On Time-Restricted Feeding

I’ve received a number of emails recently from people who, due in large part of podcasts and tweets regarding studies done on animals, are gravely concerned that their eating patterns are messing with their circadian rhythm, including when they drink their coffee. The idea that coffee during the fasting window is actually signaling the circadian pacemaker was causing undue stress. Let’s explore.


  • Briefly, the circadian rhythm refers to the ~24 hour cycle of physiological processes in humans (and animals, plants, and fungi).
  • There are patterns of brain wave activity, hormone production, cell regeneration and other biological activities linked to this daily cycle.
  • Most of this is endogenously generated.
  • Light, food, alcohol, and activity all exhibit circadian entrainment, though light is by far the most powerful and meaningful.

That last point leads into the discussion above. Much of the emerging literature has been performed in animal models, where the idea that the first sip of non-water or the first sip of coffee could entrain the clock or shift it to a new rhythm. This also contributes to certain researchers on twitter using the hashtag #afternoondiabetes, implying that since a time-restricted feeding schedule shifts all of our calories (and carbohydrates) to the afternoon, and that we’re most insulin-sensitive in the morning (or rather muscles are most insulin sensitive in the morning and fat is most insulin sensitive in the afternoon), that we’ll see a change in fasting blood sugar and adiposity long-term.

Circadian Entrainment in the Blind

The most powerful influencer of circadian entrainment is light. Those who are totally blind lack a functioning retinohypothalamic tract (RHT), which sets the circadian pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus. As noted above food, drink, and activity seem to help shift this in animal models, but a recent study in Sleep Medicine demonstrated that a targeted caffeine dose at the start of a circadian phase was unable to reset the clock of totally blind men who have non-24 hour sleep-wake disorder. Further, and what is important to me given the question I received, is that they were habitual coffee drinkers to begin with. That is, their feeding schedule, activity patterns, and coffee consumption was not shifting their circadian phases. In totally blind individuals, only melatonin seems to reliably do this. Bueno. So don’t worry that your morning coffee is somehow screwing with your circadian rhythms. Light matters much more in this regard, so get some morning sun!

Afternoon Diabetes

Speaking of coffee, it’s amazing stuff. It’s basically anti-mortality up to insane doses. However, acute studies (for example 1, 2) demonstrate that coffee and caffeine are pro-diabetic. In spite that, high coffee intake is associated with diabetes risk reduction in a dose-response manner. Huh?

It reminds me of someone looking at the tide coming in, doing some calculations and running to tell the mayor that the city will be under water in 2 weeks time. Single-dose experiments, of which many of these evening feeding studies are, are exactly the same. Or they poorly define their terms and the net result is no change in body weight.

However, controlling for feeding window and calories, as was done in a recent study, tells a different tale. Individuals maintained their weight on a 16/8 fasting/feeding schedule, with lots of carbs. Not only did they significantly improve in all of the measures that you’d like to see (increased lean mass, decreased fat mass) but they also, and this is important, saw a significant decrease in blood glucose and insulin levels with a significant increase in HOMA-IR. Considering over half of their calories were from carbohydrates, you should be questioning the notion of #afternoondiabetes from nutrient timing in isolation.

Finally, we have natural experiments for this: Argentina and Spain. The former nearly lives as long as we do (in spite being more poor and smoking more, both indicators of all-cause mortality) in spite eating dinner insanely late AND skipping breakfast. The latter lives longer than we do in spite similar lifestyle issues. Again, feeding in animal models can set circadian rhythms, but it doesn’t seem to do it well in humans.


  1. If coffee isn’t setting the circadian clock in blind folks, it isn’t in you, who I assume is reading this an has a functioning RHT.
  2. Shifting calories to the afternoon chronically results in blood sugar improvements when calories are controlled.
  3. Natural experiments have demonstrated that late night eating is really poor at making entire cultures dead sooner. Let’s start a new hashtag: #circadianparadox

So the next time someone shoots an animal model arrow at you to scare you into not drinking coffee before sunrise or fasting until past noon, repeat after Black Panther:


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