Two nights of exactly seven hours of sleep can feel completely different depending on where in a sleep cycle the alarm actually goes off, since waking up mid-cycle during deep sleep tends to feel groggier than waking at a natural cycle boundary, even with the same total hours logged. This tool does more than subtract a bedtime from a wake time: it also works out how many complete roughly ninety-minute sleep cycles fit into that window, which is the number that actually predicts how rested you're likely to feel. Enter when you fell asleep and when the alarm went off, and see both the raw duration and how many full cycles it lines up with. Useful for figuring out whether a rough night was actually short on time or just badly timed against a cycle boundary, or picking a better bedtime for tomorrow's wake-up time.