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What is a sleep cycle?
Your brain cycles through different stages of sleep roughly every 90 minutes. A full night typically contains 4–6 complete cycles. Waking up mid-cycle leaves you groggy — waking at the end of one feels natural and refreshed.
Stage 1 · Light
Transition into sleep. Easily woken. Lasts 1–7 minutes.
Stage 2 · Light
Heart rate slows, body temperature drops. Most of the night is spent here.
Stage 3 · Deep
Most restorative sleep. Hard to wake. Body repairs tissue and builds immunity.
REM · Dreaming
Brain is active. Memory consolidation and emotional processing happen here.
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Why 15 minutes is added
On average it takes a healthy adult around 10–20 minutes to fall asleep. This calculator adds 15 minutes to your bedtime so the cycles are counted from when you actually enter sleep, not when your head hits the pillow.
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How many cycles do you need?
6 cycles (9 hrs) is ideal for full recovery. 5 cycles (7.5 hrs) is the sweet spot most adults can realistically hit. Fewer than 4 cycles consistently will affect concentration, mood and physical recovery.
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REM increases through the night
Early cycles contain more deep (Stage 3) sleep — important for physical recovery. Later cycles contain more REM — important for memory and mood. This is why cutting sleep short hits your mental sharpness harder than it hits your body.
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Caffeine and sleep cycles
Caffeine blocks adenosine receptors — the chemical that makes you feel sleepy. It has a half-life of roughly 5–6 hours, meaning a 3pm coffee still has half its caffeine in your system at 9pm. This doesn't stop you sleeping but significantly reduces deep sleep quality.
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Screens before bed
Blue light from phones and screens suppresses melatonin production — the hormone that signals it's time to sleep. Even 30 minutes of screen-free time before bed can reduce the time it takes to fall asleep and improve sleep depth.
💡 Pro tip: Consistency matters more than duration. Going to bed and waking at the same time every day — even weekends — stabilises your circadian rhythm and makes waking up easier regardless of how many cycles you get.