What time should I go to bed?

Count backwards from your desired wake time. Subtract your target sleep cycles (5 cycles = 7.5 hrs for most adults) plus the time it takes you to fall asleep (~14 min). Example: wake at 7:00 AM with 14 min latency and 5 cycles means bedtime at 11:16 PM.

How many sleep cycles do I need?

Adults typically need 5–6 complete 90-minute cycles (7.5–9 hours). Teens need 5.5–6.5 cycles (8–10 hours) and children need 6–7 cycles (9–11 hours). Five cycles (7.5 hours) is the most common recommendation for adults.

Why do I feel groggy even after 8 hours?

Waking up mid-cycle causes sleep inertia — the groggy, foggy feeling that can last for hours. Aligning your alarm to a 90-minute cycle boundary means you wake at the lightest stage of sleep, feeling more refreshed even with slightly less total time.

What is sleep debt and how do I recover from it?

Sleep debt is the cumulative deficit between how much sleep you need and how much you get. One extra hour per night can help, but full recovery may take several days or even weeks. The Sleep Debt tab in this calculator estimates your deficit and recovery timeline.

Sleep Time Calculator — Bedtime & Wake Time by Sleep Cycle

What is a sleep cycle?

A sleep cycle lasts approximately 90 minutes and is made up of sequential stages. Your brain and body cycle through these stages repeatedly each night — most adults complete 4 to 6 full cycles in a healthy night's sleep.

Fall Asleep

~14 min avg

Transition from wakefulness. Brain waves slow, muscles relax.

Light Sleep

~25 min / cycle

N1 & N2 stages. Heart rate and temperature drop. Easily woken.

Deep Sleep

~35 min / cycle

N3 stage (slow-wave). Physical restoration, immune function, growth hormone release.

REM Sleep

~30 min / cycle

Rapid eye movement. Dreams occur. Memory consolidation and emotional processing.

Why align your alarm to a cycle boundary?

Waking mid-cycle — especially during deep sleep — causes sleep inertia: that heavy, foggy feeling that can last 30–60 minutes. Waking at the end of a cycle (during light sleep) is much gentler, even if total sleep time is slightly shorter. Many people report feeling more refreshed after 7.5 hours than after 8 hours, simply because the alarm hit the right moment.

How much sleep do I need?

Age Group	Recommended Hours	Ideal Cycles
Children (6–12)	9–11 hrs	6–7 cycles
Teens (13–17)	8–10 hrs	5–6.5 cycles
Adults (18–64)	7–9 hrs	5–6 cycles
Seniors (65+)	7–8 hrs	5 cycles

Tips for better cycle alignment

Keep a consistent wake time — even on weekends. Varying your wake time by more than an hour disrupts your circadian rhythm and makes it harder to fall asleep on Sunday nights. Your body will naturally adjust your bedtime over a few days. Use this calculator to set your alarm, then let your body find the right bedtime.

My usual sleep (hrs/night)

My sleep target (hrs/night)

Sleep Debt

1.0 hr deficit/night

Daily deficit

7.0 hrs/week deficit

Weekly debt

At 1 extra hr/night: ~7 days to clear debt

Note: These estimates are simplified. Real sleep debt recovery is non-linear — a few nights of extra sleep help, but full recovery from chronic deprivation takes weeks. Consistent nightly sleep is more effective than occasional long weekend sleep-ins.