How does eating before bed hurt our metabolism and increase fat gain?
TL;DR: Our bodies are most insulin-sensitive at night so tissues will store any food we eat very quickly to make sure our BGLs don’t stay elevated. Digestion and sleep also require different core body temperatures, so eating less than 2 hrs before we try to sleep disturbs our sleep, increasing cortisol levels the next day, and setting us up for metabolic imbalance. (about 7 min read)
Metabolic tips for adjusting our meals to our natural sleep-wake cycles:
Finish eating 2-3 hrs before bed to give your core body temperature time to go down after digestion.
Choose meals high in protein + fiber and low in simple carbs since you won’t be exercising while you’re asleep.
Stay hydrated after dinner to reduce the chance of snack cravings before bed.
Sleep is an important part of maintaining metabolic balance. There are various theories on why we sleep to maintain good health, but there is no question that we have to do it and that not sleeping impairs our body’s ability to function properly. One of the important things that happens during sleep is that your body gets time to “clean up” from the work you did the previous day. This includes “resetting” the chemical signals (like insulin, cortisol, and other hormones & signaling molecules) that your body used to properly balance your metabolism yesterday. Today is a new day and your body should start with a blank slate, ready for new signals.
Big meals will increase your core body temperature. There are two important metabolic differences between when we are awake and when we are asleep. When we’re awake, we’re moving around and digesting food. More blood needs to be at our intestines to help us absorb nutrients from the food we ate and to move things along from our stomachs to our guts. Blood is also flowing faster (heart rate is higher) to move the digested food from our guts to our liver where it gets processed and sorted. Our liver then decides what nutrients need to circulate through the bloodstream so that our other organs can function properly and that our muscles have the energy they need to keep us moving.
When we sleep, our core body temperature drops and our blood flows slower (heart rate is lower). One way that our core body temperature drops is by moving more of our blood towards the surface of our skin so that heat can dissipate off of us (if you’ve ever slept in the same bed with another person and they feel WARM, not cold, this is why - the part you can touch feels warm because their skin is letting heat off into the environment. But their heart is cold. I mean, their core is cold.)
Since our blood can’t be two places at once, it is hard for our bodies to digest food AND cool our cores at the same time. This is why eating a big meal before bed can lead to stomach aches and disrupts sleep. Disrupted sleep also leads to increased cortisol the next day (more below).
Your cells are more likely to store what you eat right before bed as fat. Your body is not moving a whole lot while you’re unconscious and paralyzed. Since this happens to us every single night, part of your normal daily cycle (circadian rhythm) is designed to make sure that your blood glucose levels (BGLs) don’t stay too high for too long before bed. Your BGLs will go up after eating; how high they go up depends on how many simple carbs you ate. The two ways your BGLs come back down are (1) your muscles take glucose out of the bloodstream when you move around and (2) insulin tells your other cells (especially fat and liver) to take glucose out of the bloodstream.
If you aren’t moving your muscles, then the job of bringing your BGLs down after a meal is all done by insulin. Our bodies are the most responsive to insulin (“insulin sensitive) at night, so it takes less insulin to get your cells to take glucose out of the bloodstream. Remember that insulin is a growth factor. So whatever glucose is taken out of your blood due to insulin signaling is going to get stored as new cells (healing), glycogen (animal starch), or fat. If you’ve already replaced the cells you need and your glycogen stores are full, you will store the extra glucose as fat.
Poor sleep will increase cortisol levels the next day, creating the “cortisol vs insulin” match-up, and making it harder to control BGLs without exercise. In order for us to go to sleep at night, cortisol levels need to drop throughout the day and be at their lowest before we go to bed. However, if we didn’t get enough sleep the night before, and our bodies have been releasing extra cortisol during the day to help us stay awake, then our cortisol levels won’t fall as low as they should by the time we try to go to sleep the next night. Not a great cycle to get into.
In addition to promoting “wakefulness”, cortisol interferes with insulin signaling, which means your pancreas needs to release more insulin to get your BGLs down to normal levels if you aren’t moving around. Too much insulin = too much glucose being pulled out (aka hypoglycemia), which leads to storing more fat.
So how do we avoid having too much cortisol or glucose in our blood at night? Walking around, or some other light exercise, will help bring BGLs down with less insulin (though this is more effective earlier in the day since our bodies are very insulin-sensitive and ready to STORE GLUCOSE at night). Meals that are lower in simple carbs will also provide less glucose that needs to be stored in the first place. Staying hydrated to reduce glucose and cortisol concentrations also helps our bodies prepare for sleep, so we can start the next day with a rebalanced metabolism.
Metabolic tips for adjusting our meals to our natural sleep-wake cycles:
Finish eating 2-3 hrs before bed to give your core body temperature time to go down after digestion.
Choose meals high in protein + fiber and low in simple carbs since you won’t be exercising while you’re asleep.
Stay hydrated after dinner to reduce the chance of snack cravings before bed.
