evenings, 75% of the population are exposed to artificial light at night from shortwave blue light, the kind emitted by natural sunlight…and TV’s, laptops, and smartphones. This suppresses melatonin, the hormone which is released during the biological night and signals the onset of sleep (Ibid.). When you add all this together, people now experience an intensity of light between sunset and bedtime that is over twice their level of natural daytime light exposure (43).

Why might this be a problem?

The circadian “inputs” – light/dark and feeding/fasting – are integral to circadian regulation, and sending unusual inputs, such as blue light exposure at night, late night eating or erratic meal patterns, alters the “outputs” (23). Remember the example of jetlag?

Jet lag happens when you suddenly alter the inputs; the change in light and food timing are disconnected from your current environment. Your body doesn’t know what to do for a few days while it adjusts.

Jetlag and shift-work are examples of extreme circadian desynchronization. We’ve known for some time that shift work is a primary risk factor for metabolic disease (10). However, only now is the evidence beginning to accumulate in relation to “social jetlag”, and the potential adverse effects of less extreme circadian perturbations on metabolic health.


“Social Jetlag” and Circadian Dysregulation

The average sleep duration of a working adult in the 1960s was over 8hrs: 44% of working adults now sleep an average of 6.5hrs per night, with 20-30% sleeping less than 6hrs (5). Most people then try to compensate on the weekends.

The difference between how long you would sleep on a weekend with no forced wake time, and your sleep duration during the week, is called social jetlag. The difference between circadian timing and social timing (i.e. extended wakefulness in the biological night aka partying/working late/watching infomercials at 2 am), dysregulates circadian rhythms.


The degree of artificial light exposure in the evening, both duration (>3hrs) and intensity (>500lux) has also been associated with metabolic dysfunction – increased BMI, obesity and abnormal lipid profiles (30,33).

So that’s the associative studies. The question is how could artificial light and extended evenings be having such disruptive effects on circadian function and metabolic health?

Influence of Light on Metabolism

A crucial aspect of circadian rhythmicity is that the waking/day phase is inherently tied to meal timing, nutrient intake and metabolic function. Extended illumination and artificial light disrupt these normally dichotomous alternate periods of sleep/activity, feed/fast, and energy metabolism/storage (26).

Several studies in mice exposed to light and access to food during their biological night have shown the mice consuming food under these light conditions gained significantly more fat than mice eating the same overall daily calories, but restricted eating to their waking/active phase (16-17, 2).

When our behavioral cycles are misaligned with circadian cycles, leptin levels decrease, dysregulating appetite and control of energy balance (35).

In humans, extended illumination provides greater opportunity to eat, and late night eating has been associated with increased BMI independent of sleep duration and timing (3).

Food utilization and energy expenditure are under circadian control, and late night eating alters the feed/fast period and causes the timing of systems, like the digestive system, to become offset (Oike, Oishi & Kobori, 2014).

Working with programs and coaches like those available through IIFYM can help to structure your meal planning and timing, avoiding these common diet pitfalls.


How Light May Affect Our Ability to Change Our Body Composition

Shortwave blue light emitted by electronic devices suppresses melatonin levels, and women with the lowest melatonin levels at baseline were more likely to develop T2DM when followed up 12-years later in the Nurse’s Health Study (27).

Again, the association is not causation, and there is a lack of controlled human intervention trials looking at the influence of light exposure. To date, there are only a few controlled trials in humans looking at the effects of light exposure. In one trial, 3 hrs of blue light exposure increased markers of insulin resistance, suggesting that chronic shortwave light exposure may impact metabolic function and glucose regulation (7).

In another, morning light treatment resulted in greater reductions in body fat percentage and decreased appetite in overweight women (9). It is possible that these effects may be through the physiological arousal promoting effects of light, and the ability of blue light exposure in the morning to decrease appetite through reducing hunger hormone signaling (15).

