Why Thanksgiving Makes You Sleepy (& Why Your Dog Is Staring at You While You Eat)

I think we can all admit it: Thanksgiving is basically the Olympics of eating. Every year, we sit down with confidence that this is the year we will not overdo it, and every year we end up stretched out on the couch, wondering why we thought a second plate was a good idea. But what is actually happening inside the body when the post-Thanksgiving crash hits?

Most people blame tryptophan, the amino acid in turkey that is said to make you sleepy. Tryptophan can influence serotonin and melatonin production, but turkey does not contain unusually high levels of it. So if it is not the turkey alone, what causes the sleepy feeling?

The real answer involves postprandial physiology, which refers to the normal changes that occur after eating a large meal. When we eat a feast loaded with carbohydrates, fats, and sweets, the parasympathetic nervous system begins to dominate (Wells et al., 1997). This is the system responsible for “rest and digest” activity. It slows the body down and redirects blood toward the gastrointestinal tract so that digestion can begin. Larger meals increase digestive workload and blood flow to the gut, which can contribute to reduced alertness.

Insulin also increases after a carbohydrate-heavy meal to help move glucose into cells. This change in blood glucose regulation can make you feel more tired. In addition, research suggests that inflammation-related molecules like interleukin-1 (IL-1) increase after high-calorie meals and may directly contribute to post-meal fatigue (Lehrskov et al., 2018). When combined with the energy required to digest a large meal, the sleepiness feels almost inevitable.

Here is a fun twist. Have you ever noticed how dogs stare at you during Thanksgiving dinner like they know something you do not? Dogs have an incredible sense of smell and can detect changes in human hormones related to stress, fullness, and even shifting blood glucose levels (Rooney et al., 2019). So, while they are probably just hoping for turkey scraps, their noses may also be picking up on the physiological changes happening in your body.

There is also an ethical angle to consider. Many families do not have access to large, nutritious meals. Understanding the physiology of overeating can also remind us of the privilege and responsibility tied to food access and food security.

So, if you found yourself in a classic Thanksgiving food coma this year, it was not simply because you indulged. It was the result of normal physiological processes working together to digest a very large holiday meal. And the dog staring at you probably sensed it before you did.

References:

1.     Lehrskov, L. L., Dorph, E., Widmer, A. M., Hepprich, M., Siegenthaler, J., Timper, K., & Donath, M. Y. (2018). The role of IL-1 in postprandial fatigue. Molecular Metabolism, 12, 107–112. https://doi.org/10.1016/j.molmet.2018.04.001

2.      Rooney, N. J., Guest, C. M., Swanson, L. C. M., & Morant, S. V. (2019). How effective are trained dogs at alerting their owners to changes in blood glycaemic levels?: Variations in performance of glycaemia alert dogs. PloS One, 14(1), e0210092. https://doi.org/10.1371/journal.pone.0210092

3.      Wells, A. S., Read, N. W., Uvnas-Moberg, K., & Alster, P. (1997). Influences of fat and carbohydrate on postprandial sleepiness, mood, and hormones. Physiology & Behavior, 61(5), 679–686. https://doi.org/10.1016/s0031-9384(96)00519-7