Nap or quick workout? A doctor explains the cognitive benefits after sleep deprivation

By Katia Hetter, CNN

(CNN) — Going without sleep makes it harder to concentrate and learn new information. And for people who work overnight or face an occasional period of severe sleep loss, getting enough rest may not always be possible in the moment.

Can anything help us focus besides sleep? A recent study published in the scientific journal Proceedings of the National Academy of Sciences compared two strategies for protecting memory after sleep deprivation: aerobic exercise and a nap. Researchers wanted to know whether a short bout of exercise really provides some of the same cognitive benefit as a nap. As it turns out, the answer may be yes.

So, what does this finding mean for everyday sleep and exercise habits?

To help understand how people can apply these results, I spoke with CNN wellness expert Dr. Leana Wen. Wen is an emergency physician and clinical associate professor at George Washington University. She previously was Baltimore’s health commissioner.

CNN: How did researchers go about identifying the effect of exercise versus napping?

Dr. Leana Wen: The study enrolled a small number of healthy adults ages 18 to 35 and kept them awake for 30 hours. The 54 participants were then randomly assigned to one of three groups. One performed 20 minutes of moderate-to-vigorous cycling, at 80% of their maximum heart rate. Another had a 90-minute opportunity to nap. The control group sat on a stationary bicycle for 20 minutes without exercising.

Afterward, participants viewed 150 images while researchers recorded their brain activity using electroencephalography, or EEG. They did not know there would subsequently be a memory test. Three days later, after having time to recover from the sleep deprivation, they were shown the original images mixed with new ones and asked which they remembered.

Participants who exercised correctly recognized 56% of the images, compared with 57% among those who napped and 46% in the control group. Both the exercise and nap groups performed significantly better than the control group, with memory performance about 21% higher after exercise and 23% higher after a nap. There was no significant difference in benefit between exercise and napping.

Interestingly, participants who napped reported feeling significantly less fatigued afterward, while those who exercised reported similar levels of fatigue as the control group. Despite feeling just as tired, the exercise group performed significantly better on the memory test than the control group.

CNN: How could 20 minutes of exercise help memory after someone has been awake for so long?

Wen: Sleep deprivation interferes with the brain’s ability to encode new information. After prolonged wakefulness, sleep pressure builds and the neural processes needed to form memories become less efficient.

The EEG recordings suggest that exercise helped counteract some of these effects. It reduced a marker of neural fatigue in brain regions involved in the memory task. One possible hypothesis is that exercise helps the sleep-deprived brain use its available cognitive resources more efficiently.

The 20-minute workout also fits into a broader concept known as exercise “snacking,” though it was longer than most exercise snacks. These are short bursts of physical activity typically lasting anywhere from about 30 seconds to five or 10 minutes. Activities can include climbing stairs, brisk walking, doing squats or other activities that raise the heart rate. Studies have suggested that these small doses of movements have health benefits, such as improving cardiorespiratory fitness and even reducing the risks of cancer and dementia.

CNN: Did exercise and napping help the brain in the same way?

Wen: Not exactly, and in my view, this was one of the most interesting findings. Exercise and napping produced similar memory performance benefits, but through different neural processes.

The nap changed the underlying state of the sleep-deprived brain. People in the nap group had less fatigue and lower EEG markers of sleep pressure, which is the biological drive for sleep that builds the longer someone stays awake. Measures related to this improved brain state were strongly associated with how well participants formed new memories.

Exercise did not relieve sleep pressure in the same way. Instead, it appeared to make memory processing more efficient. We can think of the distinction in this way: The nap gave the brain some of the sleep it was demanding, while exercise helped the brain function more effectively despite still being sleep-deprived.

CNN: Can someone who is sleep-deprived exercise instead of taking a nap?

Wen: No. Exercise cannot replace sleep. Sleep serves essential functions throughout the body, including regulating immune and metabolic processes and supporting many aspects of brain function. A 20-minute workout cannot provide those functions, although regular exercise does help sleep and vice versa.

The study asked a much narrower question: If someone has already experienced severe sleep loss, can an intervention protect one particular cognitive ability, the formation of new memories? For that outcome, in the specific circumstances of this study, 20 minutes of exercise and a 90-minute nap produced comparable results.

There are situations where this could have practical applications. Healthcare workers, transportation workers and others in safety-sensitive occupations can experience unavoidable sleep loss, and there may not always be time or an appropriate place for a 90-minute nap. A short bout of exercise could offer another strategy for preserving memory. But someone should not interpret these results to mean that exercise makes it safe to drive or perform other dangerous tasks while severely sleep-deprived.

CNN: Can we apply these findings to people tired from a bad night of sleep or who routinely do not get enough sleep?

Wen: We should be specific about clarifying what was tested. This was a small laboratory study of young, healthy adults who had been continuously awake in laboratory conditions for 30 hours. The researchers tested one particular exercise regimen and one aspect of cognition.

We don’t know whether the same benefit occurs after chronic sleep restriction, in older adults with medical conditions, or with other types or intensities of exercise. Nor did the study show that exercise restores attention, reaction time, judgment or the many other functions impaired by sleep deprivation. Those are questions for larger studies in real-world settings, particularly among shift workers and others who regularly experience sleep loss.

CNN: What should people do if they are sleep-deprived but need to function well that day?

Wen: The first priority is to try and get sleep. Adults should aim to get at least seven hours of sleep on a regular basis. A nap can be useful when adequate nighttime sleep has not been possible. If a nap is not possible, a short bout of aerobic exercise could be useful before a period that requires learning and memory. Someone could take a brisk walk, climb stairs or use a stationary bike.

Caffeine can temporarily increase alertness and concentration and maybe even reduce dementia risk long term, though it does not reverse the effects of sleep deprivation. People should also eat regular, balanced meals and stay hydrated. Large, heavy meals can increase sleepiness, and alcohol should be avoided because it can further impair alertness and judgment.

People also need to recognize the limits of these strategies and recognize that they do not make it safe to drive or perform other potentially dangerous tasks when severely sleep-deprived.

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