Sleep Calculator
Find the best bedtime or wake time based on 90-minute sleep cycles.
Includes 14-min average sleep onset time per cycle calculation
Sleep Quality Tips
- Keep a consistent sleep schedule — even on weekends.
- Avoid screens 30–60 minutes before bed; blue light suppresses melatonin.
- Keep your room cool (65–68°F / 18–20°C) for deeper sleep.
- Avoid caffeine after 2 PM — its half-life is about 5–6 hours.
Frequently Asked Questions
Why are sleep cycles 90 minutes long?
Human sleep progresses through 4–5 cycles per night, each lasting approximately 90 minutes (range: 80–110 min). Each cycle moves through: N1 (light sleep, 5%), N2 (light sleep, 50%), N3 (deep/slow-wave sleep, 20%), and REM (25%). Waking at the end of a cycle — during the light N1/N2 stage — minimises sleep inertia (grogginess). Waking mid-cycle during deep N3 sleep causes the worst grogginess.
How many hours of sleep do adults need?
The American Academy of Sleep Medicine (AASM) recommends: Adults (18–60): 7+ hours. Teenagers (13–18): 8–10 hours. School-age (6–12): 9–12 hours. Consistently sleeping fewer than 7 hours increases risk of obesity, cardiovascular disease, diabetes, depression, and impaired immune function. "Catching up" on weekends only partially compensates for weekday deficits.
What is sleep inertia and how can I avoid it?
Sleep inertia is the groggy, disoriented feeling immediately after waking — caused by waking during deep (N3) or REM sleep when adenosine and delta brain waves are at their strongest. To minimise it: use an alarm set to a sleep cycle boundary, use a gradual light alarm clock, get bright light exposure immediately upon waking, avoid snoozing repeatedly.
Why does the calculator add 14 minutes before calculating bedtime?
Sleep onset latency — the time from lying down to actually falling asleep — averages about 14 minutes for healthy adults. Adding this to your target wake time means your recommended bedtime is when to get into bed, accounting for the time you'll spend falling asleep. If you fall asleep faster or slower than average, adjust this offset accordingly.
Does the "wake up after N sleep cycles" method actually work?
Many people report feeling significantly more refreshed waking at a 90-minute cycle boundary compared to an arbitrary alarm time. However, sleep cycles aren't perfectly 90 minutes — they vary by age, sleep debt, alcohol consumption, and medications. The method works best as a guideline rather than a precise formula. Consistent sleep/wake times (circadian rhythm alignment) are equally important for sleep quality.