Is this the altitude, or just a bad flight?
Denver Altitude Sickness Symptoms
The tricky thing about altitude sickness in Denver is that it doesn't announce itself. There's no moment where you know. You land, you feel fine, you go out for dinner — and then somewhere in the next day, you've got a headache that won't shift, you're exhausted, and you're wondering whether it's the elevation, the flight, the drinks, or something you ate.
Usually it's the elevation, and usually the pattern gives it away. Here's what Denver altitude sickness symptoms actually look like, how they differ from a hangover or ordinary dehydration, and the short list of symptoms that mean stop reading and get help.
What does altitude sickness feel like?
Most people land on the same description without prompting: it feels like a hangover you didn't earn.
The core symptoms are:
- Headache — the defining one. Usually dull and pressing rather than sharp, often behind the eyes or across the forehead, and it doesn't respond as well as you'd expect to a glass of water.
- Fatigue — heavy, disproportionate tiredness. Sleep doesn't fix it.
- Nausea and appetite loss — mild for most people; occasionally enough to stop you eating.
- Breathlessness on exertion — a flight of stairs, a slight hill, carrying a bag. At rest you feel normal.
- Poor sleep — the first night is frequently broken, sometimes with a sensation of waking up short of breath.
- Light-headedness — particularly standing up quickly.
You don't need all of them. Headache plus any one other symptom, within a day of arriving from lower ground, is the usual working definition.
The timeline: 6 to 24 hours after you land
Symptoms rarely start immediately. The typical onset is 6 to 24 hours after arrival, which is why so many people blame the previous evening rather than the altitude.
Hours 0–6. Usually nothing. Some people feel slightly winded or notice their heart working a bit harder.
Hours 6–24. The window where it shows up. Headache builds, energy drops, sleep that first night is often poor.
Day 2. Frequently the peak for the headache, then improvement begins.
Day 3. Most people have stopped noticing.
If you're still deteriorating on day two or three rather than improving, that's not the normal pattern and it's worth medical advice.
Can you get altitude sickness in Denver?
Yes — and this genuinely surprises people, because Denver isn't a mountain town. At 5,280 feet it sits below the elevation where serious altitude illness becomes common, which is roughly 8,000 feet. But it's high enough that a meaningful share of visitors arriving from sea level feel it.
Who tends to feel it most:
- Anyone flying in from near sea level, especially in one hop
- People who've had altitude sickness before — the strongest single predictor
- Anyone who arrives and immediately exercises, drinks, or heads higher
- People sleeping poorly or arriving already dehydrated after a long travel day
Fitness doesn't protect you. Age doesn't reliably predict it either. Some very fit people feel awful and some sedentary ones feel nothing.
Altitude sickness, hangover, or dehydration?
They overlap heavily, and you can easily have all three at once.
| Altitude sickness | Hangover | Dehydration | |
|---|---|---|---|
| Timing | 6–24 hrs after arriving | Morning after drinking | Any time |
| Headache | Dull, pressing, persistent | Throbbing, eases through the day | Improves with fluids |
| Breathless on stairs | Yes | No | Not usually |
| Improves with water alone | Partly | Partly | Yes, reliably |
| Improves with a day's rest at the same elevation | Yes | Yes | Yes |
| Worsens if you go higher | Yes — the key tell | No | No |
The distinguishing test: altitude symptoms get worse when you ascend and better when you descend. Nothing else on this list behaves that way.
How long do symptoms last in Denver?
For most people, one to three days, with the first 24 hours worst. Full physiological acclimatization takes considerably longer — one to three weeks — but you stop feeling it well before that.
The detailed timeline is here: How Long to Acclimate to Denver's Altitude
Symptoms that are not ordinary altitude adjustment
Get emergency care — call 911 or go to an ER — for:
- Breathlessness while resting, not just on exertion
- Confusion, disorientation, or unusual drowsiness
- Trouble walking straight or loss of coordination
- A cough producing frothy or pink sputum, or chest tightness
- Blue or grey lips or fingertips
- Oxygen saturation persistently below 88–90% on a pulse oximeter, with symptoms
These may indicate high-altitude pulmonary edema (HAPE) or cerebral edema (HACE). They are rare at Denver's elevation and far more common higher up — but they are emergencies wherever they occur, and they are not treated with rest, water, or an IV.
Never take sleeping pills or sedatives when you have altitude symptoms — they suppress breathing at exactly the wrong moment.
What actually helps
The short list, in order of how much difference it makes:
- Don't go higher. Stay at your current elevation until symptoms improve.
- Rest. No hard exertion for the first day or two.
- Skip alcohol and sedatives.
- Drink steadily — normal amounts, not forced volumes.
- Eat carbohydrates.
- Standard painkillers — ibuprofen or acetaminophen, if they're safe for you.
- Descend if you're not improving. It's the only universally reliable treatment.
Full prevention plan: How to Avoid Altitude Sickness in Denver
If you've been vomiting or can't keep fluids down, replacing them directly is a reasonable option — with real limits worth understanding first: IV Therapy for Altitude Sickness
Frequently asked questions
Why do I feel weird in Denver?
Most likely mild altitude adjustment — slightly lower blood oxygen combined with dry air and travel dehydration. Headache, tiredness, broken sleep and feeling winded on stairs are the usual pattern in the first day or two.
Is altitude sickness in Denver dangerous?
Mild altitude sickness is unpleasant but not dangerous, and it resolves on its own. The dangerous forms — HAPE and HACE — are rare at 5,280 feet, but the red flags above are worth knowing, especially if you're continuing into the mountains.
Can altitude sickness start days after arriving?
Uncommon. Symptoms that begin after 48 hours at the same elevation are more likely something else, and worth a doctor's opinion.
Do children get altitude sickness?
Yes, and they may not describe it clearly. Watch for unusual irritability, poor sleep, appetite loss and lethargy in young children after arriving. ---
Can't keep fluids down?
A critical-care nurse can come to you anywhere in Denver. From $194, visit included.