Can chronic sleep fragmentation and insomnia blunt the morning cortisol peak and cause daytime fatigue?

The normal circadian cortisol rhythm is characterized by a sharp morning peak—the cortisol awakening response (CAR)—followed by a gradual decline throughout the day to a nadir at bedtime. Chronic sleep fragmentation and insomnia disrupt this delicate balance, causing significant hypothalamic-pituitary-adrenal (HPA) axis dysregulation.

Circadian cortisol dysregulation

• Blunted diurnal slopes: Polysomnography and actigraphy studies show that chronic sleep fragmentation and short sleep duration elevate pre-sleep and evening cortisol. This evening elevation impairs sleep efficiency, creating a bidirectional feedback loop that further fragments sleep.
• Suppressed morning peak: Over time, this chronic stress state flattens the diurnal cortisol curve. Rather than a steep, healthy morning peak, individuals experience lower waking cortisol levels and an attenuated CAR, indicating a functional HPA axis downregulation.

Fatigue and HPA axis mechanisms

• Allostatic load: A flattened diurnal cortisol slope represents a breakdown of central circadian coordination. Cortisol is essential for glucose regulation and daily energy mobilization; its blunting serves as a key biomarker of high cumulative allostatic load.
• Daytime fatigue: Clinical studies in chronic stress, burnout, and Chronic Fatigue Syndrome (CFS/ME) demonstrate that a flattened diurnal slope and mild basal hypocortisolism correlate directly with persistent daytime fatigue, non-restorative sleep, and low subjective energy.

Bottom line

• Chronic sleep fragmentation and insomnia disrupt the HPA axis, leading to elevated evening cortisol, a flattened diurnal slope, and low morning cortisol peaks, which directly contribute to daytime fatigue.