CO2 levels rise at night because the main process that removes carbon dioxide from the air, photosynthesis, shuts down when the sun goes away. At the same time, every living organism on Earth keeps breathing and releasing CO2 around the clock. This imbalance between steady production and zero removal causes CO2 to accumulate from sunset to sunrise, both outdoors and especially indoors.
Why Photosynthesis Only Works During the Day
Plants absorb CO2 and convert it into sugars using sunlight. This process, photosynthesis, is the planet’s largest CO2 sink and only operates when light is available. Once darkness falls, that absorption stops completely.
But plants don’t stop needing energy at night. They continue breaking down stored sugars to fuel growth and cellular maintenance, a process called respiration that releases CO2 as a byproduct. So plants go from being net CO2 absorbers during the day to net CO2 emitters at night. Research published in Nature confirms that despite the halt in carbon assimilation in the dark, plant demand for carbon from respiration and growth persists through the night, supported by pools of stored sugars built up during daylight hours.
This swing is significant. In a forest or a densely vegetated area, the difference between daytime and nighttime CO2 concentrations near ground level can be 50 to 100 ppm or more, peaking just before dawn when the cumulative overnight output is highest and photosynthesis hasn’t yet restarted.
Everything Else Keeps Breathing
Plants aren’t the only contributors. Soil microbes, fungi, insects, and animals all respire continuously, releasing CO2 day and night. During the day, photosynthesis offsets much of this output. At night, there’s no offset at all, so the CO2 simply accumulates in the surrounding air.
Atmospheric mixing also plays a role. During the day, the sun heats the ground, creating rising air currents that carry CO2 upward and disperse it through a tall column of atmosphere. At night, the ground cools and the air near the surface becomes stable and still, forming what meteorologists call a temperature inversion. CO2 released near the ground gets trapped in this shallow, stagnant layer rather than mixing into the broader atmosphere, concentrating it close to where you’d actually breathe it.
How Indoor CO2 Climbs While You Sleep
The nighttime CO2 rise is most dramatic indoors, especially in bedrooms. An average adult exhales about 11 liters of CO2 per hour while sleeping (men produce roughly 8% more than women). In a closed bedroom, that output has nowhere to go.
Outdoor air typically sits around 420 ppm of CO2. A bedroom with the door closed and no window cracked can climb well past 1,000 ppm within a few hours. One observational study tracking real bedroom conditions found an average CO2 concentration of 1,194 ppm across participants, with the highest readings exceeding 1,500 ppm. Two people sharing a room with poor ventilation can push levels even higher.
How Elevated CO2 Affects Your Sleep
These numbers aren’t just an air quality curiosity. Bedroom CO2 concentrations above 1,000 ppm measurably degrade sleep. In a controlled study comparing nights at 750 ppm versus 1,000 ppm, participants at the higher concentration lost 1.3% in sleep efficiency and spent an extra 5 minutes awake during the night. At 1,300 ppm, the effects worsened: sleep efficiency dropped by 1.8%, time awake increased by nearly 8 minutes, deep sleep duration decreased, and morning cortisol levels rose, a sign of increased stress.
Higher CO2 also correlates with spending more time in light sleep stages rather than the deeper, more restorative ones. Above roughly 1,200 ppm, some research suggests breathing rate slows and brief interruptions in breathing become more frequent. Participants who slept in higher-CO2 conditions also performed worse on cognitive tests the following day.
Simple Ways to Keep Bedroom CO2 Lower
The most effective fix is also the simplest: increase airflow. Opening a bedroom window even a few inches can prevent CO2 from climbing past 1,000 ppm in most cases. Leaving the bedroom door open has a similar, though somewhat smaller, effect by allowing CO2 to disperse into the rest of the home rather than pooling in one room.
If noise, outdoor air quality, or temperature make an open window impractical, a mechanical ventilation system or even a small fan drawing hallway air can help. The target to aim for is keeping average overnight CO2 below 1,000 ppm. Inexpensive CO2 monitors (around $30 to $80) can show you exactly what’s happening in your bedroom and help you figure out how much ventilation you actually need. Most people are surprised by how quickly levels climb in a sealed room, and how little airflow it takes to fix the problem.