For the study, researchers recruited 72 men and women, and had them take a 40-minute memory test that involved looking at pictures of common objects presented in 1 of 6 locations on a computer screen. Participants were asked to remember what the object was and what position they say it in.After the test, 1/3 of the participants did a 35-minute, high-intensity workout on a stationary bike, while another group waited 4 hours before participating in the same workout, and a third group did not workout at all.
Two days later, all of the subjects completed the same memory test to find out how much information they had retained. Then, they underwent brain scans while retaking the test, which allowed researchers to analyze their brain activity patterns.
The researchers found that people who waited 4 hours to work out retained the most information 2 days later. The group that didn’t work out had the second-best performance.
Dr. Guillen Fernandez, a professor of cognitive neuroscience at The Donders Institute for Brain, Cognition and Behavior at the Radboud University Medical Center in the Netherlands, said that while it’s not clear why waiting to exercise helped people retain more memories, he has .
that while exercise can release neurotransmitters like dopamine and noradrenaline, and brain chemicals help enhance memory, he suspects that the psychological effects of working out right after studying might cause some interference between the information gathered and the formation of new memories.
But he also said that researchers don’t know if working out 4 hours after studying is the optimal timefor retaining fresh information because only 2 other conditions were considered in the study. So take the findings with a grain of salt, because, according to Fernandez, it is possible the enhanced memory benefit might have been larger had the participants exercised 2 hours after studying, or by performing another type of exercise. 
Fernandez said he plans to do a follow-up study to investigate how working out at different time intervals, such as 2 or 8 hours after studying, affect long-term memory.
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