Sleep is essential for good health, and we spend up to roughly one-third of our lives asleep. Quality sleep contributes to brain health, emotional balance and physical recovery. [1] Chronic sleep deprivation is linked to many serious health problems such as cardiovascular diseases, metabolic disorders (e.g., diabetes), a weakened immune system, a decline in cognitive functions as well as an increased risk of mental health issues such as depression and anxiety. According to ResMed’s 2025 Global Sleep Survey [2], about one-third of the respondents reported difficulty falling or staying asleep three or more times per week and rated only four out of seven nights as restful.
Despite greater awareness of the importance of sleep, there is a silent disruptive factor that is often overlooked: the air we breathe at night. And it is frequently worse than thought. In modern, air-tight homes, CO₂ levels in bedrooms can rise rapidly. Levels of 1,000 ppm are reached after just one hour. In the morning, levels of 3,000 to 5,000 ppm are not uncommon in poorly ventilated rooms. This means that CO₂ concentrations are way above the recommended ranges specified in common building and IAQ standards. [3 to 7]
Unlike poor sleep, noise or light, which we immediately feel and perceive, elevated CO₂ levels are invisible and odorless. The majority of people are, therefore, unaware that this is negatively impacting their sleep. But scientists agree on one thing: Inadequate ventilation leads to increased CO₂ levels and possibly other biological risk factors and air pollutants. This is clearly linked to sleep inefficiency and reduced deep sleep and impaired cognitive performance the next day. The complete causal relationship between CO₂ levels in the environment and sleep quality is still being investigated. However, the latest findings already point to several mechanisms through which elevated CO₂ levels during sleep can negatively impact the body – from stimulation of respiration to metabolic and neurological stress responses.
CO₂ and cognitive performance
Sleep and cognitive performance are closely intertwined. Decreased sleep results in increased memory problems, poor decision-making abilities and emotional dysregulation. Inadequate ventilation and high CO₂ levels in indoor air not only impair sleep but can also have a direct impact on brain health.
In studies where the participants slept in rooms with reduced CO₂ exposure (from ~2,500 ppm to ~900 ppm), they showed improved verbal reasoning skills, better working memory and higher alertness the next morning. They also reported feeling emotionally refreshed and mentally clearer. [8] Even during waking hours, the ability to make decisions declines significantly at CO₂ levels of ~1,000 ppm and reaches an almost dysfunctional level at 2,500 ppm, a CO₂ concentration frequently observed in classrooms, offices and motor vehicles. Activities that required strategic thinking, initiative and the use of information were particularly affected. [9]
CO₂ is, therefore, not only a comfort parameter but also a tangible control quantity for demand-based ventilation and a simple proxy for nighttime interior space air quality.
More intelligent sensors for a healthier life
Sensors allow users to monitor air quality in real time, optimize HVAC systems (heating, ventilation and air conditioning) and thus create a healthier indoor climate. In the development stage, the question is not whether CO₂ is measured, but how it is measured.
Two technical approaches dominate:
- Photoacoustic NDIR (PA-NDIR): A modulated infrared beam hits CO₂ molecules, whose vibrations generate pressure fluctuations. These fluctuations are picked up by an MEMS microphone. Advantages include a high level of selectivity, long-term stability and strict compliance with standards.
- Thermal conductivity (TC): In this case, the change in thermal conductivity of the gas mixture due to the CO₂ content is measured. Advantages include an extremely compact design, low energy consumption and reduced costs. A disadvantage, on the other hand, is the lower selectivity toward gas mixtures.
Both approaches are well established but differ in terms of their integration level, form factor and suitability for standards compliant IAQ applications (indoor air quality).
Sensors for various integration scenarios
Sensirion offers an array of sensors. The SCD41 and SCD43 sensors are based on non-dispersive infrared photoacoustic technology, ensure a high level of accuracy and are ideal for certified buildings that want to satisfy green standards such as WELL, LEED, RESET and Fitwel. These sensors can be integrated into smart thermostats, air cleaners and building management systems to enable demand-driven ventilation and ensure optimal indoor air quality (see Figure 1).
When it comes to consumer electronics, the STCC4 sensor is a cost-effective solution with direct CO₂ measurement based on the principle of thermal conductivity. It can be integrated into alarm clocks, smartphones, laptops and even wearable health monitors, thereby allowing users to track their exposure to poor air quality both day and night (see Figure 2).
