The Owlet sock measures blood oxygen the same way a hospital pulse oximeter does: it shines two wavelengths of light through your baby’s foot and calculates oxygen saturation based on how much light the blood absorbs. The sensor sits against the skin on one side of the foot, and a detector picks up the light that passes through from the other side.
Red and Infrared Light Through the Foot
Inside the fabric sock is a small sensor module containing two LEDs and a photodetector. One LED emits red light at 653 nanometers, and the other emits infrared light at 931 nanometers. Both operate at very low power, under 10 milliwatts each. The light passes through the skin, tissue, and blood vessels of your baby’s foot before reaching the photodetector on the opposite side.
The key principle is that oxygenated and deoxygenated blood absorb these two wavelengths differently. Oxygen-rich hemoglobin absorbs more infrared light and lets more red light pass through. Oxygen-poor hemoglobin does the opposite, absorbing more red light. By comparing the ratio of red to infrared light that reaches the detector, the device calculates the percentage of hemoglobin carrying oxygen. This value is called SpO2, or peripheral oxygen saturation.
The sensor also detects the natural pulsing of blood through the arteries with each heartbeat. That pulsation is what separates the signal of arterial blood (the blood you actually care about) from the constant background absorption of skin, bone, and venous blood. The timing between pulses also gives the device your baby’s heart rate.
How the Sock Processes Raw Data
The sensor samples data at 1 Hz, meaning it takes a reading once per second. But a single reading can be noisy, especially on a squirming infant. To smooth things out, the Owlet uses a 10-second averaging window. It also applies a threshold for motion artifact: if the baby is moving too much for the signal to be reliable, the device filters that data out rather than reporting a potentially inaccurate number.
This smoothing is a tradeoff. It reduces false alarms from brief signal disruptions, but it also means the oxygen reading you see on your phone isn’t truly instantaneous. It reflects roughly the last 10 seconds of usable data. The FDA-cleared version of the Dream Sock displays live pulse rate alongside a 10-minute average oxygen saturation, giving a broader picture of trends rather than second-by-second fluctuations.
Why Sock Placement Matters
For the light to travel through the foot correctly, the LEDs and the photodetector need to be on opposite sides of the tissue. Owlet designs the sock with a notch that goes behind the pinky toe on the side of the foot. The toe strap wraps around the foot above the toes, and the ankle strap secures around the back of the ankle and across the top of the foot. The sensor module has a corresponding notch that should line up with the one in the sock fabric.
If the sock is too loose, the sensor can shift and the light may not pass through the foot’s blood vessels properly. If it’s too tight, it can compress the tissue and reduce blood flow to the area, which also distorts readings. Correct alignment is what makes the difference between a clean signal and a string of false notifications.
What Triggers a Notification
Healthy infants typically have oxygen saturation in the mid-to-upper 90s. The Owlet sock is designed to send a notification if the reading drops below 80%, which is well below the normal range and would suggest a meaningful drop in oxygen. The device also alerts for heart rate values outside a preset zone. These thresholds were validated through clinical studies evaluating how often they produced actionable alarms versus false ones.
The FDA Clearance Story
The oxygen monitoring feature has had a complicated regulatory path. The original Smart Sock 3 displayed live oxygen and pulse rate readings with red alerts, but in late 2021 the FDA determined it should be classified as a medical device. Owlet pulled that version from the U.S. market and launched the Dream Sock in early 2022 as a “wellness device” that initially limited its oxygen display to a 10-minute average without medical-style alerts.
On November 8, 2023, the FDA granted De Novo clearance to the Dream Sock with Health Notifications, making it the first over-the-counter infant pulse rate and oxygen saturation monitor to receive that classification. The cleared version is approved for in-home use on healthy infants and can display live SpO2 and pulse rate values while sending notifications when readings fall outside preset zones. The underlying measurement technology, red and infrared pulse oximetry, remained the same throughout these regulatory changes. What changed was whether the device was legally permitted to show you the data and act on it.