Sleep FAQ — Frequently Asked Questions
How does the Sleep Calculator work?
The calculator uses 90-minute sleep cycles to find the best times to sleep or wake up. Enter your desired wake-up time, and it suggests bedtimes that align with complete sleep cycles. Or enter your bedtime, and it calculates optimal wake-up times. It also adds about 15 minutes for the time it takes to fall asleep.
Why do I feel tired even after 8 hours of sleep?
The issue is usually when you wake up, not how long you sleep. If your alarm goes off during deep sleep or REM sleep, you’ll feel groggy regardless of total sleep time. This is called sleep inertia. Timing your alarm to the end of a sleep cycle — when you’re in light sleep — makes a big difference.
How long is one sleep cycle?
A single sleep cycle lasts about 90 minutes on average. It can range from 80 to 120 minutes depending on the individual. During one night, you typically go through 5 to 6 complete cycles.
How much sleep do I actually need?
It depends on your age:
| Age Group | Recommended Sleep |
|---|---|
| Newborn (0–3 months) | 14–17 hours |
| Infant (4–12 months) | 12–16 hours |
| Toddler (1–2 years) | 11–14 hours |
| Preschool (3–5 years) | 10–13 hours |
| School-age (6–12 years) | 9–12 hours |
| Teen (13–18 years) | 8–10 hours |
| Adult (18–64 years) | 7–9 hours |
| Older Adult (65+) | 7–8 hours |
These are guidelines from the National Sleep Foundation and CDC.
What are the four stages of sleep?
Each sleep cycle has four stages:
- Stage 1 — Light Sleep: A brief transition from wakefulness. Lasts a few minutes. Easy to wake from.
- Stage 2 — Sleep Onset: Heart rate and breathing slow down. Brain produces sleep spindles. Lasts about 20 minutes.
- Stage 3 — Deep Sleep: The most restorative stage. Your body repairs tissues, builds bone and muscle, and strengthens the immune system. Very hard to wake from.
- Stage 4 — REM Sleep: Rapid eye movement sleep. This is when vivid dreaming happens. Your brain consolidates memories and processes emotions.
What is REM sleep and why does it matter?
REM (Rapid Eye Movement) sleep is the final stage of each sleep cycle. During REM, your brain is highly active — processing memories, regulating emotions, and supporting learning. The first REM period lasts about 10 minutes, but later cycles can include REM periods of up to an hour. Cutting sleep short means missing the longest, most important REM periods.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours (5 full cycles) is almost always better than 6 hours (4 cycles). With 6 hours, you miss an entire sleep cycle — including valuable REM sleep. However, 6 hours ending at the right point in a cycle is better than 7 hours interrupted mid-cycle.
Can I make up for lost sleep on weekends?
Partially, but it’s not ideal. “Sleep debt” accumulates over time, and weekend catch-up sleep can disrupt your circadian rhythm, making Monday mornings even harder. Consistent sleep and wake times — even on weekends — produce better results.
Does napping help or hurt nighttime sleep?
Short naps (20–30 minutes) in the early afternoon can boost alertness without affecting nighttime sleep. But long naps (over 60 minutes) or napping late in the day can make it harder to fall asleep at night. If you nap, keep it short and early.
How long does it take to fall asleep?
The average person takes 10 to 20 minutes to fall asleep. Our calculator defaults to 15 minutes, but you can adjust this in Settings. If you consistently fall asleep in under 5 minutes, that may actually be a sign of sleep deprivation.
Does caffeine really affect sleep?
Yes. Caffeine has a half-life of about 5–6 hours, meaning half the caffeine from your afternoon coffee is still in your system at bedtime. For better sleep, avoid caffeine at least 6 hours before bed — ideally after 2 PM if you sleep around 10 PM.
Does screen time before bed affect sleep quality?
Blue light from screens suppresses melatonin production, the hormone that signals your body it’s time to sleep. Research suggests stopping screen use 30–60 minutes before bed improves both how quickly you fall asleep and overall sleep quality. Night mode or blue light filters help, but aren’t as effective as simply putting the screen away.
What’s the best room temperature for sleep?
Most sleep research points to 60–67°F (15–19°C) as the ideal bedroom temperature. Your core body temperature naturally drops during sleep, and a cool room supports this process. Too warm or too cold, and you’re more likely to wake up during the night.
Can exercise improve sleep?
Regular exercise significantly improves sleep quality — especially moderate aerobic activity like walking, cycling, or swimming. The key is timing: exercising in the morning or afternoon is ideal. Intense workouts within 2 hours of bedtime can actually make it harder to fall asleep.
What should I do if I can’t fall asleep?
If you’ve been lying in bed for more than 20 minutes without falling asleep, get up. Do something quiet and non-stimulating — read a book, listen to calm music, or practice deep breathing. Return to bed when you feel sleepy. Staying in bed while awake trains your brain to associate the bed with wakefulness.
How can I customize the Sleep Calculator?
Click the Settings icon in the top right corner. You can adjust two parameters:
- Sleep cycle length: Default is 90 minutes. If you know your cycles are shorter or longer, change it here (range: 60–120 minutes).
- Fall-asleep time: Default is 15 minutes. Adjust based on how long it typically takes you to fall asleep (range: 0–60 minutes).
Your settings are saved locally in your browser.