Nurse Niki

The library

How does stress physically cause fatigue?

Cortisol follows a daily rhythm — highest in the morning, tapering by night. Prolonged stress can flatten or shift that curve, which is felt as being exhausted in the morning and alert at bedtime. It also interacts with thyroid and sex hormones, so a disrupted rhythm rarely stays a single problem.

When someone tells me they are tired but wired, or that they get a second wind at ten at night and then cannot sleep, I am interested in cortisol.

The mechanism is not mysterious. Cortisol is meant to rise sharply on waking and fall through the day. Sustained demand — work, caregiving, illness, poor sleep, undereating, over-exercising — can blunt that morning rise or push the fall later, and the subjective experience of that is exactly what people describe.

It is also why stress does not stay in its lane. Cortisol influences thyroid conversion and sex hormone balance, so a flattened rhythm can drag other systems with it. That is part of why I would rather look at hormones together than one at a time.

I want to be careful with language here: this is ordinary physiology under load, not a diagnosis, and there are conditions of genuine adrenal disease that are different and require different care.

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