# Night shifts and the body clock: what a sleep evaluation looks at

Sleep timing, light exposure and work schedules help explain night-shift symptoms. Where cortisol and melatonin measurements fit.

Canonical: https://velri.co/blog/cortisol-melatonin-circadian-biomarkers-night-shift-nurse
Publisher: Velri (https://velri.co/about)
Published: 2026-06-22
Updated: 2026-09-30
Editorial policy: https://velri.co/editorial-policy

Night work can put required wakefulness and sleep out of step with the body's internal timing. That mismatch can make daytime sleep difficult and reduce alertness during a shift. [1][2]

A sleep evaluation starts with the pattern: work hours, sleep periods, symptoms and the environment. The assessment may include a sleep diary and selected tests. [3]

## Light helps set the schedule

Circadian rhythms influence sleep, hormone release, digestion and other functions across the day. Light and darkness provide strong cues to the system that coordinates those rhythms. [1]

Melatonin release is linked to the light-dark cycle and the approach of sleep. Cortisol also follows a daily pattern associated with waking. Artificial light, caffeine and changing schedules can affect how sleep timing lines up with these signals. [2]

Working through the night and trying to sleep after sunrise changes several cues at once. A free morning does not necessarily coincide with the body's strongest drive to sleep.

## The diary often supplies the missing context

NHLBI describes a sleep history that includes when, how long and how well someone sleeps, when symptoms began, caffeine use and nighttime light exposure. A provider may ask for a sleep diary if those patterns are unclear. [3]

For shift workers, workdays and days off can look very different. Recording both helps show whether the problem follows the schedule or persists across different sleep opportunities.

An examination and other history may also point to causes that disrupt sleep, such as pain or possible sleep apnea. [3]

## What testing can add

Actigraphy uses a worn motion sensor to help estimate sleep-wake patterns. A sleep study can examine sleep and the body's responses in more detail. [3]

In some evaluations, repeated measurements of melatonin, cortisol or body temperature help characterize the timing of the internal clock. The pattern over time is the relevant information; a single hormone value provides only one point in it. [3]

The resulting discussion can be specific to the person's schedule and symptoms. Persistent sleepiness, concentration problems or difficulty sleeping deserve that assessment, especially when they affect safe work or travel.

## Sources
[1] Circadian Rhythms | National Institute of General Medical Sciences. https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx
[2] How Sleep Works - Your Sleep/Wake Cycle | NHLBI, NIH. https://www.nhlbi.nih.gov/health/sleep/sleep-wake-cycle
[3] Circadian Rhythm Disorders - Diagnosis | NHLBI, NIH. https://www.nhlbi.nih.gov/health/circadian-rhythm-disorders/diagnosis

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