SCD41 – the compact all-rounder
- Measuring range: 400 – 5,000 ppm
- Accuracy: ± (50 ppm + 2.5% of reading) up to 1,000 ppm, ± (50 ppm + 3% of reading) up to 2,000 ppm, ± (40 ppm + 5% of reading) up to 5,000 ppm
- Current consumption: ~15 mA for measurement interval operation
- Supply: 2.4 – 5.5 V
- Energy-saving and single shot modes available
- Integrated temperature and humidity measurement for compensation
- Interface: I²C
- Size: 10.1 × 10.1 × 6.5 mm³
- Typical applications: DCV regulations, thermostats, IAQ monitors
SCD43 – standards compliant precision
- Measuring range: 400 – 5,000 ppm
- Improved accuracy: ± (30 ppm + 3% of reading)
- Certification: optimized for WELL, LEED, RESET, ASHRAE
- Identical footprint to SCD41 (10.1 × 10.1 × 6.5 mm³)
- Comparable power consumption to SCD41
- Single shot operation possible
- Typical applications: Building automation, certified air quality measuring devices and demand-controlled ventilation (DCV), upgrades in line with stricter standards
STCC4 – ultra-compact TC sensor
- Measuring range: 400 – 5,000 ppm
- Accuracy: ± (100 ppm + 10% of reading)
- Size: 4 × 3 × 1.2 mm³ (SMD compliant, tape and reel)
- Current consumption: ~950 µA at 1 Hz measuring rate
- Interface: I²C
- Τ63% response time: ~20 s
- Factory calibrated, simple integration
- Typical applications: Wearables, smart speakers, mobile devices and cost-sensitive IAQ products, thermostats
- Advantage: Particularly suitable for close-to-the-battery and cost-critical designs
The combination of CO₂ readings with ventilation rates, sleep parameters and cognitive performance indicators enables the use of sensors for predictive health management. If the air quality deteriorates, users can be warned without delay and take the appropriate measures. Intelligent sensor technology thus creates the basis for interior spaces that actively adapt to human needs and support health and performance in the long term.
Selection, integration and practical aspects
The selection of a suitable CO₂ sensor depends largely on the application scenario. In the field of building automation, high measuring accuracy, long-term stability and compliance with standards are essential, particularly for DCV applications and certified air quality devices. While in the smart home and consumer sector, compact designs, low power consumption and simple integration into battery-powered systems play a key role. Short response times and reduced energy consumption are also crucial for wearables and mobile devices. Accordingly, the current sensor portfolio has suitable solutions for every market segment:
- SCD41: universal solution for compact devices with a high demand for accuracy
- SCD43: for applications that need to meet strict standards and certification requirements
- STCC4: when a small footprint and low power consumption are decisive
The accuracy and long-term stability of the readings depend crucially on the level of integration into the target system. Recommended:
- Location in a lightly ventilated area with a clearly defined airflow route
- Avoidance of temperature hot spots (e.g., due to power supplies or power electronics)
- Utilization of automatic baseline and drift compensation (ASC) with PA-NDIR sensors
- Combination with temperature and humidity sensors for precise compensation
- Regular cross-checking of readings during field operation
With PA-NDIR sensors (SCD41, SCD43), fast control loops can be achieved when using short, periodic measurement intervals. In contrast, single shot operation is suitable for power-saving telemetry. TC sensors such as the STCC4, on the other hand, benefit from exact environmental compensation in order to minimize the influence of other gases or temperature gradients. This makes the CO₂ level a robust reference variable for ventilation strategies, be it in certified buildings or in mobile applications that have a direct impact on health, sleep quality and cognitive performance.
Summary
The CO₂ sensor is a technical tool that has a direct impact on health, performance and comfort. The SCD4x platform ensures precise, standards compliant solutions for building automation. The STCC4 augments the portfolio for mobile and ultra-compact applications. This allows for the implementation of demand-based ventilation – from smart home to wearable level.
References
[1] Sergio Garbarino et. al., 2021. Role of sleep deprivation in immune-related disease risk and outcomes
[2] Resmed’s 2025 Global Sleep Survey, 2025, Resmed
[3] ANSI/ASHRAE Standard 62.1, European Standard EN 13779, German norm DIN 1946-2 | Batog, P. et al. 2013. “Dynamic of Changes in Carbon Dioxide Concentration in Bedrooms”
[4] WELL V2. 2024. “Enhanced Ventilation Design”
[5] USGCB LEED V5. September 2023. “Rating System, Building Operations + Maintenance: Existing Buildings”
[6] RESET V2.0. 2018. “2.4. RESET Air Standard for Core & Shell“
[7] FITWEL V2.1 Standard. July 2021 “Reference Guide for the Fitwel Certification System”
[8] Peter Strøm-Tejsen et. al., 2014. The Effect of CO₂ Controlled Bedroom Ventilation on Sleep and Next-Day Performance
[9] Satish, U. et al., 2012. Is CO₂ an Indoor Pollutant? Direct Effects of Low-to-Moderate CO₂ Concentrations on Human Decision-Making Performance