Why HIIT Workouts Are Making You More Tired, Not Fitter

You pushed through a hard HIIT class this morning. You're proud of yourself. And by 2pm you're wiped out, irritable, and reaching for coffee number three.

Here's the mechanism nobody explains. Cortisol is supposed to follow your circadian clock — rising in the morning, dropping to its lowest point near bedtime. A healthy clock also has a second job: it shuts cortisol back down after a stressor passes, so a spike from traffic, a hard conversation, or a tough workout doesn't linger. You return to baseline.

But if your clock is already strained — from poor sleep, chronic low-grade stress, or blood sugar swings — that shutoff mechanism doesn't work as cleanly. Chronically elevated cortisol has been described as having your foot slightly on the gas pedal all day, every day. Your vessels never fully relax, and neither does the rest of you. Stack a HIIT session on top of that, and you're not building resilience. You're adding another spike to a system that hasn't come back down from the last one.

This doesn't mean intensity is the enemy. It means matching the effort to what your nervous system can absorb right now. One alternative worth knowing: Zone 2 cardio — a pace where you're working, but could still hold a conversation — actually helps activate your parasympathetic nervous system and lower that resting sympathetic tone, instead of adding to it.

Try this: for one week, swap one HIIT session for a brisk walk, easy bike ride, or swim at a conversational pace. Notice how you feel by evening compared to your usual hard-session days. Your energy will tell you which one your body actually needed.

If this resonates and you want to understand what's actually driving your fatigue, my free MasterClass walks through exactly this.