• New research published in Sports Medicine and Rehabilitation points to a link between overtraining, your mood, and heart rate variability.
• Paying attention to your mood and your heart rate variability can help you determine whether you need to dial back your training.
• Training to improve your heart rate variability may also lead to a boost in mood and performance.

You crushed your ride yesterday and went to bed feeling happy about it, but the next morning, there’s a major mood shift. What gives? Turns out this happens—and your heart might play a role in it.

A new study published in Sports Medicine and Rehabilitation looked at the relationship between cycling training one day, and heart rate variability (HRV) and mood states the following morning.

HRV is a measure of the time between each heartbeat, and it’s an indication of how well your autonomic nervous system is operating. (The autonomic nervous system is the one responsible for the bodily functions you’re not consciously activating, like breathing, heart rate, and digestion.) Higher variability means your heart can adapt to changes like stress more easily. Low variability has been linked to health issues because it shows your heart is less resilient, according to Cleveland Clinic.

In the recent study, researchers found that the higher the training power on one day, the lower the mood tended to be the next morning. A relationship between low morning HRV and mood was also found, according to lead author Carla Alfonso, Ph.D., at the Laboratory of Sport Psychology, Universitat Autónoma de Barcelona in Spain. She told Bicycling that the study points to a relationship between strenuous training and recovery levels shown through mood and HRV hours later.

One caveat is that the study was very small, involving just five road cyclists, but extensive data was collected on that handful of participants, including 123 recordings of HRV and mood over a six-week period, and 66 recordings of training power and rate of perceived exertion.

The thing researchers don’t know is why a tough training day can lead to a downer mood and low HRV. Researchers didn’t look at whether HRV stayed that way throughout the day, but that’s likely the next step in this research.

“It’s a tricky question that needs further investigation,” Alfonso said. “One possibility is that making a training session too hard for the physical abilities of the athlete can negatively alter mood. That means physical activity is great, but if you go too hard, it can destabilise mood and the system.”

The role of HRV is also important in terms of mood, she added. Previous research that goes back decades has shown a correlation between the two, said Alfonso.

For example, a 2010 study published in the journal Personality and Individual Differences found that HRV is strongly connected to perceptions of well-being and the ability to regulate emotions. That’s because HRV is a measure of autonomic nervous system function, which is directly associated with mood states. The higher your HRV, the better and more efficient your autonomic functions, and potentially, the more regulated your emotions will be.

Another bonus to paying attention to HRV: It can make you a better cyclist. In fact, a 2018 study in the International Journal of Sports Physiology and Performance suggested that training to improve HRV could be just as effective in a road cycling performance program as load progression like in periodisation training. The researchers conclude that HRV training may even result in better performance overall, based on improvements in a 40-minute time trial and peak power output.

Although the cyclists in that study had their HRV measured in a lab setting, many fitness trackers include HRV and one way to train with this in mind is to do intervals, which help to condition your cardiovascular system. Also, as Cleveland Clinic adds, managing stress levels can also improve heart rate variability.

Since lower HRV tends to be predictive of negative moods, said Alfonso, shifting toward improving your variability could be a boon on multiple levels—you’ll train more effectively, and feel happier and more relaxed along the way. The other takeaway: If your mood is down and your HRV is low, consider taking a rest day to see if they both perk up.