Meet the WHOOP Strap 2.0: a lightweight smart wearable device that you can wear on the wrist, forearm, or upper arm to monitor your performance. It has 5 sensors to measures data 100 times per second and transmit it to the WHOOP mobile app. It measures your physiological markers and exertion based on your lifestyle to help you train better.
WHOOP and its app let you know how much sleep you will need to recover after tough workout sessions. It collects over 100MB of data every day and can store it for up to 3 days. More info is available here.