Eating at Night: Does it Make You Gain Fat?
“Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house.”
Key Takeaways
Eating at night will not cause weight gain if, throughout the course of the day, you consume less calories than you burn.
Eating at night may decrease your sleep quality and negatively affect your body composition by:
Increasing hunger and decreasing satiety
Increasing stress levels
Reducing insulin sensitivity
Making you lose more muscle and less fat
The later it gets in the day, the more that “decision fatigue” will make it difficult to select foods that align with your goals.
Full Story
Until recently, you could find me saving 800-1,000 of my calories for a gigantic pre-bed meal of a protein oats, peanut butter, and a half pound of vegetables. As soon as I’d finish the last bite, I would brush my teeth, write in my journal, and head off to sleep with a belly full of protein, fat, and carbs. It just felt right - I enjoyed the relaxing reward of saving my largest meal of the day for after my work was finished.
I had been immersed in studying for nutrition for a few years and so, naturally, I considered myself an expert. It had been ingrained in my mind from reputable sources that energy balance, the difference between energy consumed and energy burned, was all that mattered in the game of fat loss and muscle gain.
Energy Balance = Calories in – Calories out
Over time, the “Energy Balance” portion must be positive to gain weight, or negative to lose weight. The “calories in” component are the foods and drinks you consume, such as carbs, fats, proteins, and alcohol. The “calories out” is the number of calories you burn, through exercise, non-exercise activity thermogenesis (NEAT), resting metabolic rate, and thermic effect of food.
Calories Out Component
Exercise- the calories burned by working out
NEAT- the calories burned by performing activity that is not considered “exercise”, such as walking, doing chores, etc.
Resting Metabolic Rate- the calories burned by simply existing, such as sitting in a chair, watching The Office, and sleeping.
Thermic Effect of Food- the calories burned to digest food. After ingesting protein, the body needs between 15-30% of those calories to digest, absorb, and transport the nutrients from the food. The same applies to carbs and fats, except the calorie burn is only 5-10% and 0-3% of the intake, respectively (1).
A calories is a calorie, whether those calories are a morning grande frappuccino or my oatmeal nightcap. It is just that simple, I had thought; until I realized that it WASN’T.
While the equation is certainly true from a bioenergetics standpoint, it doesn’t get us very far. Answering the question “How do I lose weight?” with “calories in – calories out” is analogous to answering the question “How can I make my car go faster?” with “force = mass x acceleration.”
They’re both great laws, sure, but they don’t include any application – how to turbo charge your motor or how to actually lose fat. By considering the biopsychosocial factors, i.e. your genetics, job, home environment, income, etc., this “simple” equation becomes a lot more useful, albeit a bit more complicated:
Energy Balance = [Calories in*] – [Calories out*]
*Calories In = (What food can I afford? + Should I buy organic? + Do I have the time to meal prep? + How can I make healthy food taste good? + Why does junk food taste so good? + When do I have time to eat at work? + Why do I eat when I’m stressed? + Why do I eat when I’m tired? + Should I drink with my friends? + How many calories are in this beer? + Should I try intermittent fasting? + Should I fast in the morning? + Should I fast at night? + Should I take supplements? +Am I hungry or just bored?)
*Calories Out = (Did I burn enough calories today? + Should I work out every day? + Am I working out too much? + What is the best workout to do? + Should I do cardio every day? + Should I not do any cardio? + Should I do fasted cardio? + How do I find the time to work out? + Why is my metabolism so slow? + My friend can eat whatever they want and not get fat? + Am I too stressed? + Why do I eat more when I don’t sleep enough? + Do I sit too much? + Should I walk more?)
A Physics Nightmare
You see, not unlike a complicated physics problem, one must first solve for the “calories in” and “calories out” components of the equation just as one needs to first solve for mass and acceleration in order to figure out force. The variables in play in this equation are extensive and unique to every person. This was the primary driver behind my argument that eating at night may make you gain fat.
Now you know the simplified equation, and so you understand that the calories you eat at night cannot possibly make you gain fat if you’re in a caloric deficit. There are countless studies and anecdotes that support this notion (2), including people that have successfully dieted entirely on McDonald’s and Twinkies (3, 4).
A calorie is a unit of energy, and an energy surplus results in the excess energy stored in the body as fat, muscle, or liver glycogen while an energy deficit results in a decreased amount of fat or lean body mass. For instance, if your total “calories out” part of the equation is 2500, and “calories in” is 2000 from a 100 calorie protein shake in the morning, 500 calories of beer, and 1400 calorie Big Mac and chocolate shake at 11pm, you consumed less calories than you burned and you would lose weight over time.
Mark Haub of Kansas State University ate 1 Twinkie every three hours and dropped 27lbs in two months (5),
What Does This Have To Do With Eating At Night?
