Symptom · mornings

Why do I wake up with a headache?

Quick answer: rule out one dangerous gas first; then look at your bedroom's ventilation, because a closed bedroom routinely passes 2,500 ppm of CO₂ by morning — and a cracked window measurably improved sleep in controlled research.

First, the non-negotiable: a recurring headache that improves when you leave the house is a classic sign of carbon monoxide — a different gas from CO₂, colorless, odorless, and lethal. If you have any fuel-burning appliance, fireplace, or attached garage and no CO alarm, that $25 alarm comes before anything else on this page.

Morning light falling on a bed near a window — Eight hours of exhaling into a fixed volume — the door decides the curve.

The stale-bedroom mechanism

You spend ~8 hours exhaling into a fixed volume. Strøm-Tejsen (2016) measured student bedrooms two ways: sealed (average ~2,585 ppm) and with an opened air path (~835 ppm). With fresher air, participants rated sleep better, felt less sleepy, and scored higher on next-day logical thinking. Reported morning symptoms — grogginess, heavy head — tracked the stale condition.

2,585 ppm

4008001,4003,000+

Average overnight CO₂ measured in sealed student bedrooms (Strøm-Tejsen 2016). Not rare — normal for a closed door and window.

Average overnight bedroom CO₂, measured (Strøm-Tejsen 2016). With the air path open, participants rated sleep better and scored higher the next day.

Reading your overnight curve

One night of logging tells you which bedroom you have:

Climb-and-plateau (steep rise, flattening near 2,000–3,000): classic sealed room — the plateau is where in-leakage finally balances your exhale. The door-ajar experiment will move this curve dramatically.
Slow drift to ~1,200: decent leakage or a bigger room; worth optimizing, unlikely to be the headache's whole story.
Flat near 600: your bedroom air is fine — spend your energy on the list below instead.

When it's not the air

Morning headaches are a crowded differential. Common non-air causes, worth ruling in or out:

Sleep apnea — especially with snoring, gasping, daytime sleepiness. This one needs a clinician; it's common and treatable.
Bruxism (teeth grinding), often with jaw soreness.
Dehydration or alcohol the evening before.
Caffeine withdrawal — the headache that lifts with the first coffee.
Pillow and neck position.

The cheap experiment that separates "bedroom air" from the rest: change only the ventilation for a week — door ajar or window cracked — and keep score.

Fixes, cheapest first

$0: door ajar beats door shut. A 5 cm gap changes the overnight curve.
$0: window cracked, weather and safety permitting (skip during smoke events).
$0: if you must seal the room (noise, allergies), leave the bedroom door open to the larger house volume instead.
~$25: CO alarm, if any fuel-burning appliance shares your walls. Non-optional.
~$60–250: a logging CO₂ monitor on the nightstand for one night answers the question for your exact bedroom, permanently.

Sources

Strøm-Tejsen P. et al. (2016). The effects of bedroom air quality on sleep and next-day performance. Indoor Air. doi:10.1111/ina.12254
CDC — Carbon monoxide poisoning: symptoms and prevention.
Context on CO₂ bands: our cited CO₂ reference.