- Cortisol is your stress hormone: it's essential acutely but damaging chronically.
- Cortisol has a strict daily rhythm; it peaks at awakening and falls throughout the day. Test timing matters.
- Chronically elevated cortisol impairs recovery, disrupts sleep, suppresses immunity, and blocks muscle growth.
- Training is stressful and raises cortisol; recovery (sleep, rest, nutrition) brings it down.
- If cortisol is elevated at baseline weeks after hard training, your body isn't recovering: reduce volume and prioritise sleep.
Quick Answer
Cortisol is your body's primary stress hormone. In short bursts, it's essential: it mobilises energy, sharpens focus, and helps you respond to threats. But when cortisol stays elevated for weeks or months, it impairs recovery, disrupts sleep, suppresses immunity, and blocks muscle growth. For athletes, understanding cortisol is crucial because hard training raises it acutely (which is fine), but if it stays elevated at baseline, it means your body isn't recovering well enough and training load needs adjustment. The critical detail: cortisol follows a strict circadian rhythm. It peaks at awakening and falls throughout the day. Testing cortisol at 8am gives completely different results than testing at 5pm, so timing is essential.
Why Athletes and Active People Care
Training stress and recovery: Hard training spikes cortisol as part of normal stress adaptation.[3] This acute elevation triggers adaptation and recovery. But if you're training hard without adequate sleep, nutrition, or recovery, cortisol stays elevated and prevents adaptation.
Sleep and mood: Chronically elevated cortisol disrupts sleep architecture, reduces deep sleep, and impairs mood. Poor sleep then keeps cortisol high. It's a negative feedback loop that kills performance.
Immune function: Elevated cortisol suppresses T-cell function and increases inflammation (via TNF-α and IL-6). Athletes with chronically high cortisol get sick more often and recover poorly from illness.
Muscle and metabolism: Elevated cortisol increases muscle protein breakdown and inhibits protein synthesis. It also increases appetite for refined carbohydrates and promotes fat storage around the midsection. This is the opposite of what training is supposed to achieve.
Overtraining syndrome: One hallmark of overtraining is elevated resting cortisol despite normal or high training volume.[4] It's a key marker that training load exceeds recovery capacity.
The Range Explained
RCPA reference ranges depend on time of day. Morning cortisol (8–10am): approximately 170–600 nmol/L. Afternoon cortisol (3–4pm): approximately 50–300 nmol/L.[1] The absolute numbers matter less than the pattern: cortisol should be high in the morning and low by evening. A flat, elevated curve all day suggests chronic stress or adrenal dysfunction.
| Cortisol Pattern | What it may indicate |
|---|---|
| High morning, low evening (normal) | Healthy circadian rhythm |
| Elevated all day (flat curve) | Chronic stress, poor recovery, possible adrenal dysfunction |
| Low morning, low evening | Possible cortisol insufficiency; see GP |
| Elevated at baseline weeks after hard training block | Overtraining; inadequate recovery |
How to Interpret Your Results
Normal morning cortisol, normal evening cortisol: Your circadian rhythm and stress response are intact. Good recovery.
Elevated morning and evening cortisol: Chronic elevation suggests ongoing stress (training, life stress, sleep deprivation, poor nutrition). Your body is in a prolonged alarm state. Address recovery.
Elevated baseline cortisol days or weeks after a hard training block: Your body hasn't recovered. Reduce training volume, prioritise sleep, and retest in 2–4 weeks.
Low cortisol (across the day): Rare but serious. Suggests adrenal insufficiency. See your GP to investigate pituitary or adrenal disease.
When to see a GP first: If cortisol is very high (>800 nmol/L at 8am) or very low (under 100 nmol/L), or if you're experiencing fatigue, low blood pressure, or mood changes alongside abnormal cortisol, consult your GP. They can investigate Cushing's syndrome (very high cortisol), adrenal insufficiency, or pituitary issues.
What Affects Your Cortisol
Sleep Quality and Duration
Sleep deprivation raises cortisol and flattens the circadian rhythm. Aim for 7–9 hours consistently; even one poor night elevates cortisol. Sleep is the single strongest lever for cortisol management.
Training Load and Recovery
Hard training raises cortisol acutely (normal). Excessive training without recovery keeps it elevated chronically. Balance hard sessions with recovery days and deload weeks.
Psychological Stress
Work stress, relationship strain, financial worry: all elevate cortisol independent of training. Stress management (meditation, time in nature, social connection) matters as much as training recovery.
Meal Timing and Composition
Skipping meals elevates cortisol. Eating small, frequent meals stabilises it. Refined carbohydrates spike cortisol; whole foods and fibre stabilise it.
Caffeine and Stimulants
Caffeine raises cortisol, especially if you're already stressed. Excessive stimulant use (pre-workout, energy drinks) compounds the problem.
Alcohol
Heavy alcohol consumption elevates cortisol and disrupts sleep. Moderation or abstinence lowers it.
Circadian Alignment
Light exposure, meal timing, and consistent sleep/wake schedules all regulate cortisol rhythm. Irregular sleep and eating times flatten the curve and elevate baseline.
When to Retest
If cortisol is normal, retest annually. If elevated, retest after 2–4 weeks of focused recovery (extra sleep, reduced training volume, stress management). You should see improvement if interventions are working. Retesting too soon (days later) isn't useful because cortisol takes weeks to stabilise.
FAQ
Can cortisol be healthy if it's elevated? Not long-term. Acute elevation is normal and necessary. But if cortisol is elevated at baseline weeks or months into a training block, it indicates insufficient recovery and impairs adaptation.
Is morning cortisol always the best time to test? Yes, for diagnostic purposes. Morning cortisol (8–10am) is the standard. Some athletes also test afternoon cortisol to assess circadian rhythm, but morning is the clinical reference.
Can I lower cortisol through supplements? Sleep and stress management are the primary levers. Some adaptogenic herbs (rhodiola, ashwagandha) have modest evidence for lowering cortisol, but they're far less effective than sleep and recovery optimisation. Consult your GP before starting supplements.
Why do athletes have elevated cortisol even during recovery weeks? Psychological stress can override training load. If you're anxious about performance or worried about life stress, cortisol stays elevated even with low training volume. Managing stress outside the gym is equally important.
Does cortisol-to-DHEA ratio matter? Some athletes and coaches track this ratio, but it's not standard clinically. If your cortisol is elevated and DHEA is normal, the absolute cortisol number is what matters most. Focus on recovery first.
