Question 1

How does the sleep calculator work?

Accepted Answer

The sleep calculator works backward or forward from your target time in 90-minute increments — the average length of one complete sleep cycle — and adds approximately 14 minutes to fall asleep. This means if you want to wake up at 7:00 AM, you should fall asleep at 10:46 PM (5 cycles) or 9:16 PM (6 cycles).

Question 2

How many sleep cycles do I need per night?

Accepted Answer

Most adults need 5–6 complete sleep cycles per night, which is 7.5–9 hours of sleep. Teenagers may need 6–7 cycles, and older adults 4–5 cycles. The American Academy of Sleep Medicine recommends 7–9 hours for adults aged 18–64.

Question 3

What time should I go to bed if I wake up at 7:00 AM?

Accepted Answer

If you wake up at 7:00 AM, you should go to sleep at: 9:46 PM for 6 cycles (optimal, 9 hours), 11:16 PM for 5 cycles (recommended, 7.5 hours), or 12:46 AM for 4 cycles (minimum, 6 hours). All times include 14 minutes to fall asleep.

Question 4

What is sleep inertia?

Accepted Answer

Sleep inertia is the groggy, disoriented feeling when waking mid-cycle, especially from deep sleep (N3 stage). It can impair cognitive function for 15–60 minutes. Waking at the end of a 90-minute sleep cycle dramatically minimizes sleep inertia.

Question 5

What is the best room temperature for sleep?

Accepted Answer

The optimal bedroom temperature for sleep is 65–68°F (18–20°C). Your body needs its core temperature to drop 1–2°F to initiate deep sleep. A too-warm room physically prevents this process and reduces deep sleep quality.

Question 6

How long is a sleep cycle?

Accepted Answer

A sleep cycle averages 90 minutes but can range from 70–120 minutes. Early cycles in the night have more deep sleep (N3), while later cycles have progressively more REM sleep. The 90-minute estimate is well-supported by sleep research and works as a practical guideline for most people.

Question 7

Why do I feel tired after 8 hours of sleep?

Accepted Answer

Feeling tired after 8 hours usually means you woke up in the middle of a sleep cycle, particularly during deep sleep (N3). Other causes include sleep apnea, alcohol consumption before bed (which suppresses REM sleep), accumulated sleep debt, or an inconsistent sleep schedule. Use the sleep calculator to time your alarm to the end of a cycle.

Question 8

How long should a power nap be?

Accepted Answer

The ideal power nap is 10–20 minutes, which keeps you in light sleep (N1/N2) and avoids grogginess on waking. A 30-minute nap enters some deeper sleep and may cause brief grogginess. A 90-minute nap completes one full cycle including REM and provides maximum restorative benefit. Avoid naps lasting 45–85 minutes as these often end in deep sleep.

Hours	Cycles	Notes
14–17h	—
12–15h	—
11–14h	—
10–13h	—
9–12h	6–8
8–10h	5–7
7–9h	5–6	Recommended
7–9h	5–6	Recommended
7–8h	4–5

Wake Up Refreshed Every Morning

The 4 Stages of Sleep

Light Sleep – Transition

Light Sleep – Core

Deep Sleep – Restoration

REM – Dream Sleep

How Much Sleep Do You Need?

Sleep Hygiene Tips

Frequently Asked Questions