How Cortisol Actually Works (and Why Lower Is Not Better)
By: Brian Hoeflinger, MD
September 13, 2026 | #94
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Disclaimer: Opinions are my own. Not medical advice.
Medical Trivia of the Week
Roughly what share of the world's adults cannot properly digest lactose? (the correct answer is at the end of this email)
- A) About 35 percent
- B) About 45 percent
- C) About 55 percent
- D) About 65 percent
In Today's Newsletter
- Cortisol is supposed to be high at certain hours and low at others. The internet has convinced a lot of people that it should simply be low, and that is not true.
- I will walk you through what cortisol actually does, what the daily curve looks like, and why the timing matters more than the number.
One of the Most Misunderstood Hormones
Cortisol gets called the stress hormone. That name is accurate but incomplete, and the incomplete part is where all the confusion comes from.
Cortisol does rise when you are under stress. It also rises every single morning before you open your eyes, whether your life is calm or chaotic. It is less a stress hormone than a timing hormone, and once you see it that way, the rest of it makes sense.
It is also not optional. When the adrenal glands fail to produce it, in a condition called Addison's disease, it has to be replaced with medication. This is not a hormone you can live without.
What It Actually Does
Cortisol is made by your adrenal glands, two small glands that sit on top of your kidneys. It has three main jobs.
It prepares you to get going. It pulls stored fuel into your bloodstream so your brain and muscles have energy available. And it controls inflammation.
That last one is worth pausing on. Some of the most powerful anti-inflammatory drugs in medicine, prednisone and dexamethasone, are synthetic versions of cortisol. In hospitals they are used to pull dangerous swelling out of the brain. They work because they do what your own cortisol already does, only stronger.
A hormone capable of that is not something your body produces by mistake.
Your Cortisol Runs on a Schedule
Here is the part most people don't realize. Cortisol is not meant to be low all the time. It follows a daily curve.
It sits at its lowest in the middle of the night. It begins climbing in the early hours, before you wake. Then, in the first 30 to 45 minutes after you open your eyes, it surges. Researchers call this the cortisol awakening response, and the commonly cited average is a rise of roughly 50 percent, though it varies quite a bit from person to person.
Then it declines across the day and reaches its low point again overnight so that you can sleep.
Which means if you measured your cortisol at 7 in the morning and again at 10 at night, you would be looking at two completely different numbers from a body that is working exactly as it should. The level on its own tells you almost nothing without knowing what time it was taken.
Why the Timing Matters More Than the Level
A spike is not a problem. When you have a hard conversation, run for a train, or brake to avoid an accident, cortisol rises and then comes back down. That is the system doing its job. Athletes produce large cortisol spikes in hard training sessions and are not harmed by them. I produce them every time I get on the bike.
What researchers look at instead is the shape of the curve across the whole day. When the morning rise is blunted and the evening decline never quite happens, the curve flattens out. A 2017 analysis pooling 80 studies found that flatter daily slopes were associated with worse health across most of the outcomes examined, with the strongest signal in inflammation. Those are associations rather than proof, and the effects are modest, but the direction is consistent.
So the useful question about your own cortisol is not whether it goes up. It is whether it comes back down.
What Actually Helps
Here are some basic things you can do to help with the rhythm. Wake at roughly the same time each day, weekends included, since an irregular wake time blurs the morning surge. Get outside in daylight within an hour of waking, which is the strongest signal your body has for setting its clock. And protect the back half of your evening, because alcohol and short sleep both interfere with the decline that should happen before bed.
Cortisol is not something to fight. It is something to stay in rhythm with.
Impactful Quote of the Week
"I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived."
- Henry David Thoreau
All my best,
Brian Hoeflinger
P.S. - Please reply to this email and let me know if you knew this about cortisol or what other topics you would like me to cover. I strive to read and reply to as many as I can.
Medical Trivia Answer:
The correct answer is D) About 65 percent
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*Disclaimer: This newsletter and blog is for general informational purposes only and does not constitute the practice of medicine, nursing, or other professional health care services, including the giving of medical advice, and no doctor/patient relationship is formed. The use of information on this newsletter and blog or materials linked from this newsletter and blog is at the user’s own risk. The content of this newsletter and blog is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Users should not disregard or delay seeking medical advice for any medical condition they may have and should consult their healthcare professionals for any such conditions.
