Hi
It is not normal for your Apple Watch to restart itself randomly.
The following steps may help (check after each):
Check that you are not accidentally force restarting your Apple Watch:
- If your watch is worn too close to your hand, the Digital Crown and side button may be pressed and held down by the back of your hand when flexing your wrist.
- It may help to wear your watch a little further up your arm and/or with the buttons on the other side:
- To change the configuration: on your iPhone, in the Watch app, go to: My Watch (tab) > General > Watch Orientation > choose the preferred wrist and side for the buttons.
If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:
- Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
- Update the software on your Apple Watch - Apple Support
Restart both your iPhone and your Apple Watch:
- Close the Watch app, Activity app and Health app (if open) on your iPhone:
- Double-click the Home button, then swipe up on the app preview.
- Turn both devices off together, then restart your iPhone first:
Unpair and re-pair your Apple Watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
- If the issue persists, try unpairing and setting up as a new watch, monitoring performance as you add third party apps and/or watch face complications back to your Apple Watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support
If the issue then persists, then I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: