Cortisol
| Clinical | Optimal | |
|---|---|---|
| Male | 166 - 507 nmol/L (morning 08:00-10:00) | 166–507 nmol/L (morning sample) |
| Female | 166 - 507 nmol/L (morning 08:00-10:00) | 166–507 nmol/L (morning sample) |
What is Cortisol?
Cortisol is a steroid hormone produced by the adrenal glands in response to signals from the hypothalamus and pituitary gland — a system known as the HPA axis. Often called the 'stress hormone,' cortisol plays essential roles that extend far beyond the stress response, including regulating blood sugar, metabolism, inflammation, and the sleep-wake cycle.
Cortisol follows a strong diurnal rhythm: levels are highest in the early morning (helping you wake up and feel alert) and gradually decline throughout the day, reaching their lowest point around midnight. This pattern is crucial for healthy energy levels and restorative sleep.
Because of this daily fluctuation, the timing of a cortisol blood test matters significantly. Morning samples (typically taken between 8:00 and 10:00 AM) are standard for assessing baseline adrenal function.
Why Cortisol Matters for Your Health
Cortisol is one of the most important biomarkers for understanding your stress resilience and overall metabolic health. In the short term, cortisol is protective — it mobilises energy, sharpens focus, and modulates the immune response. However, chronic elevation or dysregulation of cortisol is strongly linked to accelerated ageing and numerous health concerns.
Persistently elevated cortisol is associated with central obesity, insulin resistance, impaired immune function, bone loss, muscle wasting, poor sleep, and cognitive decline. It can also suppress other vital hormones including thyroid hormones, testosterone, and DHEA-S.
Conversely, insufficient cortisol production can lead to profound fatigue, low blood pressure, and an inability to cope with physical or emotional stress. For anyone focused on longevity, optimising cortisol regulation — rather than simply lowering it — is key to long-term health.
Cortisol& Your Wearable Data
Cortisol is the body's primary stress hormone, following a strong circadian rhythm: highest in the early morning and lowest at midnight. Your wearable provides extensive cortisol-relevant data. Heart rate variability (HRV) is an excellent proxy for cortisol status: low HRV patterns on your wearable typically correlate with elevated cortisol from physical or psychological stress.
Exercise is a potent cortisol stimulator, with the magnitude proportional to intensity and duration tracked by your wearable. Moderate exercise briefly elevates cortisol, which is healthy, but chronic overtraining (identifiable through wearable data showing excessive training load, declining HRV, elevated resting heart rate, and poor recovery scores) can lead to sustained cortisol elevation with harmful metabolic and immune consequences.
Sleep is critical for cortisol regulation. Wearable sleep data showing insufficient sleep, poor sleep quality, or irregular sleep timing is associated with disrupted cortisol rhythms. Night shift work or jet lag, trackable through wearable sleep-wake patterns, can flatten the cortisol curve, with levels remaining elevated when they should be low. Optimising sleep consistency on your wearable is one of the most effective strategies for healthy cortisol patterns.
What High Cortisol May Suggest
Elevated morning cortisol may suggest chronic stress, Cushing's syndrome, or adrenal overactivity. High cortisol can also result from acute illness, significant sleep deprivation, or excessive alcohol consumption. Certain medications, including oral corticosteroids and oestrogen-containing contraceptives, can raise cortisol-binding globulin and affect results.
Chronic cortisol elevation may be associated with weight gain (particularly around the midsection), difficulty sleeping, anxiety, elevated blood sugar, high blood pressure, and frequent infections due to immune suppression.
If results consistently suggest elevated cortisol, your GP may recommend further testing such as a 24-hour urinary cortisol, late-night salivary cortisol, or a dexamethasone suppression test to investigate further.
What Low Cortisol May Suggest
Low morning cortisol may suggest adrenal insufficiency (Addison's disease), secondary adrenal insufficiency from pituitary problems, or what is sometimes referred to as HPA axis dysfunction following prolonged stress. Results should be interpreted alongside clinical symptoms.
Symptoms associated with low cortisol can include severe fatigue, dizziness upon standing, salt cravings, unexplained weight loss, darkening of the skin (in primary adrenal insufficiency), and difficulty recovering from illness or exercise.
Low cortisol is a potentially serious finding. If your morning cortisol falls below 100 nmol/L, further investigation with a Synacthen stimulation test may be recommended to assess adrenal reserve.
How to Optimise Your Cortisol
Food
Support healthy cortisol regulation with a balanced, anti-inflammatory diet. Include complex carbohydrates from sweet potatoes, oats, and brown rice, which help modulate cortisol through steady blood sugar regulation. Magnesium-rich foods such as dark leafy greens, almonds, and dark chocolate support the stress response. Vitamin C-rich foods like bell peppers, kiwi, and citrus fruits help the adrenal glands recover. Omega-3 fatty acids from oily fish reduce the inflammatory effects of cortisol. Avoid excessive caffeine, particularly after midday, as it can amplify cortisol release.
Lifestyle
Establish a consistent sleep schedule, aiming for 7–9 hours of quality sleep to restore the cortisol circadian rhythm. Practise daily stress-reduction techniques such as meditation, deep breathing, or mindfulness — even 10 minutes can measurably reduce cortisol. Engage in moderate exercise (walking, swimming, yoga) rather than chronic high-intensity training, which can elevate cortisol. Spend time in nature, as studies show that 20 minutes outdoors can lower cortisol. Limit screen time before bed and reduce exposure to artificial light in the evening.
Supplements
Ashwagandha (KSM-66 or Sensoril extracts) has robust clinical evidence for reducing cortisol by 20–30% in chronically stressed individuals. Phosphatidylserine (300–800 mg daily) has been shown to blunt excessive cortisol response, particularly after exercise. L-theanine from green tea extract promotes calm without drowsiness and may help modulate the cortisol response.
When to Speak to Your GP
See your GP if morning cortisol is significantly elevated or low, particularly if accompanied by symptoms. Seek urgent attention if you experience severe fatigue, dizziness, fainting, unexplained weight loss, or skin darkening, as these may suggest adrenal insufficiency. Persistent features of high cortisol such as central weight gain, purple stretch marks, easy bruising, and muscle weakness should be investigated promptly.
References
- NHS. Addison's disease — Diagnosis. Updated 2024. nhs.uk
- NICE. Cushing's syndrome — Clinical Knowledge Summaries. cks.nice.org.uk
- Endocrine Society. Nieman LK, et al. The diagnosis of Cushing's syndrome. J Clin Endocrinol Metab. 2008;93(5):1526-1540. pubmed.ncbi.nlm.nih.gov
- Lancet. Newell-Price J, et al. Cushing's syndrome. Lancet. 2006;367(9522):1605-1617. pubmed.ncbi.nlm.nih.gov
Medical Disclaimer— This content is for general educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Omniwo Ltd is a wellness information service and is not a medical device, clinical laboratory, or regulated healthcare provider under MHRA guidelines. The “optimal ranges” presented on this page are based on published clinical guidelines (WHO, NICE, NHS) and peer-reviewed research; they represent functional wellness targets and may differ from standard laboratory reference ranges. Individual results should always be interpreted by a qualified healthcare professional (such as your GP) who understands your full medical history. Do not start, stop, or change any medication or supplement based solely on this information. If you are experiencing symptoms, seek medical attention promptly.