I think it is important to emphasize that energy balance is the fundamental determinant of success on a weight-loss program. A calorie deficit won’t be wiped out by watching TV, and the nutrition plans at IIFYM can help set you up with the right controlled calorie intake for your needs. Why the circadian aspect is important is because if your sleep and circadian patterns are erratic, you’re making hard work of your fat loss goals.

The timing of Food and Circadian Regulation of Hunger/Appetite

The circadian system is entrained to fixed feeding patterns. Ghrelin, which stimulates hunger from the gut, is entrained in response to meal timing and provides signaling between the peripheral circadian clocks (entrained by feeding) and the central clock (entrained by light) (42).

Food motivation and reward systems are synchronized to the central circadian clock, allowing an organism to anticipate food foraging and intake (44). When our behavioral cycles are misaligned with circadian cycles, leptin levels decrease, dysregulating appetite and control of energy balance (35).

Regular meal timing and frequency are thus essential to the integrity of the circadian system. In fact, irregular meal frequency decreases diet-induced thermogenesis compared with a regular meal frequency at the same level of calorie intake (13).

Don’t jump the gun here, this isn’t a return to “6 meals a day” fitness industry bioscience – what this is demonstrating is that erratic eating patterns are disruptive to circadian rhythmicity and metabolism (25).


How These Consumption Patterns Affect Our Circadian Rhythm

Hunger patterns also display circadian rhythmicity, with hunger signaling lowest in the morning and highest in the evening (36). In fact, human circadian rhythms appear to be hardwired to consume more food in the latter part of the day. Hunger peaks in the evening, correlating with peak ghrelin concentrations (Ibid.), while leptin concentrations are highest during the biological night and into the early half of the day, suppressing appetite (37).

Curtailing your appetite at night while dieting isn’t easy, but let one of our coaches create your Macro Blueprint so you can decrease the cravings.

In sleep curtailment to 5.5hrs, leptin decreases by 18% – comparable to the decrease when healthy subjects are underfed by 900 calories per day – and ghrelin increases by 28%, stimulating food seeking behaviors (8,29).

Disruption of leptin patterns has been shown to contribute to a 22% increase in calorie intake the following day (29). Light exposure may, however, attenuate the effects of disrupted circadian hunger/appetite regulation. In subjects restricted to 5hrs sleep, morning shortwave blue light exposure modulated the deleterious effects of sleep restriction on appetite and hunger; leptin increased while ghrelin decreased (15).

A Circadian Call to Reconsider Nutrient Timing?

We’ve seen earlier how late eating is associated with higher BMI, but early eating is as much an issue. Glucose tolerance is impaired when melatonin is elevated and advancing wake time by 2hrs results in reduced insulin sensitivity during very early morning wakefulness (11). The key point to note, if work/kids/life has you up at 5 am, is that while you may be awake, it remains your biological night – and melatonin will be elevated (Ibid.).

And the circadian rhythmicity of cortisol may explain why the Lucky Charms and Pop Tarts in your macros are probably best avoided at breakfast. Experimental evidence in humans shows that cortisol augments insulin responses to glucose, promoting rapid carbohydrate metabolism (41).

Low-carb dieting decreases sleep quality, while evening carb intake can increase brain concentrations of the amino acid tryptophan, the precursor to serotonin (1, 20).

A study comparing a higher-carb, low-fat breakfast [55% carbs] against a lower carb, higher fat [43% carbs] (which controlled for protein intake at 18%), found a quicker return of hunger and appetite in the higher carb breakfast; the lower carb/higher fat breakfast had a lower insulin response, and less appetite 4 hours after the breakfast (6).

While this may not reflect a circadian influence per se, it reflects an approach to nutrition which syncs with natural circadian hormonal oscillations. These hormonal oscillations may explain why one study found greater weight loss during an energy-restricted diet with a majority of carbohydrates eaten in the evening meal (38).

This eating pattern attenuated the drop in leptin that accompanies restrictive dieting but more importantly increased adiponectin during the day (Ibid.). Adiponectin displays circadian rhythmicity and acts as an insulin sensitizer (14). It is low in the obese and higher in lean/normal weight individuals.


