Have you ever heard of Pomodoro? It’s a great technique to help you keep track of time and stay on task during your studies or work. Read more about it on Wikipedia.
TomatoWatch is world's neatest Pomodoro timer for your watch.
All the essential features are here - configurable work and rest intervals, sound and vibration.

The App costs one time, no subscription, no data collection, no overhead, no account needed. Just a simple and reliable Pomodoro timer for your watch. It is a standalone app for your watch and does not require an iPhone.

Acknowledgements to Ilya Voronin for his macOS TomatoBar App.