Cortisol is your main stress hormone, and it follows a strong daily rhythm: high within 30 to 45 minutes of waking, low around midnight. If you want to know what stress does to your body, a cortisol measurement at a fixed morning moment is the starting point. Your blood value shows whether your stress system is in balance or whether the load is starting to undermine your recovery.
I notice athletes often see cortisol as the enemy. More cortisol is bad, so as low as possible is good. It is not that simple.
Cortisol is a healthy, necessary hormone that gets you out of bed and drives your training. It is about the pattern, not about a single number.
This guide walks through your daily rhythm, shows how and when to measure cortisol, and links high and low to what you can do about it.
What is cortisol and what does it do?
Cortisol is a hormone your adrenal glands make under the control of your brain, the so-called HPA axis. It regulates your blood sugar, your blood pressure, your inflammatory response and your energy through the day. Without cortisol you do not get out of bed, with too much cortisol you sleep poorly and recover slowly.
For an athlete cortisol is two-sided. During a session it rises to release energy, and that is exactly the point.
The problem only starts when the load stays structurally too high. Then your system gets no chance to settle back to rest.
Cortisol works together with other hormones, such as testosterone and DHEA. The balance between build-up and breakdown sets how well you recover.
Which complaint points to which blood value?
Stress complaints rarely hang on cortisol alone, but a pattern can point you to the right value. Poor sleep, growing belly fat, or fatigue after a hard block each point to a different corner of your stress system. The table below links your complaint to the value to check and the in-depth guide.
| Complaint or pattern | Check this blood value | Read on |
|---|---|---|
| Wired and restless, hard to fall asleep | Morning cortisol, evening cortisol | High cortisol |
| Growing belly fat despite the same training | Cortisol, glucose, insulin | Lower cortisol |
| Drained, cynical, empty after months of pressure | Cortisol, DHEA-S | Recognise burnout |
| Performance drops, tired after hard training blocks | Cortisol, CK, testosterone | Recognise overtraining |
| Poor sleep and daytime energy dips | Cortisol rhythm, glucose | Sleep and your hormones |
Below I work through these corners, starting with the daily rhythm that connects them all.
The daily rhythm of cortisol: why timing is everything
Cortisol has a strong daily rhythm, and that rhythm is exactly what makes it hard to measure. Your value peaks shortly after waking and falls the rest of the day to a low point in the evening. Measure at the wrong moment, and you read a number that says little.
That morning peak is called the cortisol awakening response. Research does not fully agree yet on how that peak arises, whether it comes from waking itself or continues your underlying rhythm (Velazquez Sanchez, 2025).
For you as an athlete the practical lesson is simple. A single cortisol value without a time is almost worthless.
What does work is measuring at a fixed moment, so your measurements stay comparable. That way you build a trend instead of loose snapshots.
How and when do you test cortisol?
You preferably measure cortisol in blood in the early morning, usually between 8 and 9 am, when the peak occurs. Draw at a fixed time and under calm conditions, because stress just before the draw can lift your value temporarily. Avoid a measurement right after a hard session.
If you want the evening rhythm in view, a second measurement later in the day is useful. The difference between morning and evening says more than a single morning number.
Besides blood there are saliva tests and 24-hour urine tests, which track the rhythm across the day. For a first picture, a morning blood measurement is often a logical starting point.
High cortisol: what do your blood values say?
A raised cortisol usually points to ongoing load, whether that is work, sleep loss or too much training. You recognise it in trouble falling asleep while tired, a wired feeling and sometimes growing belly fat. A single high morning value is not immediately a problem, but a persistent pattern is a signal.
The context matters. A high value after a period of deadlines or a heavy competition block is partly logical.
It becomes more concerning when the value stays high while your life has calmed down. Then your system no longer seems to settle back to rest.
If you recognise a wired, driven feeling, read high cortisol for the symptoms and what your blood values show exactly.
