9to5 Mac writes that it has heard from several Apple Watch Series 3 users that their watches are randomly rebooting when visiting, or working in, the intensive care units (ICUs) of hospitals.
According to the 9to5 Mac reader who reported about the issue, his wife got an Apple Watch Series 3 for Christmas and started using it to check messages while working in the ICU. She found that the Apple Watch started rebooting approximately every 60-90 minutes when she was in the ICU. She got the Apple Watch replaced, but that did not solve the problem.
Over the past few days other reports of similar issues have surfaced, and there has been a thread on the matter at Apple Support since as far back as October.
As yet there has been no official response from Apple about this, or any official solution put forward, but apparently using the watch in Airplane Mode seems to stop the rebooting, but it also stops you receiving alerts, of course, so not the ideal solution.
Source: Some Apple Watch users are reporting unexpected reboots possibly caused by ICU equipment