What if that late-night Ronald McDonald calorie bomb enacts an insidious series of events that alters the outcomes of the complex daily decision components of our simple equation - thereby causing weight gain not directly from that day’s events, but from its compounding impact on your food and physical activity decisions in the coming days, weeks, and years? If my theory is correct, consuming a large meal before bed may decrease your sleep quality and adversely affect your body composition by:
Increasing hunger and decreasing satiety
Increasing stress levels
Reducing your insulin sensitivity
Making you lose more muscle and less fat
Additionally, eating late at night makes one more susceptible to “decision fatigue”, a phenomenon that decreases the quality of your decisions with each one that is made throughout the day.
At this point, I know you’re either thinking “Sounds reasonable, but prove it!” or “Boring! Gotta get back to scrolling Instagram.” If you fall in the former camp, allow me to explain.
1. Increasing Hunger and Decreasing Satiety
“Eating can make you hungrier” is an oxymoron - it can’t possibly be true, right? Actually, if eating a large meal at night may leads to poor-quality sleep, it will indeed make you feel significantly hungrier, and make it tougher for you to feel full, the following day. A poor night’s sleep wreaks havoc on our hormones, powerful signals that affect our day to day mood, perception, and actions, tugging on our strings as if we were puppets controlled entirely by biology and physics (which we are) (6, 7).
You see, poor quality sleep raises levels of Ghrelin, the “hunger hormone”, and decreases level of Leptin, the “satiety hormone” (8, 9). You may take a hard pass on the second donut after a night of 8 hours of sleep, whereas you may scarf down three without even realizing it after an all-nighter. Additionally, lack of sleep raises the level of endocannabinoid, the lipid responsible for inducing the “munchies” after people smoke or ingest marijuana. Not only does endocannabinoid make you hungrier, it makes you crave ultra-processed calorically dense foods, like cookies, pizza, and those donuts (10). I
n fact, this study found that sleep deprivation may cause an increased consumption of upwards of 300 calories per day! (11). Without a doubt, an extra 300 calories per day over time has the potential to culminate in a ballooned waistline and increased rates of metabolic disease.
2. Increasing Stress Levels
Disrupted sleep caused by late night eating will increase circulating levels of Cortisol, the “stress hormone” (12). Cortisol is a hormone produced by the adrenal glands that plays many important roles in the body, such as regulating blood sugar levels, reducing inflammation, and controlling blood pressure (13, 14). In the body composition world, on the other hand, cortisol is regarded as an evil chemical that causes excess fat gain, especially in the midsection.
This study that assessed levels of cortisol and patterns of fat distribution in women found an association between Cortisol levels and levels of abdominal fat storage (15). In other words, more stress = more belly fat. Fat distribution in people with Cushing’s Disease further supports this relationship. Cushing’s Disease results in constantly elevated levels of cortisol, and people with this condition generally present with excess abdominal fat.
Unsurprisingly, there is a relationship between stress levels and obesity that manifests as a vicious cycle (16). Think about it – if you’re stressed:
You stuff your face with Doritos.
You stuff your face with Doritos, you stress out about eating so much and your sleep quality declines even more.
You wake up in a cloud of Dorito dust, fingers and lips stained, and the poor sleep and shame makes you even more stressed.
You furiously drive to the store and pick up another bag of Doritos, this time cool ranch, and the cycle repeats itself.
Need I go on? To set yourself up for success for the following day, close the kitchen a few hours before bedtime to keep sleep quality high and stress levels low.
3. Reducing Insulin Sensitivity
Insulin sensitivity is the ability of cells to utilize glucose. High insulin sensitivity is good, as the cells are able to more efficiently use blood glucose and lower blood sugar levels more quickly. Low insulin sensitivity, or insulin resistance, is bad, as it results in excess inflammation, higher risk for Heart Disease and Dementia, and could lead directly to the development of metabolic disease and Type 2 diabetes.
Reasons that one may have decreased insulin sensitivity include excess body fat, physical inactivity, age, smoking, and, you guessed it, poor sleep (17). In this study that compared 8.5 hours to 4 hours of sleep, reduced insulin sensitivity is observed after just one night of sleep deprivation (18).
And for people that believe they can get by on sleeping 5 hours a night, insulin sensitivity is even reduced after one week of “mild” sleep deprivation of 5 hours of a sleep/night (19). This small study found that poor sleep significantly decreased levels of an enzyme that plays a pivotal role in a blood sugar regulation and resulted in reduced insulin sensitivity in healthy subjects by 30%. This means that after a night of severe sleep deprivation or after a week of moderate sleep deprivation, our body’s ability to regulate blood sugar is reduced to a level that is typically observed in diabetics (20).
4. Making You Lose More Muscle and Less Fat
Did you know that it’s possible to lose weight and end up with a higher body fat percentage? Well, it is! If you lose more muscle than fat over a period of time, the proportion of fat on your body will be higher than before you began dieting. Nobody wants that. One tried and true way to optimize body composition while losing weight is to get quality sleep!
