Sleep occurs in cycles of approximately 90 minutes, and waking up in the middle of a deep sleep cycle often leaves you feeling groggy, while waking up between cycles tends to feel more refreshing. This sleep cycle calculator helps you plan your bedtime or wake time based on these 90-minute cycles, suggesting optimal times to fall asleep or wake up to align with the end of a complete sleep cycle. It's useful for anyone wanting to feel more rested, whether you're setting an alarm for the morning or calculating what time to go to bed given a fixed wake-up time. While individual sleep cycles vary somewhat, this calculator provides evidence-based guidance for better sleep timing.