Sleep Calculator

• Stage 1 (5-10 min) — light sleep, easy to wake.
• Stage 2 (20-25 min) — slightly deeper light sleep.
• Stage 3 (30-40 min) — deep slow-wave sleep. Hardest to wake from; waking during deep sleep is the source of "ugh" mornings.
• REM (10-30 min) — dream sleep. Brain active, body paralyzed. Cognitive consolidation happens here.
• Brief awakening (you don't remember) — then back to Stage 1 of the next cycle.

Across a night, early cycles are heavier on deep sleep; later cycles are heavier on REM. The deepest deep sleep happens in the first two cycles. A 4-hour sleep gets you 80% of deep sleep needs but skips most REM. A 7.5-hour sleep (five full cycles) catches both.

• Newborns (0-3 mo): 14-17 hours, with frequent waking.
• Infants (4-11 mo): 12-15 hours.
• Toddlers (1-2 yr): 11-14 hours.
• Preschool (3-5 yr): 10-13 hours.
• School age (6-13 yr): 9-11 hours.
• Teens (14-17 yr): 8-10 hours. The biological circadian shift in adolescence pushes natural bedtime ~2 hours later; early school start times are at odds with this.
• Adults (18-64 yr): 7-9 hours. Most adults at the lower end function fine; many at 6 hours think they are fine but show measurable cognitive deficits in lab studies.
• Older adults (65+): 7-8 hours. Sleep becomes more fragmented; quality declines even when quantity is maintained.

• Consistent timing — going to bed and waking at the same time every day (including weekends) is the single biggest improver of sleep quality. "Social jetlag" from weekend late nights mimics travel jet lag.
• Cool, dark room — body temperature must drop to initiate deep sleep. Bedroom 16-19°C (60-67°F) is the typical recommendation. Light (even from a phone in standby) disrupts melatonin production.
• No alcohol within 3 hours of bed — alcohol fragments sleep architecture, particularly suppressing REM in the first half of the night.
• No caffeine within 8 hours of bed — caffeine has a 5-hour half-life; afternoon coffee still has detectable levels at midnight.
• No screens 30-60 min before bed — both blue light and the cognitive arousal from content delay sleep onset.
• Morning light exposure — 10-15 minutes of bright outdoor light within an hour of waking calibrates your circadian rhythm. The single highest-leverage habit for chronic insomnia.

• You have a fixed wake time and want to pick a bedtime that does not leave you groggy.
• You have a flexible wake time (weekend) and want to pick a wake time that aligns with cycle boundaries given when you fell asleep.
• You are planning a strategic nap — 20 minutes (just stage 1-2, no grogginess) or 90 minutes (a full cycle, refreshing) but never 45 minutes (waking mid-deep-sleep is brutal).
• You are diagnosing chronic morning grogginess — is it really sleep duration, or are you waking mid-cycle every day?

• It will not diagnose sleep disorders. Snoring + daytime sleepiness despite 8h sleep = possible obstructive sleep apnea (about 25% of adult men have it; many undiagnosed). See a sleep doctor; a calculator cannot tell.
• It will not measure your actual cycles. Cycle length varies between people (75-110 minutes) and across the night. Wearables (Oura, Whoop, Apple Watch) estimate cycles via heart rate and motion; their accuracy is okay-to-bad compared to polysomnography.
• It will not solve insomnia. Sleep timing math helps with structure; insomnia treatment requires CBT-I (cognitive behavioral therapy for insomnia), light therapy, possibly medical evaluation.