Circadian rhythms

When You Fast, Exercise Impacts Sleep & Longevity, Per New Research

by Mike Mutzel



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The science linking disruption of circadian rhythms with increased risk of various diseases has been known for quite some time. (I wrote extensively about this in Belly Fat Effect back in 2013.) What is lesser known, thus far in 2021, is how the misalignment of circadian rhythms impacts the pathways regulating the aging process.


The Time When You Start and Stop Your Fast Influences Your Circadian Clock & Aging Process


I feel this is important to review because many people seem to be unaware of how meal timing impacts the peripheral circadian clocks. In brief, light and darkness powerfully influence central circadian clocks, but as is often the case with nature, there’s built in redundancy and peripheral clocks also influence circadian rhythms. Meal timing, it turns out, is a powerful influencer of these peripherals that help set (entrain) circadian time.


Many clients I’ve worked with are practicing some form of intermittent fasting (IF) but are often fasting all day and bookending their day with a bolus meal. Now, I’m openly biased and an admitted fan of time-restricted feeding (TRF), but I find that clients with these eating/fasting patterns often don’t experience the results they were hoping for.


The study that cemented the “eat early, sleep early” mantra in my mind was published in June 2019. Scientists found that early time-restricted feeding (a feeding window of 8:00 to 4:00 PM) more favorably influenced subjects’ metabolic- and longevity-associated pathways compared to subjects who had the same overall energy intake but who didn’t stop their fast until later in the day (June 2019). Various other time-restricted feeding studies of subjects eating late (after 4:00 PM) have shown similar effects, such as worsening of glucose levels and blood pressure.


In the fall of 2020, several studies conducted in women showed that late-night snacking negatively impacted body composition when compared to not snacking before bed in a calorie-matched study. There seems to be a connection between the timing of food intake: later eaters tend to have more body fat compared to earlier eaters.


What is lesser known is how circadian rhythms and meal timing impact the aging process.


Thankfully, a new review article summarized the available research linking circadian rhythms and age-associated “longevity” pathways of interest, namely the sirtuins, insulin/IGF-1 and mTOR. Let’s review the practical takeaways from this article with a focus on how fasting and caloric restriction ought to be viewed with respect to the circadian clock system.


Nutrient-Sensing Pathways Are Linked with Aging and All Are Linked to Circadian Rhythms


Since levels of glucose, fatty acids and even cholesterol are influenced by the circadian clock system, it should come as no surprise that associated nutrient sensing (AKA longevity pathways) are also under some circadian influence.


It turns out that key circadian clock genes rhythmically activate the rate-limiting enzyme in the longevity molecule NAD+. In layman terms, this may be in part why we feel tired when we disturb our sleep/wake cycles and how prolonged sleep-wake cycle disruption may accelerate aging and worsen metabolic diseases linked with insufficient cellular NAD+.


Even more, the NAD+ dependent longevity enzyme SIRT1 directly binds to a core circadian clock maintenance protein PER2 (period circadian regulator 2). PER2 contributes to the robustness of circadian rhythms, the lack of which is linked with aging. The nutrient sensor AMPK impacts a circadian rhythm protein that is similar to PER2 called CRY1.


Since exercise, fasting and calorie restriction enhance AMPK and sirtuin activity, it’s thought that these modalities can similarly help to support the health and maintenance of circadian rhythms.


Signaling of insulin/IGF-1 and mTOR have long been known to impact aging and longevity. Research in animals suggests that IGF-1 may be one of the key signals that help synchronize feeding time with circadian rhythms. Likewise, mTOR levels rhythmically rise and fall throughout the day implying that they too are linked to circadian rhythms, but the research has been relatively unexplored up to now.


What Is the Ideal Fasting Window for Weight Loss and Longevity?


The ideal feeding/fasting window is the one that you can consistently adhere to. Just because there’s science to suggest that eating earlier in the day can favorably influence circadian rhythms and longevity pathways, listen to your body. If your schedule doesn’t allow eating early, you don’t feel your best if you eat early, or it simply doesn’t work for you, perhaps a feeding window of 12-6 (-ish) is better for you.


Practically speaking, starting your fast at 4:00 PM is impractical—especially if it’s the summer or you have a social life or young children at home.


However, routinely bookending your day with the majority of your daily energy requirements may negatively impact your sleep, digestion and slow weight loss progress.


Perhaps a happy medium is making your mid-day meal the most energetically dense, giving your body more time to digest and assimilate the nutrients, and making dinner a lighter fare.


Everyone’s body is unique, but evidence supporting the mantra that athletes have lived by for years, eat early and sleep early, is gathering more scientific evidence. My general advice is to finish eating no later than three hours prior to your regular bedtime 80 percent of the time.


Socializing with friends and family have additional health benefits, so if you’re eating slightly later than normal a few days a week, it’s probably a wash from a long-term heath perspective.



1.Acosta-Rodríguez, V. A., Rijo-Ferreira, F., Green, C. B. & Takahashi, J. S. Importance of circadian timing for aging and longevity. Nat Commun 12, 2862 (2021).





  1. Hi Mike,
    I am a shift worker. I work one week early shift Mon -Thursday
    6am – 4.30pm then a week of night shift Mon – Thursday 4.30 pm – 3 am.
    My feeding window is 12pm – 8pm every day regardless of shifts.
    Is there a better time frame to fast to suit my shifts?
    Thank you in advance,


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