The Android app "Sleep as Android" [1][2] is awesome as well. Uses ultrasound to detect your movement/breathing and wakes you up at the correct part of the sleep cycle, even if your calculation goes wrong due to falling asleep later.
I always thought that 8 hours was the best amount of time to sleep but so often I'd still wake up feeling groggy and tired.
Sometimes I'd go to bed at 1AM and wake up around 7AM and feel so refreshed and awake. What's up with that? Turns out, getting 8+ hours of sleep is less important than waking up aligned with your body's sleep cycles, which for most people is every 90 minutes.
I created Wake Up Time so I could quickly see the best wake up times before setting my alarm each night. Because time math is hard... Especially when you're tired.
[1]: https://sleep.urbandroid.org/
[2]: https://play.google.com/store/apps/details?id=com.urbandroid...