Exercise is critical for maintaining optimal health; however, vigorous exercise also comes with some important caveats. For example, it is important to ensure critical nutrients are present—nutrients to support muscle, bones and energy levels. Now a new study has found that the most important exercise "nutrient" a person needs is one that is "absorbed" while lying in bed: sufficient sleep. In fact, it turns out that insufficient sleep can actually make strenuous exercise backfire. Due to the way inadequate sleep so profoundly affects cortisol levels, the researchers determined that a gym workout after getting less than six hours of sleep does more damage to the heart—and waistline—than if a person had just stayed in bed. New type of study The research, published in Nature Medicine in January 2026, was spearheaded by Dr. Emmanuel Mignot at Stanford Medicine. Dr. Mignot is recognized as a world-renowned expert in sleep medicine, and is famous for discovering the cause of narcolepsy. For this research Dr. Mignot and his team collaborated with other international researchers specializing in chronobiology. The researchers utilized a new AI model called SleepFM. The researchers determined that the use of AI modeling allowed them to turn sleep tracking into a crystal ball for long-term health. With the help of SleepFM, the researchers essentially treated accumulated sleep data as a "language." By feeding the model over 585,000 hours of physiological data from 65,000 people, they created a foundation model that doesn't just look at one metric, but integrates everything at once. The "integrated metrics" included brain waves (EEG), heart activity (ECG), muscle movements (EMG), and respiratory airflow. This modeling allowed the researchers to predict the likelihood of 130 diseases based on a person's sleep patterns. And, as part of the study, the researchers also made startling discoveries regarding the way sleep affected exercise The cortisol connection The body uses cortisol as a mechanism to stay alert, so when a person is significantly sleep-deprived the body is already in a state of elevated baseline cortisol. However, when a high-intensity workout is added to that mix, the stress/cortisol component is compounded to a point that is dangerously counterproductive. Typically, exercise causes a "temporary" cortisol spike, followed by a healthy drop. However, in sleep-deprived subjects, the study found that exercising while "exhausted" prevented cortisol from returning to baseline. Instead, it stayed elevated for up to 24 hours, putting the body in a chronic state of stress. The "Exercise Paradox" The researchers refer to this as the "Exercise Paradox." People often exercise to "wake up" or "feel better" when tired; however, the study showed that while a person might get a temporary endorphin hit, the underlying hormonal cost actually inhibits muscle recovery and increases fat storage. This is particularly true relative to visceral fat--meaning the benefits of the workout has effectively been reversed. The researchers concluded that a person who has had less than six hours of sleep would be better off replacing a workout with an extra hour of sleep. In this situation, the researchers noted, a person would receive better metabolic and cardiovascular results than "pushing through" at the gym. Too much sleep bad for health too The fact that an insufficient sleep is detrimental to health doesn't mean overdoing it in the opposite direction is good for health. Too much sleep is also detrimental. According to researchers, "sufficient sleep" has a sweet spot of roughly seven to eight hours. Some people do fine with as little as six hours, but for most people a minimum seven hours per night are needed for the "integrated metrics" and metabolic markers to show positive indications. This thinking corroborates a 2020 study from the American College of Cardiology. In that study, artery walls were found to suffer plaque buildups in both the patients who slept too little and too much each night. - - - Sources for this article: Nature Medicine, January 2026; Stanford Medicine; American College of Cardiology (2020 annual meeting).
