What is the Quantified Self movement?
Quantified Self is an idea that you can measure yourself. Measure your sleep, measure your heart rate, productivity, sent-out emails, location, mood, water intake, and so on. Then you analyze it to gain insights into yourself. To improve oneself, as in self-help. The amount of data can be vast, thus it may require big-data-like analysis. The movement is made of hobbyists, early adopters and innovators.

Rise and Fall of Quantified Self
The movement began shaping itself, in 70’s in the form of personal surveillance. One blog post I remember is how the CEO of Wolfram – Stephen Wolfram, in 1990 started logging his life. However, question emerges, how much is it really insightful, valuable for self-improvement, and how much is it “data fetishism”, hobbyism as described on Wikipedia.

As the Google Trends trend currently unveils itself, the peak of the Quantified Self movement was in 2013. Since then the idea declined, the movement slowed down.

The idea lives on
However, the idea lives on. Self-tracking stands strong. The best example would be a device like Oura ring https://ouraring.com/ – ring device for tracking your sleep, or FOCI https://fociai.com/ (currently V2 (as of 02.01.2022) on Kickstarter), which helps people to stay focused by tracking breath.
What is the future of QS? One blogger on the QS forum (Eric Jain) described it as being in the low expectations cycle, in the hype cycle

Is that true? That is a question to be answered by the future, however, self-tracking will certainly live on, while the name of Quantified Self might be forgotten. There are certainly opportunities that present themselves, by solving needs for private self-tracking of medical data and productivity data.
If you are interested in this topic, here are some reading suggestions:
https://www.cs.swarthmore.edu/~jwaterman/cs97/f14/uploads/Main/qs.pdf – Quantified Self: Fundamental Disruption in Big Data Science and Biological Discovery (2013) Melanie Swan
https://www.schumacherinstitute.org.uk/download/pubs/res/202002-The-Quantified-Self-Hadi-Sami.pdf – The Quantified Self, Hadi Sami (2020) – tackles negative issues of applying the movement as well as the future and its past.
https://blogs.gartner.com/mike-gotta/your-sensored-life-an-expanded-view-of-quantified-self/ – Your Sensored Life – an expanded view of Quantified Self – Mike Gotta, 2014
https://writings.stephenwolfram.com/2012/03/the-personal-analytics-of-my-life/ – The Personal Analytics of My Life, Stephen Wolfram, 2012
As a person who really cares about the biological processes of my organism, I am really curious to see my results and learn about my physical and mental health. I am an owner of the Apple Watch, which gives me a great possibility to measure my heart rate and oxygen saturation; my daily calories burned; steps walked and minutes of exercise per day and to be honest I really enjoy checking all those results before sleep. I feel that I am more in control of my body and that I am getting to know myself better and it helps me optimize my life. But I am not quite convinced that these estimations are absolutely reliable and accurate. As far as I understand quantified self can also measure your mood and emotions, which is quite intriguing but still not fully reliable in my opinion because surely many factors can falsify and bias your results, leading to inaccuracy and even wrong judgment. Very catchy article! Thanks!