We presented a wearable smart ring, which can provide accurate and long-term information about blood oxygen saturation. Duty-cycling of sensor activities in combination with onboard processing and solar energy harvesting, ensure self-sustainability, which was demonstrated in realistic scenarios. By measuring the blood oxygenation once every minute with a sampling rate of 100 samples/s, accurate results were achieved with a daily energy consumption of only 28 Joule, including hourly BLE transmissions of processed measurement results. This allows the system to be self-sustainable with just 64 minutes of sunlight per day or 12h of indoor home light.