Excessive caffeine intake sustains activation of the sympathetic nervous system and suppresses parasympathetic tone.

Study: Caffeine on the mind: EEG and cardiovascular signatures of cortical arousal revealed by wearable sensors and machine learning—a pilot study on a male group

This study found that even a moderate amount of caffeine makes your brain more alert and your body more wired, which means your fight-or-flight system stays on and your rest-and-digest system slows down — just like the claim says.

Study: Coffee intake may promote sudomotor function activation via the contribution of caffeine

This study found that drinking coffee makes your body sweat more quickly and more intensely, which is a sign your fight-or-flight system is turned on. That means caffeine keeps your body in alert mode and likely slows down your rest-and-digest system.

Study: Multi-ingredient energy dietary supplement with a small amount of caffeine modulates central and autonomic nervous system after a single use.

Even a small amount of caffeine, like in an energy drink, can keep your body in 'alert mode' and calm down your 'relax mode,' which is exactly what the claim says happens with too much caffeine.

Study: Effects of Low Dose Caffeine on Post-Exercise Heart Rate Variability: A Double-Blind Placebo-Controlled Trial.

This study found that a moderate amount of caffeine after exercise didn’t slow down the body’s ability to calm down — in fact, it helped it recover faster. That’s the opposite of what the claim says, which is that caffeine keeps your body in 'fight-or-flight' mode.