Low cortisol: exhaustion instead of overdrive
A too-low cortisol gets less attention, but it belongs to chronic stress just as much. After a long period of overload your system can flatten out, and then you see a lower morning peak instead of a higher one. You then feel empty and drained rather than wired.
This pattern often plays out later in the story. Where early stress often goes with raised cortisol, the picture shifts toward a flattened rhythm with long-term exhaustion (Kudielka, 2006).
For an athlete this is tricky, because a low value can feel like calm while you are actually running on your reserves.
A genuinely low cortisol value always needs medical review, because an underlying cause can sit behind it. So discuss a deviating value with your GP.
Cortisol, belly fat and insulin: the metabolic side
Cortisol touches not only your mood, but also your fat distribution and your blood sugar. Persistently raised cortisol is associated in research with more belly fat and reduced insulin sensitivity (Drapeau, 2003). That explains why stress sometimes seems to hold on to stubborn belly fat.
The mechanism is logical. Cortisol releases blood sugar to prime you for action, but without that action the sugar keeps circulating.
For a trained athlete this is an underrated corner. You can train hard and still hold belly fat if your stress and sleep do not move with you.
If you want to see this, read your glucose and insulin alongside your cortisol. In lower cortisol you read which adjustments really have an effect and what you can measure of it.
Overtraining versus burnout: where cortisol overlaps
Overtraining and burnout resemble each other, but they are not synonyms. Overtraining comes from an imbalance between training load and recovery, burnout from prolonged mental and emotional exhaustion. Cortisol plays a role in both, but the blood values are rarely black and white.
In overtraining the resting hormones are often still normal, and only under exertion do you see deviating responses (Cadegiani, 2017). A single resting cortisol measurement therefore does not always catch overtraining.
That is exactly why you never read cortisol on its own. You place it next to your performance, your sleep, your mood and your training load.
If your fatigue sits mainly in your training, read recognise overtraining. If it feels broader and more mental, then recognise burnout is the better starting point.
The DHEA-S and cortisol ratio: build-up versus breakdown
Cortisol works catabolically, DHEA-S works anabolically, and the ratio between the two says something about your balance. A high cortisol with a low DHEA-S points to a system that breaks down more than it builds up. That ratio is therefore sometimes used as a rough measure of chronic stress load.
For an athlete this is interesting because recovery is about that balance. Training breaks down, rest and sleep build up.
The ratio is not a diagnosis and has considerable individual variation. See it as an extra puzzle piece, not a verdict.
So read a cortisol-DHEA ratio next to your complaints and your training history, and discuss a striking result with your doctor.
Cortisol and sleep: the biggest lever
Sleep and cortisol influence each other in both directions. Poor sleep lifts your cortisol the next day, and a raised evening cortisol makes falling asleep harder. That creates a vicious circle that erodes your recovery step by step.
For most athletes sleep is the cheapest and strongest lever on their cortisol. Before supplements or adaptogens comes simply sleeping consistently and enough.
Poor sleep also touches your blood sugar, which worsens your daytime energy dips. We describe that connection in sleep and your hormones.
If you want to tackle your cortisol, start with your sleep before you change anything else. Measure your rhythm, improve your nights, and measure again.
What if your cortisol is normal but you still have complaints?
A cortisol within the reference range does not rule out stress complaints. Reference values are broad population averages, and a snapshot only half-captures your rhythm. You can have a normal morning value and still have a disturbed day-night pattern.
According to Thuisarts and the NHG guidelines, blood testing is used in a targeted way for complaints, not as a loose check of everything at once. Testing cortisol is therefore mainly useful if you have a concrete complaint or question.
My advice is simple. Follow the pattern across the day, not a single number.
If your complaints persist while your lifestyle is sound, discuss with your GP whether further testing makes sense. They weigh your whole picture.
What you can do about your cortisol yourself
The biggest gain sits not in a pill, but in your daily load and recovery. Consistent sleep, rest moments, and enough recovery between hard sessions do more for your cortisol than any supplement. Measuring helps you see whether your adjustments really have an effect.
