Cardiovascular exercise, commonly known as cardio, has long been a staple in fitness routines. However, there has been some debate about its effects on cortisol, the "stress hormone." There’s quite a bit of misunderstanding around this which has led some people to argue that because cardio increases cortisol levels it can lead to negative health outcomes and an increase in stress.
And whilst it is true, cortisol is responsible for stress, the story about it’s relationship with exercise is not quite so simple. Other hormones are also released and it is their interaction which ultimately matters.
What is Cortisol?
Cortisol, produced by the adrenal glands, plays a vital role in the body's stress response. Aside from stress, it also helps regulate blood sugar levels, metabolism, blood pressure, and even sleep-wake cycles. 
Cortisol is released in response to both physical and psychological stressors, enabling the body to cope with challenging situations. While cortisol is essential for our survival, chronically elevated levels can have detrimental effects on our health.
The Role of Cardio in Cortisol Levels
One prevailing myth is that cardio increases cortisol levels, leading to negative health consequences. However, this belief fails to consider the complexity of the relationship between cardio and cortisol.  While it is true that intense aerobic exercise causes a temporary spike in cortisol levels, this does not necessarily translate to long-term harm. Other hormones are also released, for example testosterone spikes, and cortisol and testosterone interact in unique ways.
Secondly, chronic cortisol increase is what causes harm,  whereas, a short term spike is nothing to be concerned about and can actually be beneficial.
Overtraining however, is another matter which we’ll come onto later.
The Importance of Exercise Intensity
The intensity of exercise plays a crucial role in determining cortisol response.
High-intensity aerobic exercise, such as interval training, can indeed cause a short-term increase in cortisol levels.
However, moderate-intensity cardio, such as jogging or brisk walking, does not elicit the same cortisol response. It is important to choose the appropriate intensity of exercise based on individual fitness levels and goals.
The Impact of Duration and Frequency
Another factor to consider is the duration and frequency of cardio sessions. Prolonged endurance training, such as long-distance running, can lead to elevated cortisol levels for several days or even longer. However, this primarily affects elite athletes or individuals engaging in excessive endurance training. For the average person, regular moderate-intensity cardio workouts of reasonable duration do not pose a significant risk of chronically elevated cortisol levels. 
Managing Cortisol Levels During Cardio Workouts
While moderate-intensity cardio does not pose a significant risk of chronically elevated cortisol levels, it is still important to manage cortisol levels effectively. Here are some strategies to help regulate cortisol during cardio workouts:
Proper Recovery and Rest
Allowing adequate recovery and rest between cardio sessions is crucial for maintaining healthy cortisol levels. Overtraining or engaging in excessive endurance training without proper recovery can lead to chronically elevated cortisol levels. Prioritize rest days and listen to your body's signals to avoid overexertion. Not getting proper rest and allowing your body’s cortisol levels to lower, is where the problems begin to creep in.
Mindfulness and Stress Reduction Techniques
Incorporating mindfulness and stress reduction techniques into your daily routine can help manage cortisol levels. Practices such as meditation, deep breathing exercises, and yoga promote relaxation and reduce stress, contributing to balanced cortisol levels.
Maintaining a well-balanced diet rich in nutrients is essential for hormone regulation, including cortisol. Consuming adequate protein, healthy fats, and complex carbohydrates supports overall hormonal balance. Avoiding excessive sugar and processed foods can also help prevent hormonal imbalances.
Quality sleep is vital for cortisol regulation. Aim for seven to eight hours of uninterrupted sleep each night to optimize hormonal balance. Establishing a regular sleep schedule and creating a relaxing bedtime routine can promote better sleep quality. 
Variety in Cardio Training
Incorporating variety into your cardio training can help prevent cortisol imbalances. Mixing different types of cardio, such as interval training, steady-state cardio, and low-impact exercises, can provide a well-rounded workout routine while minimizing the risk of overexertion.
Overtraining, Cortisol and Hit
During a HIIT workout, cortisol is released into the bloodstream as we mentioned. The brain perceives the high intensity interval as a survival challenge, leading to the release of cortisol and other hormones our fight-or-flight response. This hormonal response can also have benefits such as raising metabolism, but overdoing it can have consequences.
Proper Frequency of HIIT Workouts to manage Cortisol
To ensure proper recovery and prevent overtraining, it is recommended to limit HIIT workouts to 2-3 times per week. Ideally with rest days in between each session. This doesn’t mean that you have to take a complete rest day, but you should exercise at a lower intensity.
Recognizing the Signs of Overtraining
It is essential to listen to your body and recognize the signs of overtraining or excessive cortisol levels. Some common symptoms include changes in mood, sleep disturbances, chronic fatigue, decreased exercise performance, anxiety, repressed immune system, lack of motivation, and muscle fatigue.
The easiest way to tell is if you are getting weaker in your workouts week on week without any other factors such as lack of sleep or dietary changes to explain.
1 - https://www.ncbi.nlm.nih.gov/books/NBK538239/
2 - https://pubmed.ncbi.nlm.nih.gov/18787373/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4263906/
4 - https://pubmed.ncbi.nlm.nih.gov/18787373/
5 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4688585/