Electroencephalography (EEG) is a non-invasive technique to record electrical activity in the brain. EEG is mostly used in diagnosis, such as epilepsy.
Curious Cyborg informative section on wearables; from smart watches, disability aids and wearable clinical monitoring equipment
The lights on the bottom of your smart watch are used for monitoring your heart rate. Some smart watches may even use these lights to also measure how well oxygenated your blood is. So, to be effective at measuring your heart rate and or blood oxygen saturation, your watch should fit well and not hang loose