Does cortisol reach its lowest levels near midnight and then rise in the second half of the night to prepare you to wake up?

The human body operates on a highly synchronized internal clock that dictates metabolic and hormonal activity. A central feature of this system is the circadian rhythm of cortisol, a primary hormone produced by the hypothalamic-pituitary-adrenal (HPA) axis.

Clinical and physiological evidence

Cortisol follows a predictable 24-hour cycle that is robustly supported by clinical data. In healthy individuals, cortisol levels decline throughout the day, reaching their lowest point—the "nadir"—around midnight or the first few hours of sleep.

• The Midnight Nadir: Clinical practice frequently uses midnight as the standard reference point for the circadian low. Research establishing reference ranges for healthy adults has identified midnight salivary cortisol levels at approximately 0.21 ± 0.03 µg/dl.
• Waking Preparation: Following this nadir, cortisol levels begin a steady ascent during the second half of the night. This rise occurs while the individual is still asleep and is not merely a response to the alarm clock, but an endogenous process.
• The Awakening Response: This preparatory rise culminates in the Cortisol Awakening Response (CAR), a sharp increase of 50% to 75% in cortisol levels that occurs 30–45 minutes after waking, which provides the final metabolic push for the day’s activities.

Mechanistic explanations

The orchestration of this rhythm is managed by the suprachiasmatic nucleus (SCN), the brain’s master pacemaker.

• SCN Regulation: The SCN uses light signals from the retina to synchronize the HPA axis. Near midnight, the SCN minimizes HPA activity to facilitate deep, restorative sleep.
• Energy Mobilization: As the night progresses, the SCN signals the adrenal glands to increase cortisol production. This prepares the body for arousal by stimulating gluconeogenesis (the production of glucose) and mobilizing fatty acids, ensuring that energy is readily available upon waking.
• Sleep Architecture: This rise is closely tied to Rapid Eye Movement (REM) sleep, which predominates in the second half of the night. Cortisol levels naturally fluctuate in tandem with these sleep stages to support cognitive and physical readiness.

Factors impacting the rhythm

While the baseline rhythm is genetically encoded, it is sensitive to external factors.

• Sleep Disruption: For women in midlife, hormonal shifts (such as those in perimenopause) can disrupt sleep continuity, which may result in elevated evening cortisol or a dampened morning response.
• Environmental Cues: Shift work, artificial blue light exposure late at night, and high stress can shift the timing of the nadir, potentially delaying the preparatory rise and impacting morning alertness.

Bottom line

The claim is fully supported: cortisol reaches its lowest point near midnight and rises during the latter half of the night to mobilize energy and prepare the body for the transition from sleep to wakefulness.