What a sleep cycle actually is
Sleep is not one continuous state. Across the night your brain moves through repeating cycles of light sleep, deep sleep, and REM sleep. Each full cycle takes roughly 90 minutes in adults — sometimes 70, sometimes 110, but 90 is a useful average. Waking at the end of a cycle, when you’re already in lighter sleep, feels dramatically easier than waking mid-deep-sleep — the heavy, foggy “sleep inertia” that ruins a morning. Most adults feel best after 5 or 6 complete cycles — 7.5 to 9 hours including the ~14 minutes it takes to fall asleep — which the Sleep Cycle tab highlights as the recommended options.
Napping without the grogginess
The same 90-minute-cycle logic applies to naps. A 10–20 minute power nap boosts alertness without grogginess; a 30-minute recovery nap helps after a short night; a full 90-minute nap completes a cycle and ends in light sleep. The Nap Calculator tab picks a length based on how much time you have and what you need, steering you away from the 45–80 minute zone where most people wake mid-deep-sleep.
Timing your last cup of caffeine
Caffeine has an average half-life of about 5 hours, so an afternoon coffee can still be active in your system at bedtime. The Caffeine Cutoff tab works backwards from your bedtime and your usual drink to find the latest time you can have it while keeping your caffeine load below the roughly 50 mg threshold associated with disrupted sleep.
Brown noise and other ambient sleep sounds
The Sleep Sounds tab mixes five procedurally-generated tracks — brown noise, pink noise, white noise, a rain-like texture, and slow ocean swells — synthesised live via the Web Audio API. Brown noise in particular is a favourite for sleep, deep focus, and tinnitus relief because its low, even spectrum masks sharp environmental sounds without the harsh hiss of white noise. A built-in sleep timer fades the mix out gently instead of cutting off.
What these tools can’t tell you
Sleep cycles and caffeine metabolism vary from person to person — the calculators use solid population averages, not a measurement of your own physiology. If a suggested bedtime or cutoff leaves you groggy, adjust by 10–15 minutes and track what actually works for you over a week or two.