Start with the basics. Fixed bedtimes, less caffeine late in the day, and a deload week after a hard block.
Be critical of the promise of adaptogens and cortisol supplements. The evidence is mixed, and you better discuss supplementation before you dose high yourself.
If you want to steer based on numbers, read lower cortisol, where we go into what works and what you can measure of it.
Which blood test do you choose to map your stress level?
To really understand your stress system you look wider than cortisol alone. You place it next to your thyroid, your testosterone, your blood sugar and your recovery markers, because together they tell the story. A broad hormone panel gives you that overview in one go.
For most athletes a broad test is a logical starting point. Our 360 Health blood test combines these systems in one measurement, so your cortisol does not stay isolated.
If you want to zoom in on your hormones specifically, our general hormones test is a more targeted option.
If you prefer to test in a targeted way, draw the single value that matches your complaint from the table at the top.
How often should you test your cortisol?
For most athletes without complaints, measuring occasionally around a heavy period is enough. If you notice stress complaints and adjust, a retest after eight to twelve weeks is useful to see whether your rhythm has improved. That way you build a trend instead of loose numbers.
Preferably draw at the same time each time and under comparable conditions. Only then are two measurements really comparable.
Around an intense competition season or a busy work period an extra measurement can help. That is exactly when your cortisol moves most.
Common mistakes when testing cortisol
Most mistakes sit not in the measurement, but in the interpretation. Anyone who maps their stress level seriously often runs into the same traps.
The first mistake is measuring without a fixed time. Without the time, a cortisol value is barely readable.
The second mistake is being alarmed by a single high value. A peak after a busy week is partly normal, and only a persistent pattern is a signal.
The third mistake is reading cortisol apart from the rest. You always place it next to your sleep, your training and your other hormones.
Frequently asked questions
The questions I get back most from athletes who want to measure their stress level.
How do you test cortisol? You preferably measure blood cortisol in the early morning, usually between 8 and 9 am, at a fixed time. For the daily rhythm a second measurement later in the day or a saliva test can be useful.
Can a cortisol test be done without a referral? Yes, you can have a blood test done yourself. Every result is assessed by a BIG-registered doctor, and for treatment decisions you go to your GP.
Do I need to fast for a cortisol test? For cortisol itself that is not strictly necessary, but draw calmly and at a fixed moment. If you measure glucose and insulin at the same time, fasting is wise.
Is a high cortisol always bad? No, cortisol is meant to be high in the morning and during exertion. It is about the pattern across the day, not a single number.
What does a low cortisol mean? A low cortisol can belong to prolonged exhaustion, but a medical cause can also sit behind it. So a genuinely low value you always discuss with your GP.
Can stress explain my belly fat? Persistently raised cortisol is associated with more belly fat and lower insulin sensitivity. It is rarely the only cause, so read your values next to your nutrition, sleep and training.
Is cortisol the same in overtraining and burnout? No, the patterns overlap but differ. In overtraining resting values are often still normal, while prolonged exhaustion can flatten the rhythm.
References
- Velazquez Sanchez C, Dalley JW. The cortisol awakening response: Fact or fiction? Brain and Neuroscience Advances. 2025. PMID: 40297522.
- Cadegiani FA, Kater CE. Hormonal aspects of overtraining syndrome: a systematic review. BMC Sports Science, Medicine and Rehabilitation. 2017. PMID: 28785411.
- Drapeau V, Therrien F, Richard D, et al. Is visceral obesity a physiological adaptation to stress? Panminerva Medica. 2003. PMID: 14618117.
- Kudielka BM, Bellingrath S, Hellhammer DH. Cortisol in burnout and vital exhaustion: an overview. Giornale Italiano di Medicina del Lavoro ed Ergonomia. 2006. PMID: 19031555.
- Thuisarts.nl / NHG. Blood testing. Accessed 2026.
- RIVM. Population figures and reference values. Accessed 2026.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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