Why Carbs May Increase Sleep Quality

Yet, the overweight group consuming the majority of their carbs in the evening had significant increases in their daytime adiponectin levels, which wasn’t observed in the control group (38). Up to 60% of glucose from a carb-rich meal is disposed of in skeletal muscle, and muscle glycogen synthesis peaks at the end of the active phase (44).

There is another argument for strategically portioning carbs to the evening meal: increased sleep quality. Low-carb dieting decreases sleep quality, while evening carb intake can increase brain concentrations of the amino acid tryptophan, the precursor to serotonin (1,20).

Carbs can attribute to a well-balanced diet, find out your ideal intake with your Macro Blueprint built by one of our skilled coaches.

What is emerging, however, from both the animal data and nascent human data is the benefits of time-restricted feeding (21,25,29). Restricting feeding time in mice to their biological day/active phase protects against metabolic dysregulation from an obesity-inducing high-fat diet (21).

In humans, regularizing erratic eating patterns and shortening the habitual feeding period from 14hrs to 10-11hrs decreased energy intake, body mass, and interestingly increased sleep quality (18).

Forget the hoopla about intermittent fasting, this isn’t about that. From a circadian chronobiology perspective, time-restricted feeding provides a clear feeding/fasting cycle that is consistent with the waking/sleeping and light/dark phases, syncing peripheral circadian metabolic functions with our central, light-driven circadian clock (45).

Practical Recommendations

So, what can you so you preserve circadian rhythmicity? Manage your light environment and time-restrict your feeding.

In the evening, you want to prevent melatonin from the suppressing effects of short-wave blue light emitted from electronic devices. Exposure to blue light suppresses melatonin onset by 90mins. You have a couple of options here:

a) Purchase blue-light blocking glasses, which prevent melatonin suppression (40).
b) Turn all blue-light emitting devices – TV, smartphones, laptop – off 90mins before bed.
c) Download the software f.lux for your laptop, which naturally dims the blue light from your screen in accordance with the sunset in your timezone.
d) Blackout your bedroom. Make it a tech-free environment.
e) A mix of the above.

You’ll recall that light in the morning is a good thing, and can even attenuate the effects of the inevitable nights where we don’t get enough sleep. During the daytime is when you want that shortwave, high-intensity light. The first option is to make sure you get 30mins outdoors in the morning or at some stage during the day.

However, not every climate is suited to this, so the fall back is to use blue light therapy. The best is probably the Philips ‘GoLite Blu’, which emits 4 different blue light intensities. This is equivalent to a clear summer’s daytime light level. Use it for anything between 15 mins up to 1 hour per day in the morning.


Concluding the Best Methods For Circadian Regulation

Along with avoiding blue light in the evening, morning light exposure will help reset your circadian rhythm, especially if you work in an office. A note on caffeine: caffeine can cause a phase-delay in your circadian rhythm. Work on only consuming it during the morning, limiting it in the evening.

The last element is to be consistent with your meal timing and time-restrict your feeding. This doesn’t have to be extreme fasting – even a set 10 or 11-hour window will suffice. The aim is to regularize your meal timing and align it with your biologically active/waking period.

If you have a history of erratic eating, it helps to have guidance. Our coaches at IIFYM can help you establish regular eating patterns to help you avoid circadian dysregulation.

For all the dieting products that IIFYM offers, view their programs so you can change your body composition for the better!

And the whole intermittent fasting thing? Do it if you want. Breakfast is neither essential or non-essential. As for macros? From a circadian perspective, there is support for emphasizing protein and fat with lower overall carbs in the early part of the day, and for consuming a large meal in the evening.

However, this is inconsistent and there is also support for front-loading calories in those who are overweight/obese. Again, in metabolically healthy individuals this may be irrelevant.

The take-home point here is that there is flexibility to set up a diet plan for success. At IIFYM, your specific variables will be used to provide you with proper caloric intake and structure your macros to suit your personal preferences. But never eat during your biological night, whether that involves very early mornings – or late evenings – there should be no such thing as IIFYM at 2 am.