Is That Greasy Cheeseburger Making You Tired?

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There might be a less obvious reason than poor sleep for why you’re feeling tired and sluggish. Daytime drowsiness could have as much to do with a love for fatty foods (the greasy, processed variety, not good fats, such as avocado) as it does with how much sleep you get, suggests a new study.

The findings of the researchers at the University of Adelaide in Australia reflect a growing body of evidence that supports a link between what we eat and how we sleep. This latest study of 1,800 men with a median age of 60 that looks at this connection appeared online in the journal Nutrients this month. Here are its key takeaways:

People with the fattiest diets were nearly three times as likely to suffer from sleep apnea.

Electronic monitoring of study participants while they slept revealed that the fattiest eaters were much more likely to suffer from obstructive sleep apnea — a common disorder in which people have pauses in their breathing during sleep — when compared to the men who reported the lowest intake of fat.

A 2014 study reached similar conclusions, the Australian researchers noted. How fat makes you more prone to snoring and other apnea symptoms isn’t entirely clear. But research has suggested that fat intake might alter the body’s regulation of hormonal, metabolic, and central nervous systems, which are all involved in regulating the body’s circadian rhythms and therefore can alter sleep, says study lead author Dr. Yingting Cao, a Ph.D. candidate at the University of Adelaide School of Medicine.

Eating lots of fatty foods might make you feel sleepy the next day.

When compared with the group who ate the least fat overall, men who ate enough fat to put them in the top quarter of all those surveyed were almost 80 percent more likely to report feeling sleepy during the day. The measurement of sleepiness was less precise than the electronic monitoring of sleep apnea, however: Participants were asked whether they felt sleepy when sitting quietly during the day or evening and answered either yes, no, or sometimes, Cao says. The authors note that self-reported data, which can be less accurate than information controlled by researchers, was one of the limitations of this study.

As for how much fat these guys were eating, Cao says they averaged about 158 grams per day, most likely in the form of snacks and processed foods. Although there aren’t specific government guidelines for how much fat people can eat and still be healthy, that’s pretty high. A healthy adult eating 2,000 calories per day probably shouldn’t eat more than 78 grams of fat, according to estimates.

Fat might be a bigger factor than carbs in increasing daytime sleepiness.

Previous studies have suggested a link between high carbohydrate intake and drowsiness, Cao says. Carbs are thought to let more tryptophan, an amino acid, into the brain where it produces serotonin and melatonin, which can make you feel sleepy. But Cao adds, however, that fat intake appears to have an even stronger association to less alertness the following day than eating high-carb snacks or meals. More precise information about the timing of fatty meals is needed to better understand the connection between fat and drowsiness, she says.

The number of overall calories didn’t appear to affect daytime sleepiness.

Also surprising is that the total energy intake, i.e., calories consumed, of study participants didn’t appear to have a significant role in how tired they felt during the day. But even though the emphasis in this study was on daily fat intake, Cao points out that their data was gleaned from fat intake reported over a year. “So it is rather an eating habit than a particular day or meal,” she says, meaning that their results more reflect the effects of a pattern of eating.

Men with high BMIs suffer from sleep apnea and were often sleepy during the day, but high fat intake didn’t appear to make it worse.

Men with higher BMIs are more likely to suffer from sleep apnea, research has suggested, and that correlation was noted in this study as well. But fatty diets didn’t make men with higher BMIs more likely than thinner men to feel sleepy; the numbers were pretty consistent across the board. “This is consistent with our data that participants in the obese group had a higher risk of daytime sleepiness after adjusting for lifestyle factors,” the authors wrote. “However, obesity does not seem to be a mediator of the association between fat intake and daytime sleepiness.” Of course, determining the long-term effects of diet on sleep will require more research.

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