Cortisol: The Daily Curve and What Flattens It

Cortisol has a reputation as the stress hormone, and it does rise under stress. But its main job is a daily rhythm. It starts climbing in the last hours of sleep, peaks within about 30 to 45 minutes of waking, and then falls through the day to its lowest point around midnight. The morning peak is what gets you out of bed. It raises blood sugar and blood pressure and sharpens attention before you have had any coffee.

What flattens the curve

A healthy curve is steep. A flattened one has a weaker morning peak and a higher evening level, so the person feels foggy at 8 am and wired at 11 pm. Several things push it that direction:

• Short sleep. Even a few nights of 5 to 6 hours raise evening cortisol.
• Shift work and irregular wake times. The morning peak is set by waking and by light, and a wake time that moves around moves the peak with it.
• Chronic overtraining. Hard sessions raise cortisol in the moment, which is normal. Without enough recovery, the rhythm can lose its shape, though the research on overtraining and cortisol is mixed.
• Persistent psychological stress and depression.

A flat curve is associated with worse sleep, higher blood sugar, and more fat stored around the organs. That is one plausible reason a stressed, underslept person tends to gain weight at the waist.

How we test

The standard first look is a morning serum cortisol, drawn between 7 and 9 am. Timing is the whole test. The same person drawn at 1 pm will read much lower, and that reading is normal, not a problem. The morning value is mainly used to catch cortisol that is truly too low, which is adrenal insufficiency, while cortisol that is truly too high, Cushing's syndrome, is screened with other tests.

Saliva testing answers a different question. Several samples across one day sketch the shape of the curve, which can add context when the complaint is rhythm, such as flat mornings and alert nights. A late night salivary cortisol is also an accepted screen when Cushing's is the concern.

About adrenal fatigue

Adrenal fatigue is not a recognized diagnosis, and I will say that plainly. The adrenal glands do not wear out from a busy life. Real adrenal insufficiency is a specific, testable, serious condition, and it looks nothing like being tired after a stressful year.

The symptoms people attach to that label are real. Fatigue, poor sleep, salt and sugar cravings, and feeling flat in the morning all deserve an answer. The answer is usually sleep debt, a flattened rhythm, overtraining, low iron, a thyroid problem, or depression, and each of those can be measured and treated. A consistent wake time, morning light, enough sleep, and a lighter training week do more for the curve than any supplement sold for the adrenals.

If nobody has looked at this for you, it's worth a conversation.

This article is educational and is not medical advice.

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