What time should you go to bed to sleep well?

What is a sleep cycle?

Your night isn't a uniform block of rest. It's organized into sleep cycles that repeat 4 to 6 times between going to bed and waking up. Each cycle lasts about 90 minutes and takes you through three distinct phases, each with a specific role for your body and brain.

Picture your night as a series of waves: each wave gradually carries you toward deeper and deeper sleep, before rising back to a lighter phase. It's between two waves — between two cycles — that waking is most natural and least difficult.

The 3 phases of a sleep cycle

Each 90-minute cycle breaks down into three phases that follow one another in a precise order:

1. Light sleep (about 50% of the night) — This is the transition phase. Your breathing slows, your muscles relax, but a noise is still enough to wake you. It's a kind of gateway toward restorative sleep.

2. Deep sleep (about 20-25% of the night) — This is the most restorative phase. Your body regenerates: muscles recover, the immune system strengthens, and growth hormone is released. If your alarm rings during this phase, you feel "foggy" — this is called sleep inertia. Deep sleep is mostly present early in the night and decreases across the cycles.

3. REM sleep (about 20% of the night) — This is the dream phase. Your brain becomes as active as when awake, your eyes move rapidly under the eyelids, but your muscles are as if paralyzed. This phase consolidates your memories, sorts your emotions and boosts creativity. Unlike deep sleep, REM sleep lengthens through the night: your last cycles before waking contain the most.

Why waking between two cycles changes everything

This is the core principle of this calculator. Waking between two cycles — when sleep is at its lightest — lets you come out of sleep gently. Conversely, waking in the middle of a cycle, especially during deep sleep, causes that feeling of intense fatigue even after a long night.

That's why sleeping 7h30 (5 full cycles) can leave you more rested than sleeping 8h if waking falls in the middle of deep sleep. It's not only duration that matters, it's the moment of waking.

How many sleep cycles per night?

• 4 cycles (6h): the functional minimum, enough occasionally but not advised long term
• 5 cycles (7h30): the optimal duration for most adults
• 6 cycles (9h): ideal during recovery, illness, or for teenagers

Why time to fall asleep matters

Most sleep calculators ignore an important detail: you don't fall asleep the second you put your head on the pillow. Sleep onset latency — those minutes when you're lying down but still awake — varies from 5 to 30 minutes depending on the person. Our calculator factors it in so the suggested times match your reality.

The importance of consistency

Research in chronobiology shows that consistent sleep timing matters more than total duration. Going to bed and getting up at the same time, including on weekends, synchronizes your biological clock and improves the quality of each cycle. A shift of more than two hours on the weekend is enough to create a "social jet lag" that disrupts falling asleep on the following days.