Short answer
Cortisol is a hormone from the adrenal glands that follows a strong daily rhythm: lowest around the start of night-time sleep and rising sharply to a peak shortly after you wake. That morning surge helps mobilise energy and alertness for the day. After crossing time zones, cortisol keeps peaking on home time, which adds to daytime grogginess.
Simple explanation
If melatonin is the body’s night-time signal, cortisol is closer to a morning one. It is often described as a “stress hormone”, and it does rise in response to stress, but its steadiest job is timekeeping. Every day it bottoms out around the time you fall asleep and climbs to a peak in the first half hour or so after you wake, helping switch the body from rest into active mode. You feel this as the difference between the fog of the first minute awake and being properly up and going soon after.
Because this rhythm is set by the same master clock that runs everything else, it does not adjust the moment you land somewhere new. Your cortisol can be surging at what is now the middle of the night, or flat when the local day needs you sharp.
Deep dive
The daily cortisol curve was mapped in detail by Weitzman and colleagues in 1971: plasma cortisol reaches its lowest point (its nadir) around the onset of night sleep, begins rising in the second half of the night, and peaks roughly 30 to 45 minutes after waking – a feature later studied in its own right as the “cortisol awakening response.” Levels then decline gradually across the day back toward the night-time low.
This rhythm is orchestrated by the master clock acting through the hypothalamic-pituitary-adrenal (HPA) axis – a chain of signals from the brain down to the adrenal glands. That makes cortisol a genuine output of the circadian system, alongside melatonin and core body temperature, and it is one reason the whole system takes days rather than hours to shift: each of these outputs has to be dragged around to the new schedule.
Cortisol also interacts with how tired you feel. Sleepiness at any moment reflects both the clock and a separate sleep-pressure system. When cortisol is peaking on home time but the destination says it is late evening, the “wake-up” signal is firing at the wrong moment – and when it is flat during the local morning, the push into alertness that normally helps you start the day is missing.
Illustration (planned graphic)
A single 24-hour curve (inline SVG) tracing cortisol: a low, flat stretch around the start of night sleep (the nadir), a steepening climb through the last hours of sleep, a labelled peak about half an hour after waking (the cortisol awakening response), and a slow decline across the day. Placing it on the same clock face as the melatonin band from the melatonin page shows the two hormones as near-opposites – melatonin high at night, cortisol high in the morning. Data-accurate SVG, since the timing of the peak is the point.
What it means for travel
You cannot dose your own cortisol, and you should not try – it is driven by the clock. The practical move is the same as for the rest of the system: shift the whole clock, and the cortisol rhythm follows. Getting bright light at the right local time and anchoring meals and activity to the destination day helps the peak migrate to local morning over several days. Understanding cortisol mainly helps you interpret the symptoms: the wave of grogginess at the wrong time of day, or a jolt of alertness in the small hours, is partly this hormone still keeping home time.
For the full symptom picture see jet lag symptoms, and to estimate how many days your rhythm needs to catch up, use the jet lag calculator.
Common questions
Is cortisol just a stress hormone?
No. It rises with stress, but it also follows a strong daily rhythm set by the body clock, peaking after waking and helping drive daytime alertness and metabolism.
When is cortisol highest?
Shortly after you wake, typically within about 30 to 45 minutes – the cortisol awakening response. It is lowest around the beginning of night-time sleep.
Does jet lag change my cortisol?
Jet lag does not change how much cortisol you make so much as when it peaks. The rhythm stays on your old time zone at first and takes days to realign, which contributes to feeling alert or foggy at the wrong local hours.
Sources
- Weitzman ED, Fukushima D, Nogeire C, et al. (1971). “Twenty-four hour pattern of the episodic secretion of cortisol in normal subjects.” Journal of Clinical Endocrinology & Metabolism, 33(1). Mapped the daily cortisol curve.
- Pruessner JC, Wolf OT, Hellhammer DH, et al. (1997). Work characterising the cortisol awakening response – the rise in free cortisol in the first ~30-45 minutes after waking.
- National Institute of General Medical Sciences (NIGMS/NIH). “Circadian Rhythms” fact sheet – hormonal outputs of the body clock.
- Sleep Foundation – general educational overview of hormones and the sleep-wake cycle.
The shape and timing of the daily cortisol rhythm are well established from the peer-reviewed sources above; broader links to alertness reflect standard, widely accepted physiology.