Meal Timing and Circadian Entrainment

Our master body clock is entrained by morning sunlight. The first light to hit our eyes in the morning should come from the sun. This will help regulate our circadian rhythms correctly.

The master switch to our master body clock is light from the sun in the mornings whereas darkness later in the day is what allows secretion of sleep hormone (melatonin), hence why we should be blocking artificial blue light in the evenings. Light is what governs the central body clocks correct regulation but our bodies have peripheral clocks as well. Peripheral clocks are found in almost every cell in every mammal on earth and influence all aspects of behaviour and physiology. It has been shown that the master clock is the key driver of the peripheral clocks. This would be in so much that when light is passed through the retina the timing information would be sent from the master clock to the peripheral clocks and everything would be nice and in sync. 



However, it appears that there are other factors at play when it comes to entrainment of peripheral circadian clocks. A 2010 a study by Hirao et al. showed that food can entrain peripheral clocks, in this specific experiment the peripheral live clock. The study, albeit in mice, showed that the liver peripheral clock was entrained to the exact time of a feeding, should that feeding be once per day. The study also went on to feed the mice two meals per day, one large meal and one small meal. The findings were that the peripheral clocks were entrained better after the larger of the meals.

For optimal circadian management we could beg the question of meal timing in relation to synchronization of the master clock to the peripheral clocks. We could suggest that as the master clock is entrained by sunlight first thing in the morning, we should be eating our largest meal at breakfast followed by smaller and smaller meals as the day draws on. (Probably not this meal though....)



There was more compelling data from the Hirao study that showed that the intensity of entrainment was greater after a large meal was administered following a prolonged period of fasting. Disregarding what we know about the master clock and how sunlight entrains it, some people may suggest that you should just eat a large meal after an extended fasting period for optimal peripheral clock entrainment. However, we know that the master clock passes timing information to the peripheral clocks through light hitting the retina, so this just would not be optimal for circadian alignment.

Therefore in order to synchronize our entire circadian system we need to be eating our largest meal at first light following the longest period of fasting in a 24 hour period. Maybe we should stop eating when it’s dark and fast through to first light when we would then take our largest meal of the day?

Another thought would be that if smaller meals entrain the peripheral clocks, to some extent, should we be eating just one meal per day and that meal be breakfast?

It would also be interesting to travel back to Paleolithic times and see how a hunter gatherers behaved in relation to their food timing.  Maybe they wouldn’t have cared given the hostile feast and famine environment they lived in. Maybe eating in the evenings was more advantageous due to the increased likelihood of adiposity expansion, which would increase survival chances in an era thwart with extended periods of famine. Given the inconsistencies of food availability for hunter gatherers maybe light had the greater influence on the entrainment of peripheral clocks, given some days food could have been non-existent or scarce.

However, we live in a modern era where food, for the most of us, is abundant and given meal timing has an effect on clock synchronisation maybe we should be thinking more about meal timings in relation to light/dark cycles. Also bear in mind “too much” body fat in our era is not beneficial for survival.



In summary, stop eating as the sun goes down and eat your largest meal first thing in the morning for optimal body clock entrainment and health

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