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One observation from my Garmin watch is that my stress score goes up after meals. No matter what I eat, how healthy the food I’m eating, my stress levels go up after the meal and stay elevated for at least a couple hours or longer, depending of course on activity and whatever else I’m doing. This is even true if I eat before bed: My stress levels can remain high for up to 2 hours after I drift off to sleep, following a relatively late meal.
My body only shows as resting (meaning a low stress score) in the morning if I have yet to eat breakfast. Despite any hunger pangs, it’s less stressful for me (according to heart rate variability) to be hungry than it is for me to digest a meal after eating. I find I record more restful periods when I intermittent-fast, aka skip breakfast and eat my first meal in the afternoon. Even with the added stimulus of coffee, my stress levels remain in a low resting state.
Garmin’s stress score is a function of heart rate variability, which can indicate activation or rest of your body’s sympathetic nervous system, which activates the body for activity. When the sympathetic nervous system is regularly activated, that indicates your body is under stress. A heart rate that does not vary much is indicative of the sympathetic nervous system being activated.
What does this have to do with eating? The sympathetic nervous system is a component of the autonomic nervous system, which passively operates our organs and hormonal glands. When you eat food, the autonomic/sympathetic nervous system begins diverting blood from other organs to the stomach and other relevant digestive organs to digest your ingested food. This activation of your sympathetic nervous system will continue until your food has been suitably digested and absorbed.
Even if you are laying down and doing absolutely nothing, your sympathetic nervous system during digestion is at work and therefore your heart rate variability at rest is likely small enough to indicate a level of stress to your Garmin. That doesn’t seem fair, but welcome to human biology.
If you live a relatively low-stress existence, eat only 2-3 meals a day, and you’re in good health, this is likely not a big deal. Your heart rate will eventually return to normal variability in a couple hours, and your resting time will read to your tracker as being at rest.
Of course, the vast majority of humanity doesn’t fall into the very thin demographic I just outlined. Most of us deal with some substantial degree of regular stress. Many of us have different meal habits, and many snack or eat enough meals a day that their bodies are digesting food not just throughout the entire day but even after going to sleep. And, of course, most people are not in optimal health.
This never minds people who endurance train, and are already subjecting their bodies to substantial stress through their training. The irony is that, depending on their eating habits, their fueling after workouts may in fact be contributing to their overall (already high) stress levels.
Science incidentally hasn’t laid a hand on this subject in over a decade, so we don’t have a ton of data on why this needs to be a stress reaction let alone if we can change the body’s sympathetic nervous reaction to eating food. So we have to accept that this is reality and work within that.
This incidentally is an underlying reason why intermittent fasting and the old “eat dinner like a pauper” rule* works so well. Fasting by skipping breakfast leads to generally lower stress levels, which improves overall hormonal function. Eating light limits the stress affect on your sleep time, which can improve the quality of that sleep.
Of course, this should not be taken as license to starve yourself and not eat at all. At some points during the day you do need to eventually take in quality nutrition and “take the sack” (so to speak) on the resulting sympathetic stress, because your body needs that nutrition.
This merely points out how the timing of that nutrition can affect your overall sympathetic stress, which in turn can affect your overall health.
Though this was never an intent of the rule, this is one benefit to making sure to eat quality protein/carbs as soon after a hard workout as possible, e.g. the 30 minute and 2 hour windows. Your body undergoes a similar sympathetic stress response after a workout, though the stress ripple effect can last longer than your meals (often, for example, a long run leaves you in a high stress state for the entire rest of the day, even if you spend all day laying down).
Eating as soon as possible and triggering that sympathetic nervous reaction can effectively piggy-back off the other sympathetic nervous reaction recovering from the workout itself. Eating much later could effectively re-start the sympathetic stress reaction, whereas eating right after one has began saves you the trouble of an extra stress reaction, or an extended period of elevated sympathetic stress. You can get back to a normal resting state more quickly, and spend more time in that low-stress rested state than if you had eaten later and had two separate stress-creating episodes for your sympathetic nervous system.
This lends credence to the following ideas:
- Unless you work out in the morning, or you have health-related reasons not to do so, it’s probably best to intermittent fast by skipping breakfast, nothing but coffee and water.
- Probably only eat breakfast if doing a morning workout, and probably following that workout.
- It’s important to consume nutrition within 30 minutes of finishing tougher workouts, and to eat a meal within two hours of finishing those workouts.
- Regardless of the size of dinner, you want to buffer a couple of hours between the end of dinner and bedtime, to allow digestion and its stress reaction to finish as early in the sleep cycle as possible.
- Avoid snacking, as it restarts the sympathetic nervous stress reaction. Eat full meals and only full meals, 2-4 times a day.
Back in 2011, famous 
Well, eventually he did write that book. 
Matt Fitzgerald’s book 
