It's important to note that "sleeping while inactive" is not quite paying for what you use. These Dynos will start up to serve a request, and stay active constantly until they receive no traffic for 30 minutes.

This means, if you want to run a small cron job once an hour, you'll still pay for half a month of usage. Similarly, a very small amount of traffic spread evenly will still use up these hours quickly. It also doesn't come with any Postgres, that's another $5.

$5-10 a month isn't a lot, but it's very different to Free, and if you're going to pay it might be worth considering hobbyist-friendly alternatives. A $5 VM goes a long way for services with a little traffic or a bunch of hourly cron jobs.