How meal timing impacts your daily cortisol rhythm

Meal timing is a useful tool for optimizing your lifestyle and improving your overall health. Meal quantity and quality are bigger levers to pull, but meal timing is a third dietary lever at your disposal.

Now, this doesn’t mean that everyone needs to micromanage their diet to this degree. In fact, it may cause some people more anxiety or push them toward disordered eating. But if approached properly and with nuance, it’s an excellent tool.

Many people pay no attention to meal timing whatsoever. If you feel great doing what you’re doing and are otherwise healthy, it’s probably not worth the hassle. But if you feel cruddy or have metabolic issues to deal with, addressing all of these 3 dietary levers can cause huge changes.

Many people have jumped into intermittent fasting or time-restricted eating(TRE) as a means of improving their health. Again, the same rules apply to these tools: they’re great for some, not so great for others.

If you’ve seen excellent gains with TRE, addressing your meal timing by shifting your feeding window can really improve other things. Moving around your eating window and finding what’s right for you can improve:

  • Alertness in the morning
  • Daytime energy levels
  • Cognitive and physical performance
  • Your ability to fall and stay asleep

In today’s blog we’ll discuss how meal timing changes your physiology and what the limited data in humans shows.

Circadian rhythms drive behavioral rhythms drive biological rhythms

You’ve likely heard of the term circadian rhythms, particularly if you’ve been around the blog for a while. Bascially, circadian rhythms are variations in biological processes that follow an approximate 24 hour period. Sleep is the classic example of a process controlled by circadian rhythms.

Within each one of our cells is an internal timekeeper that helps the cell keep time with themselves and other cells. Each of us has our own endogenous rhythm that is unique to us that is normally close to, but not exactly 24 hours.

Like a watch that keeps bad time, this can actually cause us to become fragmented from the day/night cycle. Imagine a clock that loses or gains 15 minutes per day. In 48 days, 4pm to you would actually be 4am, or vice versa.

Fortunately, there’s a resetting mechanism within these clocks that helps set our clocks to the day/night cycle every day. We have a master clock in our brain that helps keep us in time with the day/night cycle via light exposure and interacting with clocks in all organs and tissues throughout the body. Additionally, body temperature and feeding rhythms add another layer of time-setting cues that keep our clocks on time.

Our circadian rhythms help us keep time with our environment and organize our physiology to promote biological success. Circadian rhythms drive behavioral rhythms and the outcomes of those behaviors create more robust biological rhythms.

As a result, this creates anticipatory rhythms that breed better outcomes. For example, when our meal times are consistent over time, we get anticipatory increases in the hormone ghrelin, which motivates us to seek out foods at those times. It also alters GI motility and gastric acid secretion to promote better digestion.

Additionally, we get anticipatory bumps in insulin and cortisol, which help coordinate things like blood glucose and metabolism. But to get these anticipatory responses, meal times have to be consistent on the daily.

Meal timing and the cortisol rhythm

As far as hormones go, cortisol is a very important part of our physiology. Under circadian control, it plays a role in making us more alert when we need to be, and mobilizes energy to power the fight or flight response.

Generally speaking, the circadian rhythm of cortisol follows a consistent rhythm whereby cortisol is high in the morning and low at night. This makes us feel more alert when we wake up and helps us wind down and fall asleep at night.

But it’s important to point out that our behaviors exert strong control over our cortisol rhythm. If cortisol simply followed a circadian rhythm where it was high in the morning and low at night, we’d be highly susceptible predation or attack at night when it’s low.

Fortunately, cortisol also responds to our environment directly. That way, if chased by a hyena at night, we’d at least stand a fighting chance; we wouldn’t just sit there and take it.

This is why stress at night negatively impacts our sleep. If you ruminate over psychologically stressful things at bedtime, cortisol increases alertness, and alertness prevents sound sleep.

As mentioned above, our mealtimes also play a role in our cortisol levels. When meal times are consistent, we get an anticipatory cortisol bump before these regular mealtimes. Taking meal times and the daily rhythm in to account, the daily cortisol profile will tend to look like this:

With this in mind, let’s take a look at a new review on meal timing in time-restricted eating.

Meal timing in TRE: Skipping breakfast or skipping dinner

A recent review on TRE looked at how different meal schedules impact cortisol levels in the few human studies that covered the topic. Though there is a limited amount of data, it gives us some interesting clues.

Overall, the data indicates that skipping breakfast actually lowered cortisol levels in the morning and increased cortisol during midday and beyond. This is somewhat counterintuitive because one would think that fasting longer would increase cortisol to mobilize energy stores.

Furthermore, the converse also appeared to be true: skipping dinner lowered cortisol levels at night. Overall, breakfast skippers had a more flattened cortisol rhythm while dinner skippers saw a more robust, natural rhythm with high cortisol in the morning and lower cortisol at night.

Note for the ladies: A study in women found that skipping breakfast increased cortisol later in the day, increased blood pressure, and dampened the cortisol rhythm. This is one of the rare instances where we have decent data specifically in women. Check out the changes in cortisol rhythm between breakfast skippers and controls:

In this paper, simply shifting their mealtimes around caused the circadian cortisol rhythm of these women to flatten. A flatter curve may decrease energy levels in the morning, and make falling asleep more difficult at night.

It’s important to point out that everyone is different, and there’s such a small amount of data available. But based on the information we have, there are trends that point us in a specific direction:

  • If you want to be more alert in the morning, observe an earlier feeding window
  • To be more alert in the afternoon/early evening, observe a later feeding window
  • If you have trouble falling asleep, shift your last meal earlier
  • If you want to be more alert in the morning, eat your first meal earlier

As always, the best approach is to try these changes and see how they impact your sleep and energy levels throughout the day. Not everyone sees the same results from a given intervention, but trends in the data give us good starting points.


Meal quantity and quality are often factors people approach to improve their health. While both are clearly important, meal timing is another factor that can help streamline your daily routine.

Time restricted eating is a great way to get the ball rolling on meal timing. However, if simply sticking to a given eating window doesn’t give you the benefits you’re looking for, you can also shift your eating window around.

When I initially started time-restricted eating back in 2014, I did the standard 12pm-8pm eating window. This is a great way to start as it’s pretty easy to skip breakfast. But over time, I shifted to a 7am to 7pm window, counting my 7am coffee and eating my first meal at 10am.

This worked better for me, and I did see less restlessness before falling asleep. Truth be told, I’ve never had an issue with having energy in the morning, so not much changed on that front. In most studies, it also seems to lead to better insulin sensitivity and glycemic control, as insulin sensitivity is highest in the morning. I think this bears itself in my wonderful HbA1c of 4.9%, despite a high carb diet.

But what works for me may not be what’s best for you. It’s important to understand that even though studies point in a certain direction, you may not be at the top of the bell curve.

That’s why it’s important to tinker with your meal timing and see how your body responds. Ultimately, if you’re looking to gain the most benefit from circadian rhythms, keeping meal times consistent is more important than the specific times you’re eating.

As long as those meal times yield good sleep, high energy levels, and a healthy weight.

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