Caffeine, Coffee, and Cortisol: Understanding the Morning Jitters

Many people rely on a morning cup of coffee to kickstart their day, but that immediate burst of alertness comes with a physiological cost. Caffeine directly stimulates your central nervous system, triggering the release of cortisol — the body's primary stress hormone. While this hormonal spike increases alertness, drinking coffee or energy drinks can accentuate your body's stress response and induce unwanted jitters.

What is cortisol?

Cortisol is most recognized as the body's “stress hormone,” but it helps orchestrate everything from your metabolism to your immune response. Produced by your adrenal glands, cortisol plays a key role in regulating your metabolism, immune system, and energy levels throughout the day.

Think of it as your body's adaptive tool: it helps you wake up in the morning, powers you through stressful situations, and even gives you a boost to tackle midday dips in energy.

The cortisol connection and circadian rhythm

Cortisol follows a natural daily rhythm, known as the circadian rhythm. It typically peaks in the early morning (around 6–9 a.m.), helping you wake up and face the day, then gradually declines throughout the day, reaching its lowest point at night as your body and mind prepare for rest and recovery (Fries, 2008).

However, when stress — whether physical, emotional, or environmental — triggers a cortisol release, it can throw this balance out of whack. Short-term cortisol spikes are an essential part of your body's alarm system, helping you respond to emergencies. But combining stress with your natural morning peak and caffeine-induced cortisol secretion can drive your cortisol levels exceptionally high.

A comprehensive 2025 comparative review of 10 studies involving approximately 2,500 subjects highlighted just how powerful this effect can be. Researchers found that coffee caused the strongest cortisol increase among the beverages studied — spiking levels by 50% above baseline.

Beyond stress: the blood sugar crash

Caffeine doesn't just affect cortisol and perceived stress levels — it can also lead to a short-term spike in blood sugar by decreasing insulin sensitivity (Shi, 2016; van Dam, 2004). In simpler terms, caffeine makes it harder for your body to process sugar efficiently. This has implications for people who already have problems with insulin sensitivity, as well as for seasons of life when cortisol already runs higher, like perimenopause and menopause.

Long-term effects and tolerance

Small, daily fluctuations in cortisol are completely natural. However, chronically high cortisol over a long period can contribute to symptoms like weight gain (especially around the midsection), mood swings, and a weakened immune system.

And while the body does adapt to regular caffeine intake, research shows it still exhibits heightened cortisol reactivity during stressful situations — tolerance blunts the buzz, not the stress response. If that describes your mornings, the gentlest place to start is simply reducing how much caffeine you drink; our guide to caffeine withdrawal covers how to taper without the headache.