I have been setting up some RESTful sensors for Home Assistant and to verify/fine tune their functionality, it dawned on me that I could use watch to continuously poll my endpoint to watch for a value change:

watch curl --no-progress-meter http://domain.tld/path/to/sensor/

If you're familiar with watch, you'll know that the default "polling rate" will be every 2 seconds. This can be altered by passing in the option -n x, where "x" is the number of seconds you'd like between command runs.