Now that it’s time for Thanksgiving, we can gather round as family and friends and concentrate on what’s really important to us – food. While this celebration is also a moment to spend in the company of loved ones, I’d like to bet that food will occupy a special place in your heart (and most of your stomach) for this November 24th.

We’ve all heard stories of Turkey and tryptophan, of carbs and colossal amounts of food, all seducing you into an early slumber. But this isn’t good enough for the iMotions team – we want the facts.

So, I set out to know – just how sleepy does an overload of food make your brain?

This study first required finding people who would be willing to overeat for the sake of science. Sometimes the things we do in the pursuit of scientific truth are difficult to do – sometimes it requires courage. This was not one of those times. Barely had I uttered the words about lunch and people were already loosening their belts.

But what would I be asking people to do, other than give themselves away to gluttony (for noble goals)? In the morning they would watch a relaxing video of lake and mountain scenes, allowing them to admire nature’s beauty, and take it easy for a while. While they watch the vista unfold, I would watch their brains.

eeg scenery

It Comes in Waves

Using electroencephalography (EEG), I can detect the speed of their neural oscillations. These waves of neural activity reveal many things, amongst them a measure of wakefulness. Alpha waves (8-15 Hz) are most commonly displayed during relaxed wakefulness, while theta waves (4-7 Hz) likely mean the participant has nodded off. Beta waves (16-31 Hz), on the other hand, are more associated with alertness. So which would I see the most of? Would it all be theta waves, as they fall deeper and deeper into sleep? Or would they power through the food-coma attack, all beta-waves and alert as ever?

Now, with this being iMotions, I didn’t have to stop there – I decided to carry out eye tracking while they watched the videos, and just for good measure, record their heart rate too (via electrocardiography, otherwise known as ECG).

eeg waves sleep

The Early Bird Catches the Worm

This might be tough to hear if you don’t consider yourself a morning person, but you’re at your most alert shortly after you’ve awoken (the first 2-4 hours of wakefulness). While it might be slightly too eager to plug people into the experiment immediately after their alarm has gone off (believe me, I tried), I did however want to catch them well before they would get to lunch.

Luring the participants in with the promise of beautiful panoramas and a few minutes of rest was at least half-true. The other half of course included conductive gel, EEG headsets, and the psychomotor vigilance test (PVT) – just for good measure.

This latter test is a rather mundane measure of wakefulness, referred to as the “gold standard” of alertness testing. Within this, participants simply stare at a screen and wait for something to happen. When it does, they click with their mouse as fast as possible. Bored already? That’s part of it. Maintaining concentration when nothing else is going on is a great way to see how far your focus really stretches. (If you want to try it out, there’s an online version here – but don’t say I didn’t warn you).

After the participants were lulled by the calming scenes and the riveting rigor of the PVT, they were asked to indicate how tired or alert they felt, on a 7-point Likert scale (shown below).

likert scale

gamma waves abmThe first participants ambled in and were summarily connected to our ABM B-Alert X10, which can give us information about the waves of neural activity while allowing a great amount of free movement (and will obviously be next season’s hottest fashion trend).

The North American mountain scenes are a pleasant interlude, and allow enough stimulation that the participants aren’t necessarily bored, but aren’t exactly on the edge of their seat either. The duration of the film also allows enough time to collect a briefly representative picture of their neural oscillations – enough to know how active their brains are at that time.

The PVT is less stimulating, but as a two-minute task it was bearable (most experimental settings require a longer exposure time, but this was reduced for brevity’s sake). After a few recordings and tests, the willing subjects are presented with the biggest decision of the day – how much to overeat (I may have tried to lead the way with this – as an act of solidarity, of course).

Overall the morning session revealed that the participants were awake and alert with a large degree of oscillations occurring around the Beta range. The eye movements were fast and seemingly alert, the heart rate normal. They also reported feeling awake, so at least they were starting off on the right foot. But the big test was yet to come.

Out to Lunch

I waited an hour after lunch before grabbing theeeg alpha waves willing subjects, hoping to collide their afternoon lethargy with the recordings. The results were soon clear – the post-lunch dip had set in strongly. The data showed, over the course of the video, a slowing of neural oscillations, a slackening of EEG waves, and a steady but sure increase of drowsiness.

Alpha waves have long been known to be related to drowsiness. Several studies have also shown that gorging, or even eating normal amounts of food can be related to a decline in alertness, increased sleepiness, and reduced cognitive vigilance.

The eye tracking data additionally painted a picture of passivity – increased blinks and eye closing occurrences are related to tiredness, and this bore out in the data too. The heart rate variability (a measure of the uniformity of the heart beat rate) was found to be increased, which has also been linked to increased sleepiness.

AOI alpha eeg

Falling asleep fashionably.


Curiously, there didn’t appear to be any difference in the performance of the PVT, despite an apparent increase of psychophysiological indicators of sleepiness. This might have been due to the shorter time for data collection, but it could also have resulted from an increase in motivation from participants – which has been associated with maintaining peak performance with reaction times. They pulled through in some ways after all.

eeg motivation

The Proof is in the Pudding

One of the great advantages about using biometric, or psychophysiological measurements, is that they are ableto tell an unbiased, and unfiltered story about an individual’s mental state. The sensors can’t be fooled – so when we see an increase of alpha wave activity, or a lowered heart rate, we know that the data is truly reflective of the physiology.

While efforts can be made to temporarily compensate for the drowsiness in a task, the measurements defined how the person felt more generally, which can ultimately be more informative. It seemed clear from the results that the baseline of neural activity was reverting to a calmer state than in the morning, even if it the PVT could be performed just as well.

Of course, there are a few caveats to this study. While the cognitive slowdown following food is well documented, the study carried out here had no control group to compare to, only a few participants, and the time of day affecting alertness was not balanced for. Overall though, it follows a long line of research that shows the very same thing – the food coma is real.thanksgiving science study

Scientifically Sleeping

So why is this happening? People have often pointed to the tryptophan-rich turkey as the reason for the intensity of their drowsiness on Thanksgiving day, but most meats contain similar levels of this amino acid (suggesting that most people would experience the same intensity of feelings on any other day).

The so-called “postprandial somnolence” / food coma is an established phenomena, which results from activation of the sympathetic nervous system, allowing your body to digest effectively, and also has the side-effect of sending you straight to sleep.thanksgiving science sleep

Further to this, a a mixture of over-consumption and over-exhaustion all-round is surely – and I don’t think a scientific study is required to prove this – a recipe for drowsiness. There are things that can be done to mitigate the effects of this however, so if you find yourself desperate to null the nap, then you could try exposing yourself to blue lights, and some form of light exercise.

The best way to fight off the postprandial somnolence might however have more to do with eating so much in the first place. But that doesn’t exactly sound as much fun. So, maybe don’t resist, and let the nap begin. It’s tradition after all.

I hope you’ve enjoyed our Thanksgiving study, and hopefully you’re working up an appetite. If you’re hungry for more science (with apologies for that pun), then download our free pocket guide of experimental design by clicking below, or get in touch if you’d like to know more. Happy Thanksgiving!

experimental design guide