Can insomnia cause nighttime anxiety and palpitations even when morning cortisol is normal?

The physiological state of chronic insomnia is increasingly understood as a disorder of 24-hour hyperarousal, where the sympathetic nervous system (SNS) remains overactive during the night, regardless of whether standard hormonal markers like cortisol appear elevated in the morning.

Clinical evidence and manifestations

Research indicates that individuals with chronic insomnia demonstrate objective markers of SNS dominance during sleep. This sympathetic overdrive is characterized by:

• Cardiovascular symptoms: Studies using muscle sympathetic nerve activity (MSNA) and heart rate variability (HRV) analysis show that patients with insomnia have a higher low-frequency to high-frequency (LF/HF) ratio, indicating a failure of the parasympathetic "rest and digest" system to take over. This results in an increased mean heart rate and perceived palpitations or a "racing heart."
• Psychological symptoms: This lack of autonomic recovery and blunted baroreflex sensitivity increases stress reactivity. The hyperarousal theory posits that nocturnal anxiety is a core pathogenic driver of the disorder, reinforced by this physiological feedback loop.

Mechanistic explanations

The manifestation of these symptoms despite "normal" morning cortisol is explained by the functional decoupling of the SNS and the hypothalamic-pituitary-adrenal (HPA) axis:

• Differential timescales: The SNS responds rapidly via catecholamine release (norepinephrine and epinephrine), causing immediate physical sensations like a racing heart. In contrast, the HPA axis produces a slower, sustained cortisol response.
• System decoupling: In chronic sleep disruption, nocturnal sympathetic activity (often measured by salivary alpha-amylase or norepinephrine) can be significantly elevated while cortisol rhythms are disrupted, phase-shifted, or have reduced amplitude. Consequently, a single morning cortisol measurement may fall within the "normal" clinical range while the patient remains in a state of intense nocturnal sympathetic arousal.

Bottom line

Insomnia-related sympathetic overactivation frequently presents as nighttime anxiety and palpitations. These symptoms are driven by elevated nocturnal catecholamines and can occur even when morning cortisol is normal, as the sympathetic and hormonal stress systems often operate independently during chronic sleep loss.