Sleep Quality Calculator
Optimise your sleep by calculating the ideal bedtime or wake-up time based on 90-minute sleep cycles so you never wake mid-cycle groggy again.
Frequently Asked Questions
Why are sleep cycles 90 minutes long?
A complete sleep cycle progresses through four stages: N1 (light sleep, 5%), N2 (light sleep, 45%), N3 (deep/slow-wave sleep, 25%), and REM (rapid eye movement, 25%). Each cycle takes approximately 90 minutes. Waking mid-cycle — particularly during deep N3 sleep — causes sleep inertia (that groggy, disoriented feeling). Waking at the end of a cycle, when sleep is naturally lightest, makes rising much easier.
How many hours of sleep do I actually need?
The CDC and sleep experts recommend: teenagers (14–17) 8–10 hours, adults (18–64) 7–9 hours, older adults (65+) 7–8 hours. Sleeping 5–6 cycles (7.5–9 hours) is optimal for most adults. Chronically sleeping under 6 hours is linked to impaired cognition, weakened immunity, weight gain, cardiovascular disease, and a 3× higher risk of catching a cold. Only ~3% of people are genuine "short sleepers" who thrive on 6 hours due to a rare genetic mutation.
What is the best time to wake up for maximum energy?
The ideal wake time depends on when you fell asleep plus a multiple of 90 minutes. For example: asleep at 11:00 pm → ideal wake times are 6:00 am (7 cycles/7.5 hours) or 7:30 am (6 cycles/9 hours). Waking in alignment with your circadian rhythm (shortly after sunrise for most people) also helps. Consistency matters most — keeping the same wake time 7 days a week anchors your body clock.
What are the stages of sleep and why do they matter?
N1 and N2 (light sleep) consolidate memories and restore energy. N3 (deep sleep) is physically restorative — it repairs tissue, strengthens the immune system, and releases growth hormone. REM sleep is cognitively restorative — it processes emotions, consolidates learning, and fuels creativity. N3 sleep is concentrated in the first half of the night; REM in the second half, which is why cutting sleep short disproportionately reduces REM.
How can I improve my sleep quality?
Evidence-based tips: maintain a consistent sleep-wake schedule (even weekends); keep your bedroom cool (18–20°C/65–68°F), dark, and quiet; avoid screens 1 hour before bed (blue light suppresses melatonin); cut caffeine after 2 pm (caffeine half-life is 5–7 hours); avoid alcohol within 3 hours of bedtime (alcohol reduces REM sleep quality); exercise regularly but not within 2 hours of bed.