Hmm I do appreciate the complexity involved in implementing a max number of hours per day you can grow a star. But I think it may be worth it. The more you clamp down on ways to exploit the system the more player's will respect their achievement.

The minimum of one hour is easy to circumvent. I wake up, turn off the alarm and then set another for 12 hours from now..

Theoretically if this is to track healthy sleeping then 6-8 hours max per day is the target. If users start logging 23 hours of sleep a day you probably shouldn't be rewarding them unless its a 'Master Sloth' achievement. haha

Maybe a simple solution is to prevent the player from receiving constellation growth unless at least X hours have passed since the last alarm completed? X could be a hardcoded 8 hours or maybe its the size of the previous alarm's duration?

Do you plan on having leaderboards or ways to share your constellation? If not I think that could be a feature to make the app more viral. I don't know if it would ever happen but imagine people sharing Nebula links in their Tinder bio as a way to signal healthy living...? Maybe there is a world where you pair users based on their constellation similarities. Could Nebula evolve into a dating app because maybe 'sleep patterns are actually one of the best indicators of compatibility' ? Probably far fetched and poisoning to the core idea but thought I'd share in case it plants any seeds for you.