Many scientists have focused on understanding how sleep works, but fewer have looked at sleep’s opposite number: alertness.
Daytime sleepiness is a factor in thousands of car crashes every year and countless workplace accidents. It affects younger and older people alike, impacting learning, decision-making, and work performance.
Even if we avoid serious accidents, many of us just want to feel more awake during the day. We’re all busy, and not feeling alert can be a real drain.
A recent study investigates which factors increase daytime alertness. And, contrary to what you might think, it doesn’t appear to have much to do with your genes.
The research was carried out by ZOE’s scientists in collaboration with the neuroscientists and sleep experts at Prof. Matt Walker’s lab.
What did they do?
The scientists used data from ZOE’s Personalized Responses to Dietary Composition Trial (PREDICT 1), including more than 800 participants aged 18–65.
As part of PREDICT 1, the researchers asked participants to eat some standardized meals — muffins with varying amounts of fat, carbohydrates, and protein. Each participant also wore a monitor to measure their sleep and activity levels.
Every day, the participants reported what they ate and how alert they felt when they woke up.
They also completed a questionnaire about how well they had slept over the past month and when they typically went to bed and woke up.
What did they find?
The scientists found that your sleep habits, diet, exercise levels, and blood glucose responses to breakfast are all related to how alert you feel during the day.
Let’s jump into the details of how you might tweak your routine to improve your alertness.
First, the researchers found that when people slept longer than usual, they reported feeling more alert the next morning.
This makes sense — more sleep equals more alertness.
More surprisingly, the researchers found that when you sleep matters, too.
For the participants, waking up a little later than they did on average was linked to feeling more alert — even if their sleep duration was the same as usual.
The scientists showed that when people were more active during the day, they felt more alert the next morning.
So, making sure that you fit some exercise in today is likely to help you feel more alert tomorrow.
Conversely, people who were more active during the night felt less alert the following morning. This also makes sense — if you're active a lot at night, your sleep is likely disrupted.
What’s for breakfast?
The scientists found that what you eat in the morning can affect your alertness during the day.
As part of the PREDICT 1 study, the researchers asked participants to have pure glucose syrup for breakfast on some days. On these days, alertness was lowest.
According to lead author Raphael Vallat, Ph.D., this was one of the “strongest effects” they measured in the study.
Also, a high-protein breakfast was associated with feeling less alert. But alertness was still higher than it was after a breakfast of pure glucose.
Counter to what you might expect, the scientists found that a breakfast relatively high in carbohydrates was linked to increased alertness, but only for participants with good blood sugar control. More on this in a moment.
So, why did the glucose solution — a carb — cause a slump in alertness, when the high-carb breakfast did the opposite?
The authors have a few theories. For instance, they explain that the other components in the high-carb breakfast, like fat, fiber, and protein, change how the body responds to the carbs.
It might also be because the high-carb breakfast included sucrose. This, they suggest, might not affect blood glucose levels as much as pure glucose.
Another surprising finding came when participants consumed a high-calorie breakfast rich in carbs, fat, and protein. It contained 900 calories, compared with the 500 calories in the other breakfasts.
“We were expecting to see a decrease in alertness following this meal (think ‘food coma’),” explained Raphael. “However, there was no such decrease, and this meal was not significantly different from the other meals, in terms of post-breakfast alertness.”
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When investigating human biology, it’s rarely a one-size-fits-all deal. We’re all unique.
This study delved into a few of these interpersonal differences: blood sugar responses, genetics, and how levels of alertness differ between individuals. Let’s look at each factor in turn.
When you eat, your body breaks down carbs into its preferred energy source, glucose, which enters your blood. This causes a rise in blood sugar after eating — that’s normal.
However, as past ZOE research has shown, everyone’s blood sugar response differs. After eating the same meal, some people may only have a small uptick, while others have a much larger spike.
In this study, the researchers showed that a smaller blood sugar response after breakfast was linked to feeling more alert.
But Raphael explained to us that even after accounting for individual blood sugar responses, pure glucose for breakfast still had a strong negative effect on alertness.
In other words, it doesn’t matter how good your glucose control is, pure sugar first thing will make you feel less alert in the hours that follow.
What about genes?
Beyond looking at how different factors impact alertness, the authors wanted to understand why there are differences between people’s baseline alertness.
Put another way, why are some people more alert, on average, than others?
Thanks to the inclusion of identical twins in the dataset, the researchers could investigate whether genes play a part.
Interestingly, they didn’t identify a strong relationship between genes and levels of alertness.
So, what’s driving this difference? Why are some people just more alert than others if genes don’t have much to do with it? The scientists used machine learning techniques to investigate.
Differences in individual alertness
The scientists identified several factors linked to a person’s average level of alertness. The four main ones were:
Age: Average alertness increased with age in this group of people aged 18–65.
Mood: People who reported being happier felt more alert. And conversely, people with depression or an anxiety disorder, or those who had experienced them in the past tended to feel less alert.
Sleep quality: Perhaps unsurprisingly, people with better-quality sleep felt more alert. However, people who routinely slept for longer didn’t have better baseline alertness. So, quality trumps quantity.
Food frequency: People who ate five or more times a day were less alert than people who ate four times or fewer.
We asked Raphael why people tend to feel more alert as they age. He said:
“I think that it might be related to another strong relationship we found in this study, that of happiness and alertness. The happier you are, the more alert you feel, on average. It is a well-known observation that older adults are happier than younger adults.”
“In more scientific terms,” he continued, “there is a shift from negativity bias in younger age to positive thinking later in life, something referred to as the positivity effect.”
Raphael explained that this is why healthy older adults “consistently rate their sleep quality higher than healthy young adults, even though they objectively sleep far worse than the latter.”
The take-home message
The researchers hope that some of their findings might help shape public health recommendations. They note that one group, in particular, who might benefit are students.
Avoiding breakfasts that might lead to large blood sugar spikes could help students be more alert and take in information more easily.
The authors write: “[T]his issue is particularly noteworthy, considering the rapidly growing trend for teenagers and young adults to consume sugar-sweetened energy drinks as an alternative to a whole-foods breakfast — a trend that would only serve to increase sleepiness in morning classes.”
Overall, the results of this study are good news. The researchers identified modifiable factors — sleep, diet, and exercise — that might help us feel more alert.
There’s also some solace in knowing that, as we age, our levels of alertness (and mood) might also improve.
Center for human sleep science. (n.d.). https://www.humansleepscience.com/
Happiness across the life span: Not a slippery slope after all. (2019). https://dornsife.usc.edu/news/stories/3117/happiness-across-the-life-span-not-a-slippery-slope-after-all/
How people wake up is associated with previous night’s sleep together with physical activity and food intake. Nature Communications. (2022). https://www.nature.com/articles/s41467-022-34503-2
Lifestyle factors and sleep health across the lifespan. Environmental Research and Public Health. (2021). https://www.mdpi.com/1660-4601/18/12/6626
Predicting personal metabolic responses to food using multi-omics machine learning in over 1,000 twins and singletons from the UK and US: The PREDICT I Study (OR31-01-19). Current Developments in Nutrition. (2019). https://academic.oup.com/cdn/article/3/Supplement_1/nzz037.OR31-01-19/5517817
Prevalence of motor vehicle crashes involving drowsy drivers, United States, 1999-2008. Accident Analysis and Prevention. (2012). https://pubmed.ncbi.nlm.nih.gov/22269499/
Sleep inertia: Current insights. Nature and Science of Sleep. (2019). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6710480/
The positivity effect: A negativity bias in youth fades with age. (2018). Current Opinion in Behavioral Sciences. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6186441/