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A salivary measure of your body's active stress hormone just before sleep, revealing whether cortisol has reached the low point necessary for healthy sleep onset.
The bedtime cortisol sample is the last reading of the day and should show cortisol approaching its lowest level. In a healthy diurnal rhythm, cortisol is near its floor at bedtime, allowing melatonin to rise and sleep to initiate without interference.
This time point carries particular clinical weight. In Cushing's disease, a rare but serious condition of cortisol excess, bedtime cortisol is elevated and often more than four times above the upper reference range limit, with cortisol and cortisone staying high throughout the day with no normal daily rhythm. If your bedtime cortisol is markedly elevated, further testing may be warranted. Oral hydrocortisone taken throughout the day may produce a similar pattern on testing.
High bedtime cortisol may be associated with insomnia, anxiety, blood sugar dysregulation, and difficulty falling or staying asleep. Cortisol that is elevated at the end of the day can suppress melatonin production. High waking cortisol the following morning is often associated with higher overnight cortisol levels, which may also suppress melatonin. If your report shows both high bedtime cortisol and low melatonin, the cortisol elevation may be directly contributing.
Other potential causes for elevated bedtime cortisol include psychological stress, pain, acute inflammation, caffeine or alcohol consumed earlier in the day, and blood sugar instability. Because cortisol is an acute stress response hormone, a single elevated bedtime reading could also reflect an unusually stressful collection day rather than a chronic pattern.
Always compare this reading with your bedtime cortisone. If cortisol is very high but cortisone is not, consider the possibility of hydrocortisone cream contamination. Hydrocortisone is bioidentical to active cortisol. If even a small amount of cream is on your hands when you handle the sample, it can raise free cortisol without affecting cortisone. In this case, the cortisone pattern is a more accurate representation of your true HPA axis function.
The strategies for lowering bedtime cortisol overlap with those for dinner cortisol but with added emphasis on sleep-adjacent habits. Limiting caffeine well before evening, avoiding alcohol before bed, and reducing blue light exposure support the natural cortisol decline. Calming herbs (passionflower, valerian, lemon balm, chamomile, California poppy, milky oats), GABA support, and parasympathetic nervous system practices (breath work, meditation, vagal nerve stimulation) are particularly relevant at this time of day.
Blood sugar regulation is important here. Overnight blood sugar drops can trigger cortisol release, so ensuring stable blood sugar at bedtime through appropriate meal composition may prevent cortisol from rising during sleep. Addressing chronic inflammation, thyroid issues, gut dysbiosis, and low progesterone, all of which are listed as potential contributors to insomnia and nighttime cortisol elevation, may also help.