Sleep Cycle Calculator

To wake up feeling refreshed, you should aim to complete 5-6 full sleep cycles. The times below are based on a 90-minute cycle and include 15 minutes to fall asleep.

How is the Best Sleep Time Calculated?

This calculator works on the principle that our sleep is not one long, continuous state. Instead, we cycle through different stages of sleep—from light sleep to deep sleep and then REM (Rapid Eye Movement) sleep.

• The 90-Minute Cycle: One full sleep cycle lasts approximately 90 minutes. Waking up in the middle of a deep sleep cycle can make you feel groggy and tired, a feeling known as sleep inertia.
• The Goal: The key to waking up refreshed is to wake up at the *end* of a sleep cycle, when your body is in a much lighter stage of sleep.
• The Calculation: The calculator takes your desired wake-up time and works backward in 90-minute increments to find the best times to fall asleep. It also adds a 15-minute buffer, as this is how long it takes the average person to fall asleep. Similarly, if you know your bedtime, it calculates the best wake-up times by adding cycles to it.

The Surprising History of Our 8-Hour Sleep

The idea of a single, uninterrupted 8-hour block of sleep is actually a relatively modern invention. For much of human history, a pattern of "biphasic sleep" was the norm.

Historians and researchers have found countless references in literature, court records, and diaries from pre-industrial Europe describing a "first sleep" and a "second sleep." People would go to bed shortly after dusk, sleep for about four hours, and then wake up for a one-to-two-hour period in the middle of the night. This waking period was used for everything from prayer and quiet reflection to chores and socializing with neighbors. Afterward, they would go back to bed for their "second sleep" of another few hours. It was the advent of industrialization and affordable artificial lighting that consolidated our sleep into the single block we consider normal today.

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