This is because I am usign the free tier and will need to either upgrade to using the paid tier / use one of the app available to make requests to the app at regular intervals.
Tried Kaffeine(http://kaffeine.herokuapp.com/) and it doesn't seem to be working. Looking into the others.
Additional info:
It takes about 5 mins for the app to come up when a request is sent to it while asleep.But this doesn't match up with Slack's response times, so definitely need some sort of cron job/scheduler running, all while ensuring the app sleeps for 6 hours a day.