Losing fat while retaining muscle is a tall order to ask of physiology. Muscle is a metabolically expensive tissue, and if your body isn’t constantly receiving the signals to hold onto it - resistance training, and the building blocks to build muscle, protein - it will preferentially keep the fat. Poor sleep, i.e. the one you have after smashing a box of Thin Mints before bed, has been shown to result in significantly more muscle loss and less fat loss (21).
In fact, this study that compared two groups of dieters that either slept 5.5 hours or 8.5 hours, those that slept 5.5 hours showed a decreased proportion of fat mass lost by 55% and increased proportion of muscle lost by 60% in only two weeks! (22). Over the course of an extended caloric restriction period, this difference in fat and muscle loss would amount to radically differing body compositions. I’m not saying that you’re completely sabotaging your body composition goals by eating late at night, but I am saying that it will not accelerate your progress.
Side Note - Decision Fatigue
In addition to wreaking havoc on sleep quality and stacking the deck against your body composition goals, eating late at night is unwise due to decision fatigue. Wikipedia defines decision fatigue as “the deteriorating quality of decisions made by an individual after a long session of decision making” (23).
A common example is very important people, such as Barrack Obama and Mark Zuckerberg, deciding to wear the same one or two outfits every day so that they can reserve their precious mental facilities for more important choices. It’s the reason judges are much more lenient in the beginning of the day and after breaks than later in the afternoon (24). Exhaustion of self-restraint is similar to resistance training -the first few reps are easy, but, after a while, you run out of reserves and are physically unable to perform another rep (25).
Similarly, after successfully passing on donuts in the office, pizza at lunch, and birthday cake in the afternoon, come nightfall it becomes nearly impossible to resist that tub of Ben and Jerry’s in the freezer. By getting in the habit of not eating late at night, the choice will have already been made long before your tired brain persuades you to spoon Rocky Road down your gullet.
Barrack Obama and Mark Zuckerberg have been known to limit their wardrobe to 1 outfit to minimize decision fatigue (26),
So, What Exactly Have I Changed My Mind About?
You may be surprised to learn that I have frequently changed my mind on this topic.
At first, , I had believed that eating at night was harmless as long as you consumed the correct amount of calories for your goals.
Next, I believed that eating anything at night would wreck you sleep and cause you to overeat, both the same night and the next day, increase stress levels, and my body composition.
Now, my thinking is somewhere in the middle!
But Then, It All Changed
After a two week n=1 trial in which I observed the effect a large pre-bed meal had on my sleep, I am skeptical, to say the least, about the significance of any negative effects. It turns out that, over these past 2 weeks, a large pre-bed meal did NOT prove to have a significant effect on my sleep quality, recovery, or weight.
It’s safe to say that late night eating did not cause weight gain for me - but that doesn’t mean that it won’t for everyone. Even if you, like me, are skeptical of the significance of these effects, we can’t let the midnight snack off the hook just yet.
For more on the long term health effects of eating late at night, check in next week for the release of my next article, “Eating at Night Part 2: Does Your Midnight Snack Cause Cancer?”
The difference a large pre-bed meal (~900 cals) made on my resting heart rate, heart rate variability, sleep efficiency, deep sleep, and REM sleep.
Wrap Up
Eating late at night will not directly cause fat gain. That being said, it may very well have an adverse impact on the biopsychosocial aspect of food consumption and physical activity. Consuming large, pre-bed meals may disturb your sleep and will likely have unfavorable effects on hunger, stress, insulin sensitivity, and body composition.
At night, you are also less likely to have the mental fortitude to make adventitious food choices compared to earlier in the day. This article does NOT mean that you should never enjoy eating and drinking late into the night on weekends and special occasions. Instead, please reference this information as a tool in your arsenal to help you establish sustainable healthy habits.
Sources
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC524030/
https://www.businessinsider.com/how-to-lose-weight-eating-only-mcdonalds-2015-10
https://www.cnn.com/2010/HEALTH/11/08/twinkie.diet.professor/
https://www.cnn.com/2010/HEALTH/11/08/twinkie.diet.professor/index.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC535701/
https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/cortisol
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4868668/
https://www.sciencedaily.com/releases/2000/11/001120072314.htm
https://link.springer.com/article/10.1007%2Fs13679-018-0306-y
https://www.webmd.com/diabetes/insulin-resistance-syndrome
https://pubmed.ncbi.nlm.nih.gov/20371664/
https://www.acpjournals.org/doi/10.7326/0003-4819-157-8-201210160-00005?articleid=1379773
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6262283/
https://www.acpjournals.org/doi/10.7326/0003-4819-153-7-201010050-00006?aimhp=
https://en.wikipedia.org/wiki/Decision_fatigue
https://www.pnas.org/content/108/17/6889#ref-7
https://www.businessinsider.com/barack-obama-mark-zuckerberg-wear-the-same-outfit-2015-4
