Because subscriptions are handled by iTunes and the Apple App Store, in most instances simply logging into your iTunes account in your iPhone's settings will re-enable your Sleepiest+ subscription.
We do however also offer a "Restore Purchases" facility in the event this doesn't work right away.
To restore your purchases please follow the steps below:
Navigate to the "Home" tab.
Press the settings cog in the top-right corner of the screen.
Press "Account Settings".
Press "Restore Purchases".
Enter your iTunes credentials if prompted to do so.