In this era where robustness is in high demand, relying on medical professionals for your regular health checkup is a bit tedious. One of the major concerns in today’s world is the stress experienced by a user, which can have adverse effects on health, also known as cognitive stress, psychological stress or psychosocial stress. There are a lot of devices present to monitor and record rudimentary information of a user’s physiology. Groundbreaking and advanced technologies such as Polyplethysmography (PPG) which provides a wide spectrum of features which can be used to observe physiological changes and compute the stress level of a person. Use of wearable devices in health monitoring has increased exponentially over the past few years. In all these devices PPG sensor has been a key component. In this paper uses of PPG sensors are discussed for obtaining values for parameters such as blood flow, heartbeat, oxygen consumption etc. These features are further used to derive complex features, e.g. heartbeat is used to get heart rate variability which in turn can be used to detect sleep stages. Other sensors in smart watches can provide skin conductance which when collaborated with features like body temperature can provide hydration level of the body. In this paper, multiple algorithms and state-of-the-art researches that use PPG technology in wearables to monitor the above mentioned features are mentioned. It is discussed that stress can be detected using sleep history, hydration, heart rate variability and oxygen consumption.