For a patient who goes to bed at 7 pm, should the late‑night salivary cortisol sample for Cushing syndrome screening be collected at midnight by staying awake, or at the bedtime of 7 pm?

Collect Saliva at 7 PM Bedtime, Not at Midnight

For a patient whose usual bedtime is 7 pm, the late-night salivary cortisol sample should be collected at their actual bedtime (7 pm), not at midnight. This approach reduces false positive results and aligns with the physiological basis of the test.

Rationale Based on Guidelines

The 2021 Lancet Diabetes & Endocrinology consensus guideline explicitly addresses this scenario: "Sampling saliva at usual bedtime rather than at midnight could decrease false positive results, as cortisol nadir is tightly entrained to sleep onset" 1. This is the most recent and authoritative guidance on this specific question.

The diagnostic utility of late-night salivary cortisol depends on detecting the loss of normal circadian rhythm in Cushing syndrome patients. In healthy individuals, cortisol reaches its nadir shortly after sleep onset, not at a fixed clock time 1. Therefore:

• The test measures cortisol at the expected physiological nadir
• This nadir occurs with sleep onset, not at a specific hour
• Collecting at midnight when the patient normally sleeps at 7 pm would miss the true nadir

Supporting Evidence

Multiple studies confirm that bedtime collection is appropriate:

• A 2002 study specifically used "bedtime for outpatients" rather than fixed midnight timing, achieving 93% sensitivity for Cushing syndrome detection 2
• Pediatric research demonstrated that bedtime salivary cortisol (not midnight) successfully screened for Cushing syndrome 3
• A 2021 pediatric validation study used "bedtime salivary cortisol" as the standard approach 4

Critical Caveat

This recommendation does NOT apply to shift workers or anyone with disrupted day/night cycles 1. The test should not be performed in these populations as the circadian rhythm disruption invalidates the results.

Practical Implementation

• Have the patient collect saliva at their usual 7 pm bedtime
• Ensure at least 2-3 samples on different nights 1
• The patient should be following their normal sleep schedule during collection
• Avoid the test if the patient has recently changed sleep patterns

This bedtime-based approach maximizes specificity while maintaining the high sensitivity needed for Cushing syndrome screening.