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Category Health & Nutrition

Wake-up time Set the time your alarm should ring

Bedtime Set the time you plan to go to bed

Calculating from the current time...

Sleep onset latency

min

Average time to fall asleep (default 14 min)

Sleep Architecture Preview

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About

Waking mid-cycle - during deep N3 or REM sleep - produces sleep inertia: grogginess, impaired cognition, and elevated cortisol that can persist for 30 - 60 minutes. Each human sleep cycle averages 90 min and repeats 4 - 6 times per night, cycling through stages N1, N2, N3 (slow-wave), and REM. This calculator factors in an average sleep onset latency of 14 min (the time a healthy adult needs to transition from wakefulness to N1) and computes alarm or bedtime targets that coincide with the end of a complete cycle - the lightest sleep phase, where arousal threshold is lowest.

The tool assumes fixed 90-minute cycles. In practice, early cycles are dominated by slow-wave sleep (70 - 100 min) while later cycles shift toward REM (90 - 120 min). Individual variation is significant: cycle length, onset latency, and chronotype all differ. Treat these times as starting approximations, not prescriptions. The National Sleep Foundation recommends 7 - 9 hours for adults aged 18 - 64.

Formulas

The wake-up or bedtime target is computed by stepping through complete 90-minute cycles, offset by an average sleep onset latency of 14 min.

T_wake = T_sleep + L + n × 90

T_bed = T_wake − L − n × 90

Where T_sleep is the moment you intend to go to bed, T_wake is the desired alarm time, L = 14 min is the mean sleep onset latency for healthy adults, and n ∈ {3, 4, 5, 6} is the number of complete cycles. Each cycle consists of N1 → N2 → N3 → N2 → REM, averaging 90 min. The calculator evaluates all four values of n and flags n = 5 or n = 6 as optimal for adults.

Reference Data

Cycles	Sleep Duration	Total (with onset)	Rating	Dominant Late-Phase
1	1 h 30 min	1 h 44 min	Nap (emergency)	N2 / light SWS
2	3 h 0 min	3 h 14 min	Minimal rest	SWS-heavy
3	4 h 30 min	4 h 44 min	Poor - sleep debt accrues	SWS → early REM
4	6 h 0 min	6 h 14 min	Adequate (short sleepers)	Balanced SWS / REM
5	7 h 30 min	7 h 44 min	Good - recommended	REM-dominant
6	9 h 0 min	9 h 14 min	Optimal - full recovery	Extended REM
Sleep Stage Reference
Stage	Characteristics		% of Night	EEG Pattern
N1	Light sleep, hypnic jerks, easy arousal		5%	θ waves (4-7 Hz)
N2	Sleep spindles, K-complexes, body temp drops		45%	σ bursts (12-14 Hz)
N3 (SWS)	Deep sleep, growth hormone release, tissue repair		25%	δ waves (0.5-2 Hz)
REM	Dreaming, memory consolidation, muscle atonia		25%	Desynchronized (mixed)
Age-Based Sleep Recommendations (NSF)
Age Group	Recommended Hours		Min Cycles	Max Cycles
Newborn (0-3 mo)	14 - 17 h		9	11
Infant (4-11 mo)	12 - 15 h		8	10
Toddler (1-2 y)	11 - 14 h		7	9
Preschool (3-5 y)	10 - 13 h		7	9
School (6-13 y)	9 - 11 h		6	7
Teen (14-17 y)	8 - 10 h		5	7
Adult (18-64 y)	7 - 9 h		5	6
Older Adult (65+ y)	7 - 8 h		5	5

Frequently Asked Questions

The 90-minute figure is a population average derived from polysomnography studies. Individual cycle lengths range from 70 to 120 minutes and shift within the same night: early cycles are shorter and SWS-heavy, while late cycles stretch due to longer REM episodes. This calculator uses 90 minutes as the best single approximation. If you consistently wake groggy at the suggested times, try shifting your alarm by ±10 minutes to calibrate to your personal cycle length.

Sleep onset latency (SOL) is the interval between lying down with intent to sleep and reaching stage N1. The 14-minute default reflects healthy adult norms from AASM literature. If you fall asleep in under 5 minutes, you may be sleep-deprived - paradoxically, shorter SOL suggests higher sleep debt. If your SOL exceeds 25 minutes (common with insomnia or high caffeine intake), increase the value in the tool. Alcohol reduces SOL but fragments later cycles, reducing REM by up to 20%.

Caffeine has a half-life of approximately 5-6 hours. Consumed within 6 hours of bedtime, it increases SOL by 10-40 minutes and reduces N3 (deep sleep) duration. Alcohol initially acts as a sedative (reducing SOL) but causes rebound wakefulness in the second half of the night, fragmenting cycles 4-6. Neither substance changes the 90-minute cycle architecture itself, but both degrade cycle quality and may cause mid-cycle awakenings that the calculator cannot predict.

From a cycle-alignment perspective, 6 hours (4 complete cycles) will produce a cleaner wake-up than 7 hours, which interrupts the 5th cycle during deep N3 sleep at approximately the 6.5-hour mark. However, chronic 6-hour sleep accumulates sleep debt - after 10 days, cognitive impairment equals that of 24 hours of total sleep deprivation (Van Dongen et al., 2003). Use 4 cycles as a short-term strategy only.

The calculator uses modular 24-hour arithmetic. If you set a bedtime of 23:30 and request wake-up times, it correctly rolls forward past midnight. Similarly, a wake-up time of 06:00 with 6 cycles yields a bedtime of 20:46 the previous evening. All displayed times use 12-hour format with AM/PM indicators for clarity.

Set your alarm for the exact time or up to 5 minutes after. The transition from REM to the brief N1 inter-cycle window is a narrow target - approximately 5-15 minutes wide. Setting the alarm earlier risks catching the tail end of the preceding REM phase. If your phone alarm has a "smart alarm" feature (using accelerometer data), set the window to ±10 minutes around the calculated time for best results.